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* [PATCH v38 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
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filename="v38-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v27 9/9] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 601 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index a533a2153e..2561faa163 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2193,6 +2193,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3508,6 +3517,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3891,7 +3912,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..52a4d206b1 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..c29cfc19b6 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 4b2ba5a4e6..71e10ef5a1 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1090,6 +1090,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Fri__22_Apr_2022_14_58_01_+0900_MN3L/o2YUVF2g4zw--
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* [PATCH v25 10/15] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 6 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 599 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 7d5b0b1656..7c07ac8ece 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2188,6 +2188,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3474,6 +3483,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3857,7 +3878,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..c3bd46571f 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause,
+ a unique index is created on the columns of GROUP BY expressions. Also, if the
+ view has DISTINCT clause, a unique index is created on all columns in the target
+ list. Otherwise, if the view contains all primary key attritubes of its base
+ tables in the target list, a unique index is created on these attritubes. In
+ other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..e0d15fdce0 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,11 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. Also, if the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining
+ the view are created. If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 4aa4e00e01..13a64eab7f 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1090,6 +1090,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Fri__4_Feb_2022_01_48_06_+0900_N8BNZpfOR27sWrgY
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name="v25-0009-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Disposition: attachment;
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v26 10/10] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 601 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 7777d60514..990798e13d 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2188,6 +2188,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3485,6 +3494,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3868,7 +3889,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..52a4d206b1 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..c29cfc19b6 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 4aa4e00e01..13a64eab7f 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1090,6 +1090,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v24 10/15] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 6 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 599 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 00b648a433..68a1377b6f 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2188,6 +2188,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3472,6 +3481,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3855,7 +3876,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index d8c48252f4..8a440ca810 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause,
+ a unique index is created on the columns of GROUP BY expressions. Also, if the
+ view has DISTINCT clause, a unique index is created on all columns in the target
+ list. Otherwise, if the view contains all primary key attritubes of its base
+ tables in the target list, a unique index is created on these attritubes. In
+ other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -153,7 +279,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 3bf8884447..dc81853057 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,11 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. Also, if the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining
+ the view are created. If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 6065b1c2a3..264fdefc37 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1093,6 +1093,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Fri__29_Oct_2021_18_16_28_+0900_jlYRKjywLqhZ7oyk--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v24 10/15] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 6 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 599 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 2f0def9b19..5d4d5a6e5e 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2183,6 +2183,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3465,6 +3474,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3848,7 +3869,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index d8c48252f4..8a440ca810 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause,
+ a unique index is created on the columns of GROUP BY expressions. Also, if the
+ view has DISTINCT clause, a unique index is created on all columns in the target
+ list. Otherwise, if the view contains all primary key attritubes of its base
+ tables in the target list, a unique index is created on these attritubes. In
+ other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -153,7 +279,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 3bf8884447..dc81853057 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,11 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. Also, if the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining
+ the view are created. If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 6065b1c2a3..264fdefc37 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1093,6 +1093,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Thu__23_Sep_2021_04_57_30_+0900_b5pmgR1N8oMaMz.U--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v23 10/15] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 133 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 6 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 601 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 2b2c70a26e..b1ebdae501 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2183,6 +2183,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3465,6 +3474,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3848,7 +3869,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index d8c48252f4..5abccf14ba 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,134 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause,
+ a unique index is created on the columns of GROUP BY expressions. Also, if the
+ view has DISTINCT clause, a unique index is created on all columns in the target
+ list. Otherwise, if the view contains all primary key attritubes of its base
+ tables in the target list, a unique index is created on these attritubes. In
+ other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+ <listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+ <listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -153,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 3bf8884447..dc81853057 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,11 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. Also, if the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining
+ the view are created. If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 5024e4ff70..597332fe96 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1093,6 +1093,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Mon__2_Aug_2021_15_28_34_+0900_wlHCjIpnD/FrGAKu--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v37 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.43.0
--Multipart=_Fri__29_May_2026_23_14_17_+0900_Te0o73X2VqYK57Gd
Content-Type: text/x-diff;
name="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Disposition: attachment;
filename="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v37 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.43.0
--Multipart=_Fri__29_May_2026_23_14_17_+0900_Te0o73X2VqYK57Gd
Content-Type: text/x-diff;
name="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Disposition: attachment;
filename="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v38 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.43.0
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Content-Type: text/x-diff;
name="v38-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
