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help / color / mirror / Atom feedFrom: Jürgen Purtz <[email protected]>
To: David G. Johnston <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: Additional Chapter for Tutorial
Date: Fri, 30 Oct 2020 11:57:04 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKFQuwbtReW3dBO0HmZCb8rF4z-w+oFt24nPko8pbAk4Y2VWEw@mail.gmail.com>
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On 26.10.20 15:53, David G. Johnston wrote:
> Removing -docs as moderation won’t let me cross-post.
>
> On Monday, October 26, 2020, David G. Johnston
> <[email protected] <mailto:[email protected]>> wrote:
>
> On Monday, October 26, 2020, Jürgen Purtz <[email protected]
> <mailto:[email protected]>> wrote:
>
> On 21.10.20 22:33, David G. Johnston wrote:
>>
>> Two, I find the amount of detail being provided here to be on
>> the too-much side. A bit more judicious use of links into
>> the appropriate detail chapters seems warranted.
>>
> The patch is intended to give every interested person an
> overall impression of the chapter within its new position.
> Because it has moved from part 'Tutorial' to 'Internals' the
> text should be very accurate concerning technical issues -
> like all the other chapters in this part. A tutorial chapter
> has a more superficial nature.
>
> Haven’t reviewed the patches yet but...
>
> I still think that my comment applies even with the move to
> internals. The value here is putting together a coherent
> narrative and making deeper implementation details accessible. If
> those details are already covered elsewhere in the documentation
> (not source code) links should be given serious consideration.
>
> David J.
>
Please find the new patch in the attachment after integrating David's
suggestions: a) versus the last patch and b) versus master.
Notably it contains
* nearly all of his suggestions (see sgml file for comments 'DGJ')
* reduction of <firstterm>. This was a hangover from the
pre-glossary-times. I tried to emphasis standard terms. This is no
longer necessary because nowadays they are clearly defined in the
glossary.
--
J. Purtz
Attachments:
[text/x-patch] 0009-architecture-vs-0008.patch (38.3K, 3-0009-architecture-vs-0008.patch)
download | inline diff:
diff --git a/doc/src/sgml/architecture.sgml b/doc/src/sgml/architecture.sgml
index b6c0eb4a31..e547a87d08 100644
--- a/doc/src/sgml/architecture.sgml
+++ b/doc/src/sgml/architecture.sgml
@@ -13,42 +13,26 @@
<sect1 id="tutorial-ram-proc-file">
<title>Collaboration of Processes, RAM, and Files</title>
<para>
- In a client/server architecture
- clients do not have direct access to stored data. Instead,
- they send requests to the server and receive
- the requested data in response. In the case of
- <productname>PostgreSQL</productname>, the server launches a
- single process for each connected client, referred to as a
- <glossterm linkend="glossary-backend">Backend process</glossterm>.
-
-
- <!-- DGJ: this whole next paragraph bothers me but I cannot justify
- simply removing it nor have a possibly better suggestion.
- In particular I disagree with categorizing this is a
- "close" and "tightly-coupled" relationship -->
-
-
- It acts in close cooperation with the
- <glossterm linkend="glossary-instance">instance</glossterm> which
- is a group of tightly coupled server-side processes plus a
- <glossterm linkend="glossary-shared-memory">Shared Memory</glossterm>
- area located in RAM.
- Notably, PostgreSQL does not utilize application threading within its
- implementation.
- </para>
-
-
- <!-- DGJ: I've gotten the impression firstterm
- is overused, probably as a result of copy-paste -->
-
-
- <para>
- During <firstterm>instance</firstterm> startup time, the
- <glossterm linkend="glossary-postmaster">Postmaster</glossterm>
- process loads the
- configuration files, allocates
- <firstterm>Shared Memory</firstterm>,
- and starts supporting background processes:
+ In a client/server architecture clients do not have direct access
+ to database files and the data stored in them. Instead, they send
+ requests to the server and receive the requested data in the response.
+ In the case of <productname>PostgreSQL</productname>, the server
+ launches a single process for each client connection, referred to as a
+ <glossterm linkend="glossary-backend">Backend</glossterm> process.
+ Those Backend processes handle the client's requests by acting on the
+ <glossterm linkend="glossary-shared-memory">Shared Memory</glossterm>.
+ This leads to other activities (file access, WAL, vacuum, ...) of the
+ <glossterm linkend="glossary-instance">Instance</glossterm>. The
+ Instance is a group of server-side processes acting on a common
+ Shared Memory. Notably, PostgreSQL does not utilize application
+ threading within its implementation.
+ </para>
+
+ <para>
+ The first step in an Instance start is the start of the
+ <glossterm linkend="glossary-postmaster">Postmaster</glossterm>.