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^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v29 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index d17ff51e28..3de3303cac 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2224,6 +2224,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..c29cfc19b6 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index d229b94d39..22e4cad103 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1096,6 +1096,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2b35c2f91b..5366f707eb 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Mon__28_Aug_2023_11_52_52_+0900_hj6L5h176QaSGtg7--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v29 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index d17ff51e28..3de3303cac 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2224,6 +2224,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..c29cfc19b6 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index d229b94d39..22e4cad103 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1096,6 +1096,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2b35c2f91b..5366f707eb 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Mon__28_Aug_2023_16_05_30_+0900_b1OvQD_3A3ZMTGvj--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v37 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.43.0
--Multipart=_Fri__29_May_2026_23_14_17_+0900_Te0o73X2VqYK57Gd
Content-Type: text/x-diff;
name="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Disposition: attachment;
filename="v37-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Transfer-Encoding: 7bit
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v38 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 4b474c13917..9b1c88161a7 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2276,6 +2276,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 62d897931c3..f49db209558 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -157,7 +276,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade803..a4d729bdf0f 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7f23962f524..da9ad3c6316 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1103,6 +1103,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 2ebec6928d5..65d1e2041d2 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -2231,6 +2231,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.43.0
--Multipart=_Wed__1_Jul_2026_00_04_01_+0900_OVSy2WWK_9aByzDJ
Content-Type: text/x-diff;
name="v38-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Disposition: attachment;
filename="v38-0010-Add-regression-tests-for-Incremental-View-Mainte.patch"
Content-Transfer-Encoding: 7bit
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v28 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 587 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index ed32ca0349..51378358a1 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2223,6 +2223,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..52a4d206b1 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 4d79b6ae7f..4e27e29e35 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -38,9 +38,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index d229b94d39..d000beacdf 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1096,6 +1096,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.25.1
--Multipart=_Thu__1_Jun_2023_23_59_09_+0900_/G5+8nG46.f1T42K--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v30 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 880f717b10..f8df2951a8 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2224,6 +2224,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 7a019162c3..dd894a2324 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 784c16e76e..149bb99b1c 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1098,6 +1098,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index be90edd0e2..c909b2ab91 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Mon__4_Mar_2024_11_58_46_+0900_UaponF/qQhQrVCFt--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v30 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 880f717b10..f8df2951a8 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2224,6 +2224,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 7a019162c3..dd894a2324 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 784c16e76e..149bb99b1c 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1098,6 +1098,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index be90edd0e2..c909b2ab91 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Mon__4_Mar_2024_11_58_46_+0900_UaponF/qQhQrVCFt--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v30 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 880f717b10..f8df2951a8 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2224,6 +2224,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 7a019162c3..dd894a2324 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 784c16e76e..149bb99b1c 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1098,6 +1098,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index be90edd0e2..c909b2ab91 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Mon__4_Mar_2024_11_58_46_+0900_UaponF/qQhQrVCFt--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v31 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 096ddab481..d4255f1015 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2231,6 +2231,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade80..a4d729bdf0 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7a928bd7b9..73597ea7a5 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1100,6 +1100,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 3c8dca8ca3..da22606213 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Fri__29_Mar_2024_23_47_00_+0900_KGpmmDOIs1266Ib1--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v32 11/11] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 9 +
.../sgml/ref/create_materialized_view.sgml | 124 ++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 437 ++++++++++++++++++
doc/src/sgml/system-views.sgml | 9 +
5 files changed, 583 insertions(+), 4 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 096ddab481..d4255f1015 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2231,6 +2231,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if relation is incrementally maintainable materialized view
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..8c574062db 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,125 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +274,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8ed43ade80..a4d729bdf0 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -36,9 +36,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
privilege on the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 7a928bd7b9..73597ea7a5 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1100,6 +1100,443 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2 id="rules-ivm-support">
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3 id="rules-ivm-support-joins">
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3 id="rules-ivm-support-aggregates">
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4 id="rules-ivm-restrictions-aggregates">
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3 id="rules-ivm-general-restricitons">
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2 id="rules-ivm-distinct">
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-concurrent-transactions">
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table and
+ <command>INSERT</command> is performed on the table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2 id="rules-ivm-rls">
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
diff --git a/doc/src/sgml/system-views.sgml b/doc/src/sgml/system-views.sgml
index 3c8dca8ca3..da22606213 100644
--- a/doc/src/sgml/system-views.sgml
+++ b/doc/src/sgml/system-views.sgml
@@ -1787,6 +1787,15 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>isimmv</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view is incrementally maintainable
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>definition</structfield> <type>text</type>
--
2.25.1
--Multipart=_Sun__31_Mar_2024_22_59_31_+0900_msknEviJj08_wgqO--
^ permalink raw reply [nested|flat] 28+ messages in thread
* [PATCH v27 9/9] Add documentations about Incremental View Maintenance
@ 2019-12-20 01:25 Yugo Nagata <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Yugo Nagata @ 2019-12-20 01:25 UTC (permalink / raw)
---
doc/src/sgml/catalogs.sgml | 24 +-
.../sgml/ref/create_materialized_view.sgml | 131 +++++-
.../sgml/ref/refresh_materialized_view.sgml | 8 +-
doc/src/sgml/rules.sgml | 443 ++++++++++++++++++
4 files changed, 601 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index a533a2153e..2561faa163 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -2193,6 +2193,15 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para></entry>
</row>
+ <row>
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>relisivm</structfield> <type>bool</type>
+ </para>
+ <para>
+ True if materialized view enables incremental view maintenance
+ </para></entry>
+ </row>
+
<row>
<entry role="catalog_table_entry"><para role="column_definition">
<structfield>relrewrite</structfield> <type>oid</type>
@@ -3508,6 +3517,18 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><symbol>DEPENDENCY_IMMV</symbol> (<literal>m</literal>)</term>
+ <listitem>
+ <para>
+ The dependent object was created as part of creation of the Materialized
+ View with Incremental View Maintenance reference, and is really just a
+ part of its internal implementation. The dependent object must not be
+ dropped unless the materialized view is also dropped.
+ </para>
+ </listitem>
+ </varlistentry>
</variablelist>
Other dependency flavors might be needed in future.
@@ -3891,7 +3912,8 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l
<structfield>extrelocatable</structfield> <type>bool</type>
</para>
<para>
- True if extension can be relocated to another schema
+ True for materialized views which are enabled for incremental
+ view maintenance (IVM).
</para></entry>
</row>
diff --git a/doc/src/sgml/ref/create_materialized_view.sgml b/doc/src/sgml/ref/create_materialized_view.sgml
index 0d2fea2b97..52a4d206b1 100644
--- a/doc/src/sgml/ref/create_materialized_view.sgml
+++ b/doc/src/sgml/ref/create_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
-CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
+CREATE [ INCREMENTAL ] MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
[ (<replaceable>column_name</replaceable> [, ...] ) ]
[ USING <replaceable class="parameter">method</replaceable> ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
@@ -60,6 +60,132 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
<title>Parameters</title>
<variablelist>
+ <varlistentry>
+ <term><literal>INCREMENTAL</literal></term>
+ <listitem>
+ <para>
+ If specified, some triggers are automatically created so that the rows
+ of the materialized view are immediately updated when base tables of the
+ materialized view are updated. In general, this allows faster update of
+ the materialized view at a price of slower update of the base tables
+ because the triggers will be invoked. We call this form of materialized
+ view as "Incrementally Maintainable Materialized View" (IMMV).
+ </para>
+ <para>
+ When <acronym>IMMV</acronym> is defined without using <command>WITH NO DATA</command>,
+ a unique index is created on the view automatically if possible. If the view
+ definition query has a GROUP BY clause, a unique index is created on the columns
+ of GROUP BY expressions. Also, if the view has DISTINCT clause, a unique index
+ is created on all columns in the target list. Otherwise, if the view contains all
+ primary key attritubes of its base tables in the target list, a unique index is
+ created on these attritubes. In other cases, no index is created.
+ </para>
+ <para>
+ There are restrictions of query definitions allowed to use this
+ option. The following are supported in query definitions for IMMV:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ Inner joins (including self-joins).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Some built-in aggregate functions (count, sum, avg, min, max) without a HAVING
+ clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Unsupported queries with this option include the following:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Outer joins.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Sub-queries.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Aggregate functions other than built-in count, sum, avg, min and max.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Aggregate functions with a HAVING clause.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ DISTINCT ON, WINDOW, VALUES, LIMIT and OFFSET clause.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ Other restrictions include:
+ <itemizedlist>
+
+ <listitem>
+ <para>
+ IMMVs must be based on simple base tables. It's not supported to
+ create them on top of views or materialized views.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the TRUNCATE command is executed on a base table,
+ no changes are made to the materialized view.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ It is not supported to include system columns in an IMMV.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm02 AS SELECT i,j FROM mv_base_a WHERE xmin = '610';
+ERROR: system column is not supported with IVM
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Non-immutable functions are not supported.
+ <programlisting>
+CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm12 AS SELECT i,j FROM mv_base_a WHERE i = random()::int;
+ERROR: functions in IMMV must be marked IMMUTABLE
+ </programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ IMMVs do not support expressions that contains aggregates
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication does not support IMMVs.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><literal>IF NOT EXISTS</literal></term>
<listitem>
@@ -155,7 +281,8 @@ CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] <replaceable>table_name</replaceable>
This clause specifies whether or not the materialized view should be
populated at creation time. If not, the materialized view will be
flagged as unscannable and cannot be queried until <command>REFRESH
- MATERIALIZED VIEW</command> is used.
+ MATERIALIZED VIEW</command> is used. Also, if the view is IMMV,
+ triggers for maintaining the view are not created.