+ He loads the configuration files, allocates Shared Memory, and
+ starts the other processes of the Instance:
<glossterm linkend="glossary-background-writer">Background Writer</glossterm>,
<glossterm linkend="glossary-checkpointer">Checkpointer</glossterm>,
<glossterm linkend="glossary-wal-writer">WAL Writer</glossterm>,
@@ -56,8 +40,11 @@
<glossterm linkend="glossary-autovacuum">Autovacuum</glossterm>,
<glossterm linkend="glossary-stats-collector">Statistics Collector</glossterm>,
<glossterm linkend="glossary-logger">Logger</glossterm>, and more.
- <xref linkend="tutorial-ram-proc-file-figure"/> visualizes
- the main aspects of their collaboration.
+ Later, the Postmaster starts
+ <glossterm linkend="glossary-backend">Backend</glossterm> processes
+ which communicate with clients and handle their requests.
+ <xref linkend="tutorial-ram-proc-file-figure"/> visualizes the processes
+ of an Instance and the main aspects of their collaboration.
</para>
<figure id="tutorial-ram-proc-file-figure">
@@ -77,155 +64,110 @@
</figure>
<para>
-
-
- <!-- DGJ: This is detailed, but wrong. Either use less detail or correct detail.
- See header of postmaster.c
- -->
-
-
When a client application tries to connect to a
<glossterm linkend="glossary-database">database</glossterm>,
- this request is handled initially by the
- <firstterm>Postmaster</firstterm> process. It checks authorization,
- starts a new <firstterm>Backend process</firstterm>,
- and instructs the client application to connect to it. All
- further client requests go to this process and are handled
- by it.
+ this request is handled initially by the Postmaster. He
+ starts a new Backend process and instructs the client
+ application to connect to it. All further client requests
+ go to this process and are handled by it.
</para>
<para>
Client requests like <command>SELECT</command> or
<command>UPDATE</command> usually lead to the
- necessity to read or write some data. In a first attempt
- the client's <firstterm>Backend process</firstterm> tries
- to get the information out of <firstterm>Shared
- Memory</firstterm>. This <firstterm>Shared
- Memory</firstterm> is a mirror of parts of the
- <glossterm linkend="glossary-heap">heap</glossterm> and
- <glossterm linkend="glossary-index">index</glossterm> files.
- Because files are often larger than memory, it's likely that
- the desired information is not (completely) available
- in RAM. In this case the <firstterm>Backend process</firstterm>
- must transfer additional file pages to
- <firstterm>Shared Memory</firstterm>. Files are physically
- organized in pages. Every transfer between files and
- RAM is performed in units of complete pages; such transfers
- do not change the size or layout of pages.
- </para>
-
- <para>
- Reading file pages is much slower than reading
- RAM. This is the primary motivation for the usage of
- <firstterm>Shared Memory</firstterm>. As soon as one
- of the <firstterm>Backend processes</firstterm> has
- read pages into memory, those pages become available for all
- other <firstterm>Backend processes</firstterm> for direct
- access in RAM.
- </para>
-
- <para>
- <firstterm>Shared Memory</firstterm> is limited in size.
- Sooner or later, it becomes necessary to overwrite old RAM
- pages. As long as the content of such pages hasn't
- changed, this is not a problem. But in
- <firstterm>Shared Memory</firstterm> also write
- actions take place
- — performed by any of the <firstterm>Backend
- processes</firstterm> (or an
- <firstterm>autovacuum</firstterm> process,
- or other processes). Such modified pages are called
- <firstterm>dirty pages</firstterm>.
- Before <firstterm>dirty pages</firstterm> can be overwritten,
- they must be written back to disk. This is a two-step process.
- </para>
+ necessity to read or write some data. This is carried out
+ by the client's backend process. Reads involve a page-level
+ cache housed in Shared Memory (for details see:
+ <xref linkend="sysvipc"/>) for the benefit of all processes
+ in the instance. Writes also involve this cache, in additional
+ to a journal, called a write-ahead-log or WAL.
+ </para>
+
+ <para>
+ Shared Memory is limited in size. Thus, it becomes necessary
+ to evict pages. As long as the content of such pages hasn't
+ changed, this is not a problem. But in Shared Memory also
+ write actions take place. Modified pages are called dirty
+ pages or dirty buffers and before they can be evicted they
+ must be written back to disk. This happens regularly by the
+ Background Writer and the Checkpointer process to ensure
+ that the disk version of the pages are kept up-to-date.
+ The synchronisation from RAM to disk consists of two steps.
+ </para>
+
+<!-- DGJ: WAL should already be done well before page
+ eviction/overwriting comes into play.
+ This is good material but seems misplaced.
+ JuP: Really misplaced? Don't think so.
+-->
<para>
First, whenever the content of a page changes, a
<glossterm linkend="glossary-wal-record">WAL record</glossterm>
- is created out
- of the delta-information (difference between the old and
- the new content) and stored in another area of
- <firstterm>Shared Memory</firstterm>. These
- <firstterm>WAL records</firstterm> are read by the
- <firstterm>WAL Writer</firstterm> process,
- which runs in parallel to the <firstterm>Backend
- processes</firstterm> and other processes of
- the <firstterm>Instance</firstterm>. It writes
- the continuously arising <firstterm>WAL records</firstterm> to
- the end of the current
+ is created out of the delta-information (difference between the
+ old and the new content) and stored in another area of
+ Shared Memory. The parallel running WAL Writer process
+ reads them and appends them to the end of the current
<glossterm linkend="glossary-wal-record">WAL file</glossterm>.