</para>
</listitem>
</varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 675d6090f3..c29cfc19b6 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -35,9 +35,13 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
owner of the materialized view. The old contents are discarded. If
<literal>WITH DATA</literal> is specified (or defaults) the backing query
is executed to provide the new data, and the materialized view is left in a
- scannable state. If <literal>WITH NO DATA</literal> is specified no new
+ scannable state. If the view is an incrementally maintainable materialized
+ view (IMMV) and was unpopulated, triggers for maintaining the view are
+ created. Also, a unique index is created for IMMV if it is possible and the
+ view doesn't have that yet.
+ If <literal>WITH NO DATA</literal> is specified no new
data is generated and the materialized view is left in an unscannable
- state.
+ state. If the view is IMMV, the triggers are dropped.
</para>
<para>
<literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 4b2ba5a4e6..71e10ef5a1 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1090,6 +1090,449 @@ SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;
</sect1>
+<sect1 id="rules-ivm">
+<title>Incremental View Maintenance</title>
+
+<indexterm zone="rules-ivm">
+ <primary>incremental view maintenance</primary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>materialized view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<indexterm zone="rules-ivm">
+ <primary>view</primary>
+ <secondary>incremental view maintenance</secondary>
+</indexterm>
+
+<sect2 id="rules-ivm-overview">
+<title>Overview</title>
+
+<para>
+ Incremental View Maintenance (<acronym>IVM</acronym>) is a way to make
+ materialized views up-to-date in which only incremental changes are computed
+ and applied on views rather than recomputing the contents from scratch as
+ <command>REFRESH MATERIALIZED VIEW</command> does. <acronym>IVM</acronym>
+ can update materialized views more efficiently than recomputation when only
+ small parts of the view are changed.
+</para>
+
+<para>
+ There are two approaches with regard to timing of view maintenance:
+ immediate and deferred. In immediate maintenance, views are updated in the
+ same transaction that its base table is modified. In deferred maintenance,
+ views are updated after the transaction is committed, for example, when the
+ view is accessed, as a response to user command like <command>REFRESH
+ MATERIALIZED VIEW</command>, or periodically in background, and so on.
+ <productname>PostgreSQL</productname> currently implements only a kind of
+ immediate maintenance, in which materialized views are updated immediately
+ in AFTER triggers when a base table is modified.
+</para>
+
+<para>
+ To create materialized views supporting <acronym>IVM</acronym>, use the
+ <command>CREATE INCREMENTAL MATERIALIZED VIEW</command>, for example:
+<programlisting>
+CREATE <emphasis>INCREMENTAL</emphasis> MATERIALIZED VIEW mymatview AS SELECT * FROM mytab;
+</programlisting>
+ When a materialized view is created with the <literal>INCREMENTAL</literal>
+ keyword, some triggers are automatically created so that the view's contents are
+ immediately updated when its base tables are modified. We call this form
+ of materialized view an Incrementally Maintainable Materialized View
+ (<acronym>IMMV</acronym>).
+<programlisting>
+postgres=# CREATE INCREMENTAL MATERIALIZED VIEW m AS SELECT * FROM t0;
+NOTICE: could not create an index on materialized view "m" automatically
+HINT: Create an index on the materialized view for effcient incremental maintenance.
+SELECT 3
+postgres=# SELECT * FROM m;
+ i
+---
+ 1
+ 2
+ 3
+(3 rows)
+
+postgres=# INSERT INTO t0 VALUES (4);
+INSERT 0 1
+postgres=# SELECT * FROM m; -- automatically updated
+ i
+---
+ 1
+ 2
+ 3
+ 4
+(4 rows)
+</programlisting>
+</para>
+
+<para>
+ Some <acronym>IMMV</acronym>s have hidden columns which are added
+ automatically when a materialized view is created. Their name starts
+ with <literal>__ivm_</literal> and they contain information required
+ for maintaining the <acronym>IMMV</acronym>. Such columns are not visible
+ when the <acronym>IMMV</acronym> is accessed by <literal>SELECT *</literal>
+ but are visible if the column name is explicitly specified in the target
+ list. We can also see the hidden columns in <literal>\d</literal>
+ meta-commands of <command>psql</command> commands.
+</para>
+
+<para>
+ In general, <acronym>IMMV</acronym>s allow faster updates of materialized
+ views at the price of slower updates to their base tables. Updates of
+ <acronym>IMMV</acronym> is slower because triggers will be invoked and the
+ view is updated in triggers per modification statement.
+</para>
+
+<para>
+ For example, suppose a normal materialized view defined as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+SELECT 10000000
+
+</programlisting>
+
+ Updating a tuple in a base table of this materialized view is rapid but the
+ <command>REFRESH MATERIALIZED VIEW</command> command on this view takes a long time:
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 0.990 ms
+
+test=# REFRESH MATERIALIZED VIEW mv_normal ;
+REFRESH MATERIALIZED VIEW
+Time: 33533.952 ms (00:33.534)
+</programlisting>
+</para>
+
+<para>
+ On the other hand, after creating <acronym>IMMV</acronym> with the same view
+ definition as below:
+
+<programlisting>
+test=# CREATE INCREMENTAL MATERIALIZED VIEW mv_ivm AS
+ SELECT a.aid, b.bid, a.abalance, b.bbalance
+ FROM pgbench_accounts a JOIN pgbench_branches b USING(bid);
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+NOTICE: created index "mv_ivm_index" on materialized view "mv_ivm"
+</programlisting>
+
+ updating a tuple in a base table takes more than the normal view,
+ but its content is updated automatically and this is faster than the
+ <command>REFRESH MATERIALIZED VIEW</command> command.
+
+<programlisting>
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 13.068 ms
+</programlisting>
+
+</para>
+
+<para>
+ Appropriate indexes on <acronym>IMMV</acronym>s are necessary for
+ efficient <acronym>IVM</acronym> because it looks for tuples to be
+ updated in <acronym>IMMV</acronym>. If there are no indexes, it
+ will take a long time.
+</para>
+
+<para>
+ Therefore, when <acronym>IMMV</acronym> is defined, a unique index is created on the view
+ automatically if possible. If the view definition query has a GROUP BY clause, a unique
+ index is created on the columns of GROUP BY expressions. Also, if the view has DISTINCT
+ clause, a unique index is created on all columns in the target list. Otherwise, if the
+ view contains all primary key attritubes of its base tables in the target list, a unique
+ index is created on these attritubes. In other cases, no index is created.
+</para>
+
+<para>
+ In the previous example, a unique index "mv_ivm_index" is created on aid and bid
+ columns of materialized view "mv_ivm", and this enables the rapid update of the view.
+ Dropping this index make updating the view take a loger time.
+<programlisting>
+test=# DROP INDEX mv_ivm_index;
+DROP INDEX
+Time: 67.081 ms
+
+test=# UPDATE pgbench_accounts SET abalance = 1000 WHERE aid = 1;
+UPDATE 1
+Time: 16386.245 ms (00:16.386)
+</programlisting>
+
+</para>
+
+<para>
+ <acronym>IVM</acronym> is effective when we want to keep a materialized
+ view up-to-date and small fraction of a base table is modified
+ infrequently. Due to the overhead of immediate maintenance, <acronym>IVM</acronym>
+ is not effective when a base table is modified frequently. Also, when a
+ large part of a base table is modified or large data is inserted into a
+ base table, <acronym>IVM</acronym> is not effective and the cost of
+ maintenance can be larger than the <command>REFRESH MATERIALIZED VIEW</command>
+ command. In such situation, we can use <command>REFRESH MATERIALIZED VIEW</command>
+ and specify <literal>WITH NO DATA</literal> to disable immediate
+ maintenance before modifying a base table. After a base table modification,
+ execute the <command>REFRESH MATERIALIZED VIEW</command> (with <literal>WITH DATA</literal>)
+ command to refresh the view data and enable immediate maintenance.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Supported View Definitions and Restrictions</title>
+
+<para>
+ Currently, we can create <acronym>IMMV</acronym>s using inner joins, and some
+ aggregates. However, several restrictions apply to the definition of IMMV.
+</para>
+
+<sect3>
+<title>Joins</title>
+<para>
+ Inner joins including self-join are supported. Outer joins are not supported.
+</para>
+</sect3>
+
+<sect3>
+<title>Aggregates</title>
+<para>
+ Supported aggregate functions are <function>count</function>, <function>sum</function>,
+ <function>avg</function>, <function>min</function>, and <function>max</function>.
+ Currently, only built-in aggregate functions are supported and user defined
+ aggregates cannot be used. When a base table is modified, the new aggregated
+ values are incrementally calculated using the old aggregated values and values
+ of related hidden columns stored in <acronym>IMMV</acronym>.
+</para>
+
+<para>
+ Note that for <function>min</function> or <function>max</function>, the new values
+ could be re-calculated from base tables with regard to the affected groups when a
+ tuple containing the current minimal or maximal values are deleted from a base table.
+ Therefore, it can takes a long time to update an <acronym>IMMV</acronym> containing
+ these functions.
+</para>
+
+<para>
+ Also note that using <function>sum</function> or <function>avg</function> on
+ <type>real</type> (<type>float4</type>) type or <type>double precision</type>
+ (<type>float8</type>) type in <acronym>IMMV</acronym> is unsafe. This is
+ because aggregated values in <acronym>IMMV</acronym> can become different from
+ results calculated from base tables due to the limited precision of these types.