- Because this writing is sequential, it is much
- faster than the more or less random access
- to data files with <firstterm>heap</firstterm>
- and <firstterm>index</firstterm> information.
- As mentioned, this WAL-writing happens
- in an independent process. All
- <firstterm>WAL records</firstterm> created out of one
- <firstterm>dirty page</firstterm> must be transferred
- to disk before the <firstterm>dirty page</firstterm>
- itself can be transferred to disk.
- </para>
-
- <para>
- Second, the transfer of <firstterm>dirty buffers</firstterm>
- from <firstterm>Shared Memory</firstterm> to file must
- take place. This is the primary task of the
- <firstterm>Background Writer</firstterm> process. Because
- I/O activities can block other processes significantly,
- it starts periodically and acts only for a short period.
- Doing so, its expensive I/O activities are spread over
- time, avoiding debilitating I/O peaks. Also, the <firstterm>
- Checkpointer</firstterm> process transfers
- <firstterm>dirty buffers</firstterm> to file —
- see next paragraph.
- </para>
-
- <para>
- The <firstterm>Checkpointer</firstterm> creates
+ Such sequential writes are much faster than writes to random
+ positions of heap and index files. All WAL records created
+ out of one dirty page must be transferred to disk before the
+ dirty page itself can be transferred to disk in the second step.
+ </para>
+
+ <para>
+ Second, the transfer of dirty buffers from Shared Memory to
+ files must take place. This is the primary task of the
+ Background Writer process. Because I/O activities can block
+ other processes significantly, it starts periodically and
+ acts only for a short period. Doing so, its extensive (and
+ expensive) I/O activities are spread over time, avoiding
+ debilitating I/O peaks. Also, the Checkpointer process
+ transfers dirty buffers to file.
+ </para>
+
+ <para>
+ The Checkpointer creates
<glossterm linkend="glossary-checkpoint">Checkpoints</glossterm>.
- A <firstterm>Checkpoint</firstterm>
- is a point in time when all older <firstterm>dirty buffers</firstterm>,
- all older <firstterm>WAL records</firstterm>, and
- finally a special <firstterm>Checkpoint record</firstterm>
- have been written and flushed to disk.
- After a <firstterm>Checkpoint</firstterm>, we say
- data files and <firstterm>WAL files</firstterm> are in sync.
- In case of a recovery (after a crash of the instance)
- it can be relied upon that the information of all
- <firstterm>WAL records</firstterm> preceding
- the last <firstterm>Checkpoint record</firstterm>
- were already integrated into the data files. This
- speeds up the recovery.
+ A Checkpoint is a point in time when all older dirty buffers,
+ all older WAL records, and finally a special Checkpoint record
+ have been written and flushed to disk. Heap and index files
+ on the one hand and WAL files on the other hand are in sync.
+ Previous WAL is no longer required. In other words,
+ a possibly occurring recovery, which integrates the delta
+ information of WAL into heap and index files, will happen
+ by replaying only WAL past the last recorded checkpoint
+ on top of the current heap and files. This speeds up recovery.
</para>
<para>
- As a result of data changes,
- <firstterm>WAL records</firstterm> arise and get written
- to <firstterm>WAL files</firstterm>.
- Those <firstterm>WAL files</firstterm> — in combination with
- a previously taken <firstterm>Base Backup</firstterm> —
- are necessary to restore a database after a crash of the
- disk on which data files have been stored. Therefore it is
- recommended to transfer a copy of the
- <firstterm> WAL files</firstterm>
- to a second, independent place. The purpose of the
- <firstterm>WAL Archiver</firstterm> process is to perform
- this copy action.
+ While the Checkpointer ensures that a running system can crash
+ and restart itself in a valid state, the administrator needs
+ to handle the case where the heap and files themselves become
+ corrupted (and possibly the locally written WAL, though that is
+ less common). The options and details are covered extensively
+ in the backup and restore section (<xref linkend="backup"/>).
+ For our purposes here, note just that the WAL Archiver process
+ can be enabled and configured to run a script on filled WAL
+ files — usually to copy them to a remote location.
</para>
+
+<!-- DGJ: after the heavy focus on client processes and data these two
+ seem to come out of left field. Skipping over for now.
+ JuP: Don't skip, we want to explain all elements of the graphic.
+-->
+
<para>
- The <firstterm>Statistics Collector</firstterm> collects
- counters about accesses to <firstterm>SQL objects</firstterm>
- like tables, rows, indexes, pages, and more. It stores the
- obtained information in system tables.