+ To avoid this problem, use the <type>numeric</type> type instead.
+</para>
+
+ <sect4>
+ <title>Restrictions on Aggregates</title>
+ <para>
+ There are the following restrictions:
+ <itemizedlist>
+ <listitem>
+ <para>
+ If we have a <literal>GROUP BY</literal> clause, expressions specified in
+ <literal>GROUP BY</literal> must appear in the target list. This is
+ how tuples to be updated in the <acronym>IMMV</acronym> are identified.
+ These attributes are used as scan keys for searching tuples in the
+ <acronym>IMMV</acronym>, so indexes on them are required for efficient
+ <acronym>IVM</acronym>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>HAVING</literal> clause cannot be used.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </sect4>
+</sect3>
+
+<sect3>
+<title>Other General Restrictions</title>
+<para>
+ There are other restrictions which generally apply to <acronym>IMMV</acronym>:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Sub-queries cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ CTEs cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Window functions cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s must be based on simple base tables. It's not
+ supported to create them on top of views, materialized views, foreign tables, inhe.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ LIMIT and OFFSET clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain system columns.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <acronym>IMMV</acronym>s cannot contain non-immutable functions.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ UNION/INTERSECT/EXCEPT clauses cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ DISTINCT ON clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ TABLESAMPLE parameter cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ inheritance parent tables cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ VALUES clause cannnot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <literal>GROUPING SETS</literal> and <literal>FILTER</literal> clauses cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ FOR UPDATE/SHARE cannot be used.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain columns whose name start with <literal>__ivm_</literal>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ targetlist cannot contain expressions which contain an aggregate in it.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Logical replication is not supported, that is, even when a base table
+ at a publisher node is modified, <acronym>IMMV</acronym>s at subscriber
+ nodes are not updated.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When the <literal>TRUNCATE</literal> command is executed on a base table,
+ nothing is changed on the <acronym>IMMV</acronym>.
+ </para>
+ </listitem>
+
+ </itemizedlist>
+</para>
+</sect3>
+
+</sect2>
+
+<sect2>
+<title><literal>DISTINCT</literal></title>
+
+<para>
+ <productname>PostgreSQL</productname> supports <acronym>IMMV</acronym> with
+ <literal>DISTINCT</literal>. For example, suppose a <acronym>IMMV</acronym>
+ defined with <literal>DISTINCT</literal> on a base table containing duplicate
+ tuples. When tuples are deleted from the base table, a tuple in the view is
+ deleted if and only if the multiplicity of the tuple becomes zero. Moreover,
+ when tuples are inserted into the base table, a tuple is inserted into the
+ view only if the same tuple doesn't already exist in it.
+</para>
+
+<para>
+ Physically, an <acronym>IMMV</acronym> defined with <literal>DISTINCT</literal>
+ contains tuples after eliminating duplicates, and the multiplicity of each tuple
+ is stored in a hidden column named <literal>__ivm_count__</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Concurrent Transactions</title>
+<para>
+ Suppose an <acronym>IMMV</acronym> is defined on two base tables and each
+ table was modified in different a concurrent transaction simultaneously.
+ In the transaction which was committed first, <acronym>IMMV</acronym> can
+ be updated considering only the change which happened in this transaction.
+ On the other hand, in order to update the view correctly in the transaction
+ which was committed later, we need to know the changes occurred in
+ both transactions. For this reason, <literal>ExclusiveLock</literal>
+ is held on an <acronym>IMMV</acronym> immediately after a base table is
+ modified in <literal>READ COMMITTED</literal> mode to make sure that
+ the <acronym>IMMV</acronym> is updated in the latter transaction after
+ the former transaction is committed. In <literal>REPEATABLE READ</literal>
+ or <literal>SERIALIZABLE</literal> mode, an error is raised immediately
+ if lock acquisition fails because any changes which occurred in
+ other transactions are not be visible in these modes and
+ <acronym>IMMV</acronym> cannot be updated correctly in such situations.
+ However, as an exception if the view has only one base table,
+ the lock held on thew view is <literal>RowExclusiveLock</literal>.
+</para>
+</sect2>
+
+<sect2>
+<title>Row Level Security</title>
+<para>
+ If some base tables have row level security policy, rows that are not visible
+ to the materialized view's owner are excluded from the result. In addition, such
+ rows are excluded as well when views are incrementally maintained. However, if a
+ new policy is defined or policies are changed after the materialized view was created,
+ the new policy will not be applied to the view contents. To apply the new policy,
+ you need to refresh materialized views.
+</para>
+</sect2>
+
+</sect1>
+
<sect1 id="rules-update">
<title>Rules on <command>INSERT</command>, <command>UPDATE</command>, and <command>DELETE</command></title>
--
2.17.1
--Multipart=_Fri__22_Apr_2022_11_29_39_+0900_ZOAC7UMt5e8j1Nvx--
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-02-08 02:57 Julien Rouhaud <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Julien Rouhaud @ 2022-02-08 02:57 UTC (permalink / raw)
To: Tom Lane <[email protected]>; +Cc: Justin Pryzby <[email protected]>; [email protected]
On Mon, Feb 07, 2022 at 02:00:25PM -0500, Tom Lane wrote:
> Not too surprisingly, these patches broke during the commitfest.
> Here's a rebased version.
>
> I'm not sure that anyone wants to review these in detail ...
> should I just go ahead and push them?
I don't see anything shocking after a quick glance, and I don't think any
review is going to give any more confidence compared to the script-dep-testing
script, so +1 for pushing them since the cf bot is green again.
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-02-08 20:42 Tom Lane <[email protected]>
parent: Julien Rouhaud <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Tom Lane @ 2022-02-08 20:42 UTC (permalink / raw)
To: Julien Rouhaud <[email protected]>; +Cc: Justin Pryzby <[email protected]>; [email protected]
Julien Rouhaud <[email protected]> writes:
> On Mon, Feb 07, 2022 at 02:00:25PM -0500, Tom Lane wrote:
>> Not too surprisingly, these patches broke during the commitfest.
>> Here's a rebased version.
>> I'm not sure that anyone wants to review these in detail ...
>> should I just go ahead and push them?
> I don't see anything shocking after a quick glance, and I don't think any
> review is going to give any more confidence compared to the script-dep-testing
> script, so +1 for pushing them since the cf bot is green again.
Done, will watch the farm.
regards, tom lane
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-07-22 19:21 Aleksander Alekseev <[email protected]>
parent: Tom Lane <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Aleksander Alekseev @ 2022-07-22 19:21 UTC (permalink / raw)
To: PostgreSQL Hackers <[email protected]>; +Cc: Tom Lane <[email protected]>; Julien Rouhaud <[email protected]>; Justin Pryzby <[email protected]>
Hi hackers,
> > I don't see anything shocking after a quick glance, and I don't think any
> > review is going to give any more confidence compared to the script-dep-testing
> > script, so +1 for pushing them since the cf bot is green again.
>
> Done, will watch the farm.
I wanted to test one of the patches we have for the July CF on the
Raspberry Pi 3 Model B+. It runs Raspbian GNU/Linux 10 (buster) and
Linux Kernel 5.10.60-v7+.
I discovered that the PostgreSQL tests don't pass in this environment.
Using `git bisect` I was able to pinpoint the problem to cc50080a and
found this thread. Currently `REL_15_STABLE` and `master` are
affected.
To build PostgreSQL I use my regular set of scripts [1] and the
following command:
```
./quick-build.sh && ./single-install.sh && make installcheck
``
regression.diffs and regression.out are attached. The same tests pass
just fine on Ubuntu Linux and MacOS.
I didn't investigate the problem further since it's pretty late in my
timezone. I just wanted to share my current discoveries with you.
Since we have several agents on buildfarm running Raspbian that are
pretty happy with the patch I would guess this may have something to
do with particular flags and/or configure options I'm using.
[1]: https://github.com/afiskon/pgscripts/
--
Best regards,
Aleksander Alekseev
Attachments:
[application/x-gzip] regression.tgz (110.1K, ../../CAJ7c6TOYvcdOQuo=djuTXUgbJDS-BeSqnY1pTw7VpWVTb=47Lg@mail.gmail.com/2-regression.tgz)
download
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-07-22 19:42 Tom Lane <[email protected]>
parent: Aleksander Alekseev <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Tom Lane @ 2022-07-22 19:42 UTC (permalink / raw)
To: Aleksander Alekseev <[email protected]>; +Cc: PostgreSQL Hackers <[email protected]>; Julien Rouhaud <[email protected]>; Justin Pryzby <[email protected]>
Aleksander Alekseev <[email protected]> writes:
> I wanted to test one of the patches we have for the July CF on the
> Raspberry Pi 3 Model B+. It runs Raspbian GNU/Linux 10 (buster) and
> Linux Kernel 5.10.60-v7+.
> I discovered that the PostgreSQL tests don't pass in this environment.