+ The Statistics Collector collects counters about accesses to
+ SQL objects like tables, rows, indexes, pages, and more. It
+ stores the obtained information in system tables.
</para>
<para>
- The <firstterm>Logger</firstterm> writes
- text lines about serious and less serious events which can happen
- during database access, e.g., wrong password, no permission,
- long-running queries, etc.
+ The Logger writes text lines about serious and less serious
+ events which can happen during database access, e.g., wrong
+ password, no permission, long-running queries, etc.
</para>
</sect1>
@@ -286,46 +228,40 @@
such as <literal>my_db</literal>, will be copied from the
<literal>template1</literal> database. Due to the unique
role of <literal>template0</literal> as the pristine original
- of all other databases, no client
- can connect to it.
+ of all other databases, no client can connect to it.
</para>
<para>
- Every database must contain <glossterm linkend="glossary-schema">
- at least one schema</glossterm> because
- schemas contain the other
- <glossterm linkend="glossary-sql-object">SQL Objects</glossterm>.
- Schemas are namespaces for
- their SQL objects and ensure — with one
- exception — that within their scope, names are used only once across all
- types of SQL objects. E.g., it is not possible
+ Every database must contain at least one schema because all
+ <glossterm linkend="glossary-sql-object">SQL Objects</glossterm>
+ are contained in a schema.
+ Schemas are namespaces for their SQL objects and ensure
+ (with one exception) that within their scope names are used
+ only once across all types of SQL objects. E.g., it is not possible
to have a table <literal>employee</literal> and a view
- <literal>employee</literal> within the same
- schema. But it is possible to have
- two tables <literal>employee</literal> in different
- schemas. In this case, the two tables
+ <literal>employee</literal> within the same schema. But it is
+ possible to have two tables <literal>employee</literal> in
+ different schemas. In this case, the two tables
are separate objects and independent of each
other. The only exception to this cross-type uniqueness is that
<glossterm linkend="glossary-unique-constraint">unique constraints
- </glossterm> and the according <firstterm>unique index</firstterm>
- use the same name.
+ </glossterm> and the according unique index
+ (<xref linkend="indexes-unique"/>) use the same name.
</para>
<para>
Some schemas are predefined. <literal>public</literal>
- acts as the default schema and contains all
- <firstterm>SQL objects</firstterm> which are created
- within <literal>public</literal> or without using an explicit schema
- name. <literal>public</literal> should not contain user-defined
- SQL objects. Instead, it is recommended to
- create a separate schema that
- holds individual objects like application-specific tables or
- views. <literal>pg_catalog</literal> is a schema for all tables
- and views of the <glossterm linkend="glossary-system-catalog">
- System Catalog</glossterm>.
+ acts as the default schema and contains all SQL objects
+ which are created within <literal>public</literal> or
+ without using an explicit schema name. <literal>public</literal>
+ should not contain user-defined SQL objects. Instead, it is
+ recommended to create a separate schema that holds individual
+ objects like application-specific tables or views.
+ <literal>pg_catalog</literal> is a schema for all tables and views of the
+ <glossterm linkend="glossary-system-catalog">System Catalog</glossterm>.
<literal>information_schema</literal> is a schema for several
- tables and views of the <firstterm>System Catalog</firstterm>
- in a way that conforms to the SQL standard.
+ tables and views of the System Catalog in a way that conforms
+ to the SQL standard.
</para>
<para>
@@ -334,11 +270,11 @@
view, index, constraint, sequence, function, procedure,
trigger, role, data type, operator, tablespace, extension,
foreign data wrapper</firstterm>, and more. A few of them, the
- <firstterm>Global SQL Objects</firstterm>,
- are outside of the strict hierarchy:
- All database names, all tablespace names, and all role names
- are automatically known and available throughout the
- cluster, independent from
+ <firstterm>Global SQL Objects</firstterm>, are outside of the
+ strict hierarchy: All <firstterm>database names</firstterm>,
+ all <firstterm>tablespace names</firstterm>, and all
+ <firstterm>role names</firstterm> are automatically known and
+ available throughout the cluster, independent from
the database or schema in which they where defined originally.
<xref linkend="tutorial-internal-objects-hierarchy-figure"/>
shows the relation between the object types.
@@ -369,7 +305,7 @@
<productname>PostgreSQL</productname> organizes long-lasting
data as well as volatile state information about transactions
or replication actions in the file system. Every
- <firstterm>Cluster</firstterm> has its root directory
+ <xref linkend="glossary-db-cluster"/> has its root directory
somewhere in the file system. In many cases, the environment
variable <literal>PGDATA</literal> points to this directory.
The example shown in
@@ -427,8 +363,8 @@
subdirectories, there are files containing information about
<glossterm linkend="glossary-sql-object">Global SQL Objects</glossterm>.
One type of such Global SQL Objects are
- <firstterm>tablespaces</firstterm>. In
- <literal>global</literal> there is information about
+ <glossterm linkend="glossary-tablespace">tablespaces</glossterm>.