Since you haven't explained what's different about this environment,
it's hard to comment on these results. But is this really a stock
Postgres source tree, with no local modifications? The fragment of
src/test/regress/expected/copy.out that you show does not look
current.
regards, tom lane
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-07-22 19:58 Aleksander Alekseev <[email protected]>
parent: Tom Lane <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Aleksander Alekseev @ 2022-07-22 19:58 UTC (permalink / raw)
To: Tom Lane <[email protected]>; +Cc: PostgreSQL Hackers <[email protected]>; Julien Rouhaud <[email protected]>; Justin Pryzby <[email protected]>
Hi Tom,
> Since you haven't explained what's different about this environment,
> it's hard to comment on these results. But is this really a stock
> Postgres source tree, with no local modifications? The fragment of
> src/test/regress/expected/copy.out that you show does not look
> current.
Yes, this is a stock PostgreSQL source code without any modification,
with `git clean -dfx` etc.
The fragment of copy.out probably doesn't look current because I was
using `git bisect` and I'm on cc50080a82 right now. However the same
tests fail on both `master` and `REL_15_STABLE`. It takes a while on
Raspberry Pi to rebuild Postgres :)
To clarify, the step that is failing is `./quick-build.sh`, or `make
check' in this script to be precise. So postgresql.conf I'm using in
single-install.sh has nothing to do with the problem, this step is not
reached.
Sorry about the confusion regarding the environment differences. GCC
version is 8.3.0, Perl 5.28.1. All in all this is pretty much the
default Raspbian 10 environment, something you would typically get
after setting up your RPi 3 B+ using Raspberry Pi Imager and running
`apt update`, nothing exotic. Please let me know if there are any
other details of interest.
I'll continue looking for the source of the problem and will post an
update as soon as I have one.
--
Best regards,
Aleksander Alekseev
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-07-22 22:58 Tom Lane <[email protected]>
parent: Aleksander Alekseev <[email protected]>
0 siblings, 1 reply; 28+ messages in thread
From: Tom Lane @ 2022-07-22 22:58 UTC (permalink / raw)
To: Aleksander Alekseev <[email protected]>; +Cc: PostgreSQL Hackers <[email protected]>; Julien Rouhaud <[email protected]>; Justin Pryzby <[email protected]>
Aleksander Alekseev <[email protected]> writes:
> Sorry about the confusion regarding the environment differences. GCC
> version is 8.3.0, Perl 5.28.1. All in all this is pretty much the
> default Raspbian 10 environment, something you would typically get
> after setting up your RPi 3 B+ using Raspberry Pi Imager and running
> `apt update`, nothing exotic. Please let me know if there are any
> other details of interest.
FWIW, I tried to replicate this locally on my own RPi3B+, using
current Ubuntu 20.04.4 LTS (GNU/Linux 5.4.0-1066-raspi aarch64).
No luck: it all works fine for me. We have at least one Raspbian
buildfarm animal too, and it's not been unhappy either. I suspect
there is something odd about your environment settings.
regards, tom lane
^ permalink raw reply [nested|flat] 28+ messages in thread
* Re: Refactoring the regression tests for more independence
@ 2022-07-23 10:58 Aleksander Alekseev <[email protected]>
parent: Tom Lane <[email protected]>
0 siblings, 0 replies; 28+ messages in thread
From: Aleksander Alekseev @ 2022-07-23 10:58 UTC (permalink / raw)
To: PostgreSQL Hackers <[email protected]>; +Cc: Tom Lane <[email protected]>; Julien Rouhaud <[email protected]>; Justin Pryzby <[email protected]>
Hi Tom,
> FWIW, I tried to replicate this locally on my own RPi3B+, using
> current Ubuntu 20.04.4 LTS (GNU/Linux 5.4.0-1066-raspi aarch64).
> No luck: it all works fine for me. We have at least one Raspbian
> buildfarm animal too, and it's not been unhappy either. I suspect
> there is something odd about your environment settings.
Thanks for sharing this.
I repeated the experiment in a clean environment (Raspbian installed
from scratch on a brand new SD-card) and can confirm that the problem
is gone.
Sorry for the disturbance.
--
Best regards,
Aleksander Alekseev
^ permalink raw reply [nested|flat] 28+ messages in thread
end of thread, other threads:[~2022-07-23 10:58 UTC | newest]
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2022-02-08 02:57 Re: Refactoring the regression tests for more independence Julien Rouhaud <[email protected]>
2022-02-08 20:42 ` Re: Refactoring the regression tests for more independence Tom Lane <[email protected]>
2022-07-22 19:21 ` Re: Refactoring the regression tests for more independence Aleksander Alekseev <[email protected]>
2022-07-22 19:42 ` Re: Refactoring the regression tests for more independence Tom Lane <[email protected]>
2022-07-22 19:58 ` Re: Refactoring the regression tests for more independence Aleksander Alekseev <[email protected]>
2022-07-22 22:58 ` Re: Refactoring the regression tests for more independence Tom Lane <[email protected]>
2022-07-23 10:58 ` Re: Refactoring the regression tests for more independence Aleksander Alekseev <[email protected]>
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