+ In <literal>global</literal> there is information about
the tablespaces, not the tablespaces themselves.
</para>
@@ -443,14 +379,14 @@
<para>
The subdirectory <literal>pg_xact</literal> contains
information about the status of each transaction:
- in_progress, committed, aborted, or sub_committed.
+ <literal>in_progress</literal>, <literal>committed</literal>,
+ <literal>aborted</literal>, or <literal>sub_committed</literal>.
</para>
<para>
In <literal>pg_tblspc</literal>, there are symbolic links
- that point to directories containing such<firstterm>
- SQL objects</firstterm> that are created within
- tablespaces.
+ that point to directories containing such SQL objects
+ that are created within tablespaces.
</para>
<para>
@@ -474,7 +410,7 @@
<title>MVCC — Multiversion Concurrency Control</title>
<para>
- In most cases, <productname>PostgreSQL</productname> based applications
+ In most cases, <productname>PostgreSQL</productname> databases
support many clients at the same time. Therefore, it is necessary to
protect concurrently running requests from unwanted overwriting
of other's data as well as from reading inconsistent data. Imagine an
@@ -533,7 +469,7 @@
</para>
<para>
- The description in this chapter simplifies by omitting detail.
+ The description in this chapter simplifies by omitting some details.
When many transactions are running simultaneously, things can
get complicated. Sometimes transactions get aborted via
<command>ROLLBACK</command> immediately or after a lot of other activities, sometimes
@@ -644,7 +580,7 @@
xids grow, old row versions get out of scope over time.
If an old row version is no longer valid for ALL existing
transactions, it's called <firstterm>dead</firstterm>. The
- space occupied by all dead row versions is called
+ space occupied by dead row versions is part of the
<glossterm linkend="glossary-bloat">bloat</glossterm>.
</simpara>
</listitem>
@@ -680,13 +616,13 @@
<glossterm linkend="glossary-bloat">bloat</glossterm>.
This chapter explains how the SQL command
<command>VACUUM</command> and the automatically running
- <firstterm>autovacuum</firstterm> processes clean up
+ <firstterm>Autovacuum</firstterm> processes clean up
by eliminating bloat.
</para>
<note>
<para>
- <firstterm>Autovacuum</firstterm> runs automatically by
+ Autovacuum runs automatically by
default. Its default parameters as well as such for
<command>VACUUM</command> fit well for most standard
situations. Therefore a novice database manager can
@@ -696,30 +632,27 @@
</note>
<para>
- Client processes can issue the SQL command <command>VACUUM</command> at arbitrary
- points in time. DBAs do this when they recognize special situations,
- or they start it in batch jobs which run periodically.
- <firstterm>Autovacuum</firstterm> processes run as part of the
- <link linkend="glossary-instance">instance</link> at the server.
- There is a constantly running <firstterm>autovacuum</firstterm> daemon.
- It permanently controls the state of all databases based on values that
- are collected by the
+ Client processes can issue the SQL command <command>VACUUM</command>
+ at arbitrary points in time. DBAs do this when they recognize
+ special situations, or they start it in batch jobs which run
+ periodically. Autovacuum processes run as part of the
+ <link linkend="glossary-instance">Instance</link> at the server.
+ There is a constantly running Autovacuum daemon. It permanently
+ controls the state of all databases based on values that are collected by the
<link linkend="glossary-stats-collector">Statistics Collector</link>
- and starts <firstterm>autovacuum</firstterm> processes whenever it detects
+ and starts Autovacuum processes whenever it detects
certain situations. Thus, it's a dynamic behavior of
<productname>PostgreSQL</productname> with the intention to tidy
up — whenever it is appropriate.
</para>
<para>
- <command>VACUUM</command>, as well as
- <firstterm>autovacuum</firstterm>, don't just eliminate bloat.
- They perform additional tasks for minimizing future
+ <command>VACUUM</command>, as well as Autovacuum, don't just eliminate
+ bloat. They perform additional tasks for minimizing future
I/O activities of themselves as well as of other processes.
- This extra work can be done in a very efficient way
- since in most cases the expensive physical access to pages
- has taken place anyway to eliminate bloat.
- The additional operations are:
+ This extra work can be done in a very efficient way since in most
+ cases the expensive physical access to pages has taken place anyway
+ to eliminate bloat. The additional operations are:
</para>
<itemizedlist>
@@ -758,7 +691,7 @@
<emphasis>freeze</emphasis> is controlled by configuration
parameters, runtime flags, and in extreme situations by
the processes themselves. Because vacuum operations typically are I/O
- intensive, which can hinder other activities, <firstterm>autovacuum</firstterm>
+ intensive, which can hinder other activities, Autovacuum
avoids performing many vacuum operations in bulk. Instead,
it carries out many small actions with time gaps in between.
The SQL command <command>VACUUM</command> runs immediately
@@ -784,8 +717,8 @@
<listitem>
<simpara>
<literal>xmax</literal> must contain an xid which is older
- than the oldest xid of all
- currently running transactions (min(pg_stat_activity.backend_xmin)).
+ than the oldest xid of all currently running transactions
+ <literal>(min(pg_stat_activity.backend_xmin))</literal>.
This criterion guarantees that no existing or upcoming transaction
will have read or write access to this row version.
</simpara>
@@ -810,9 +743,9 @@
<para>
After the vacuum operation detects a superfluous row version, it
- marks its space as free for future use of writing
- actions. Only in rare situations (or in the case of <command>VACUUM FULL</command>),
- is this space released to the operating system. In most cases,
+ marks its space as free for future use of writing actions. Only
+ in rare situations (or in the case of <command>VACUUM FULL</command>),
+ this space is released to the operating system. In most cases,
it remains occupied by <productname>PostgreSQL</productname>
and will be used by future <command>INSERT</command> or
<command>UPDATE</command> commands concerning this row or a
@@ -860,7 +793,7 @@
<listitem>
<simpara>
- When an <firstterm>autovacuum</firstterm> process acts. For optimization
+ When an Autovacuum process acts. For optimization
purposes, it considers the Visibility Map in the same way as
<command>VACUUM</command>. Additionally, it ignores tables with few modifications;
see <xref linkend="guc-autovacuum-vacuum-threshold"/>,
@@ -877,7 +810,7 @@
This logic only applies to row versions of the heap. Index entries
don't use <literal>xmin/xmax</literal>. Nevertheless, such index
entries, which would lead to outdated row versions, are released
- accordingly. (??? more explanations ???)
+ accordingly.
</para>
<para>
@@ -996,7 +929,8 @@
<listitem>
<simpara>
- The transaction of <literal>xmin</literal> must be committed.
+ The transactions of <literal>xmin</literal> and
+ <literal>xmax</literal> must be committed.
</simpara>
</listitem>
</itemizedlist>
@@ -1029,8 +963,8 @@
</listitem>
<listitem>
<simpara>
- When an <firstterm>autovacuum</firstterm> process runs. Such
- a process acts in one of two modes:
+ When an Autovacuum process runs. Such a process acts in one
+ of two modes:
</simpara>
<itemizedlist>
@@ -1053,7 +987,7 @@
<xref linkend="guc-autovacuum-freeze-max-age"/>
(default: 200 million). The value of the oldest unfrozen
xid is stored per table in <literal>pg_class.relfrozenxid</literal>.
- In this <emphasis>aggressive mode</emphasis> <firstterm>autovacuum</firstterm>
+ In this <emphasis>aggressive mode</emphasis> Autovacuum
processes all such pages of the selected table that are marked
in the Visibility Map to potentially have bloat or unfrozen rows.
</simpara>
@@ -1065,7 +999,7 @@
</para>
<para>
- In the first two cases and with <firstterm>autovacuum</firstterm> in
+ In the first two cases and with Autovacuum in
<emphasis>aggressive mode</emphasis>, the system knows
to which value the oldest unfrozen xid has moved forward and
logs the value in <literal>pg_class.relfrozenxid</literal>.
@@ -1093,13 +1027,11 @@
<bridgehead renderas="sect2">Protection against Wraparound Failure</bridgehead>
<para>
- The <firstterm>autovacuum</firstterm> processes are initiated by the
- constantly running <firstterm>autovacuum</firstterm> daemon.
- If the daemon detects that for a table
- <firstterm>autovacuum_freeze_max_age</firstterm> is exceeded, it
- starts an <firstterm>autovacuum</firstterm> process in
- <emphasis>aggressive mode</emphasis>
- (see above) — even if <firstterm>autovacuum</firstterm> is disabled.
+ The Autovacuum processes are initiated by the constantly running
+ Autovacuum daemon. If the daemon detects that for a table
+ <literal>autovacuum_freeze_max_age</literal> is exceeded, it
+ starts an Autovacuum process in <emphasis>aggressive mode</emphasis>
+ (see above) — even if Autovacuum is disabled.
</para>
<bridgehead renderas="sect2">Visibility Map and Free Space Map</bridgehead>
@@ -1127,7 +1059,7 @@
<para>
The setting of the flags is silently done by <command>VACUUM</command>
- and <firstterm>autovacuum</firstterm> during their bloat and freeze operations.
+ and Autovacuum during their bloat and freeze operations.
This is done to speed up future vacuum actions,
regular accesses to heap pages, and some accesses to
the index. Every data-modifying operation on any row
@@ -1138,10 +1070,9 @@
The <link linkend="glossary-fsm">Free Space Map</link>
(FSM) tracks the amount of free space per page. It is
organized as a highly condensed b-tree of (rounded) sizes.
- As long as <command>VACUUM</command> or
- <firstterm>autovacuum</firstterm> change the free space
- on any processed page, they log the new values in
- the FSM in the same way as all other writing
+ As long as <command>VACUUM</command> or Autovacuum change
+ the free space on any processed page, they log the new
+ values in the FSM in the same way as all other writing
processes.
</para>
@@ -1153,11 +1084,10 @@
decisions for the generation of execution plans. This
information can be gathered with the SQL commands
<command>ANALYZE</command> or <command>VACUUM ANALYZE</command>.
- But <firstterm>autovacuum</firstterm> processes also gather
+ But also Autovacuum processes gather
such information. Depending on the percentage of changed rows
per table <xref linkend="guc-autovacuum-analyze-scale-factor"/>,
- the <firstterm>autovacuum</firstterm> daemon starts
- <firstterm>autovacuum</firstterm> processes to collect
+ the Autovacuum daemon starts Autovacuum processes to collect
statistics per table. This dynamic invocation of analyze
operations allows <productname>PostgreSQL</productname> to
adopt queries to changing circumstances.
@@ -1221,7 +1151,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
<para>
The atomicity also affects the visibility of changes. No
- connections running simultaneously to a data modifying
+ connection running simultaneously to a data modifying
transaction will ever see any change before the
transaction successfully executes a <command>COMMIT</command>
— even in the lowest
@@ -1293,7 +1223,7 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
<command>ROLLBACK</command> command instead of a
<command>COMMIT</command>. The <command>ROLLBACK</command>
cancels the transaction, and all changes made so far remain
- invisible forever; it's like they never happened. There
+ invisible forever; it is as if they had never happened. There
is no need for the application to log its activities and
undo every step of the transaction separately.
</para>
@@ -1319,10 +1249,11 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
</para>
<para>
- There is an additional feature which defines transactions'
- <link linkend="transaction-iso">isolation level</link>
- to each other in a declarative way. It automatically
- prevents applications from some strange situations.
+ There is the additional feature
+ '<link linkend="transaction-iso">isolation level</link>',
+ which separates transactions from each other in certain ways.
+ It automatically prevents applications from some strange
+ situations.
</para>
<para>
@@ -1405,19 +1336,21 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
are also in the files, but - as usual - they are never seen
by any of the following transactions because uncommited
changes are never shown. Such recovery actions run
- completely automatically, it is not necessary that you
- configure or start anything by yourself.
+ completely automatically, it is not necessary that a
+ database administrator configure or start anything by
+ himself.
</para>
<bridgehead renderas="sect3">Disk crash</bridgehead>
<para>
If a disk crashes, the course of action described previously
cannot work. It is likely that the WAL files and/or the
- data and index files are no longer available. You need
- to take special actions to overcome such situations.
+ data and index files are no longer available. The
+ database administrator must take special actions to
+ overcome such situations.
</para>
<para>
- You obviously need a backup. How to take such a backup
+ He obviously needs a backup. How to take such a backup
and use it as a starting point for a recovery of the
cluster is explained in more detail in the next
<link linkend="tutorial-backup">chapter</link>.
@@ -1428,13 +1361,12 @@ UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
It is conceivable that over time the disk gets full,
and there is no room for additional data. In this case,
<productname>PostgreSQL</productname> stops accepting
- commands which change the data or even terminates
- completely. No data loss or data corruption will
- occur.
+ data-modifying commands or even terminates completely.
+ No data loss or data corruption will occur.
</para>
<para>
- To come out of such a situation, you should remove
- unused files from this disk. But you should never
+ To come out of such a situation, the administrator should
+ remove unused files from this disk. But he should never
delete files from the
<glossterm linkend="glossary-data-directory">data directory</glossterm>.
Nearly all of them are necessary for the consistency
diff --git a/doc/src/sgml/start.sgml b/doc/src/sgml/start.sgml
index 8751410179..abb61445f2 100644
--- a/doc/src/sgml/start.sgml
+++ b/doc/src/sgml/start.sgml
@@ -76,30 +76,27 @@
A process at the server site with the name
<glossterm linkend="glossary-postmaster">Postmaster</glossterm>.
<indexterm><primary>postgres</primary></indexterm>
+ <indexterm><primary>postmaster</primary></indexterm>
It accepts connection requests from client applications, starts
(<quote>forks</quote>) a new <glossterm linkend="glossary-backend">
- Backend process</glossterm> for each of them, and passes
+ Backend</glossterm> process for each of them, and passes
the connection to it. From that point on, the client and the new
- <firstterm>Backend process</firstterm>
- communicate directly without intervention by the original
- <firstterm>postgres</firstterm> process. Thus, the
- <firstterm>postgres</firstterm> process is always running, waiting
- for new client connections, whereas clients and associated
- <firstterm>Backend processes</firstterm> come and go.
- (All of this is of course invisible to the user. We only mention it
- here for completeness.)
+ Backend process communicate directly without intervention by the original
+ Postmaster process. Thus, the Postmaster process is always running,
+ waiting for new client connections, whereas clients and associated
+ Backend processes come and go. (All of this is of course invisible
+ to the user. We only mention it here for completeness.)
</para>
</listitem>
<listitem>
<para>
A group of processes at the server site, the <glossterm
- linkend="glossary-instance">instance</glossterm>, to which also the
- <firstterm>postgres</firstterm> process belongs. Their duties are
- handling of central, common database activities like file access,
- vacuum, <glossterm linkend="glossary-checkpoint">checkpoints</glossterm>,
- replication, and more. The mentioned <firstterm>Backend processes</firstterm>
- delegate those actions to the <firstterm>instance</firstterm>.
+ linkend="glossary-instance">Instance</glossterm>, to which also
+ the Postmaster process belongs. Their duties are handling of
+ central, common database activities like file access, transaction
+ handling, vacuum, checkpoints, replication, and more. The mentioned
+ Backend processes delegate those actions to the instance.
</para>
</listitem>
@@ -127,18 +124,6 @@
file name) on the database server machine.
</para>
- <para>
- The <productname>PostgreSQL</productname> server can handle
- multiple concurrent connections from clients. To achieve this it
- starts (<quote>forks</quote>) a new process for each connection.
- From that point on, the client and the new server process
- communicate without intervention by the original
- <filename>postgres</filename> process. Thus, the
- supervisor server process is always running, waiting for
- client connections, whereas client and associated server processes
- come and go. (All of this is of course invisible to the user. We
- only mention it here for completeness.)
- </para>
</sect1>
[text/x-patch] 0009-architecture-vs-master.patch (224.4K, 4-0009-architecture-vs-master.patch)
download | inline diff:
diff --git a/doc/src/sgml/advanced.sgml b/doc/src/sgml/advanced.sgml
index 2d4ab85d45..5c3245c0ec 100644
--- a/doc/src/sgml/advanced.sgml
+++ b/doc/src/sgml/advanced.sgml
@@ -1,7 +1,7 @@
<!-- doc/src/sgml/advanced.sgml -->
<chapter id="tutorial-advanced">
- <title>Advanced Features</title>
+ <title>Advanced SQL Features</title>
<sect1 id="tutorial-advanced-intro">
<title>Introduction</title>
diff --git a/doc/src/sgml/arch-dev.sgml b/doc/src/sgml/arch-dev.sgml
index 7883c3cd82..9db0ae2c78 100644
--- a/doc/src/sgml/arch-dev.sgml
+++ b/doc/src/sgml/arch-dev.sgml
@@ -1,7 +1,7 @@
<!-- doc/src/sgml/arch-dev.sgml -->
<chapter id="overview">
- <title>Overview of PostgreSQL Internals</title>
+ <title>Overview of Query Handling</title>
<note>
<title>Author</title>
diff --git a/doc/src/sgml/architecture.sgml b/doc/src/sgml/architecture.sgml
new file mode 100644
index 0000000000..e547a87d08
--- /dev/null
+++ b/doc/src/sgml/architecture.sgml
@@ -0,0 +1,1517 @@
+<!-- doc/src/sgml/architecture.sgml -->
+
+ <chapter id="tutorial-architecture">
+ <title>Overview of Architecture and Implementation</title>
+
+ <para>
+ Every DBMS implements basic strategies to ensure a fast
+ and robust system. This chapter provides an overview of the
+ techniques <productname>PostgreSQL</productname> uses to
+ achieve this.
+ </para>
+
+ <sect1 id="tutorial-ram-proc-file">
+ <title>Collaboration of Processes, RAM, and Files</title>
+ <para>
+ In a client/server architecture clients do not have direct access
+ to database files and the data stored in them. Instead, they send
+ requests to the server and receive the requested data in the response.
+ In the case of <productname>PostgreSQL</productname>, the server
+ launches a single process for each client connection, referred to as a
+ <glossterm linkend="glossary-backend">Backend</glossterm> process.
+ Those Backend processes handle the client's requests by acting on the
+ <glossterm linkend="glossary-shared-memory">Shared Memory</glossterm>.
+ This leads to other activities (file access, WAL, vacuum, ...) of the
+ <glossterm linkend="glossary-instance">Instance</glossterm>. The
+ Instance is a group of server-side processes acting on a common
+ Shared Memory. Notably, PostgreSQL does not utilize application
+ threading within its implementation.
+ </para>
+
+ <para>
+ The first step in an Instance start is the start of the
+ <glossterm linkend="glossary-postmaster">Postmaster</glossterm>.
+ He loads the configuration files, allocates Shared Memory, and
+ starts the other processes of the Instance:
+ <glossterm linkend="glossary-background-writer">Background Writer</glossterm>,
+ <glossterm linkend="glossary-checkpointer">Checkpointer</glossterm>,
+ <glossterm linkend="glossary-wal-writer">WAL Writer</glossterm>,
+ <glossterm linkend="glossary-wal-archiver">WAL Archiver</glossterm>,
+ <glossterm linkend="glossary-autovacuum">Autovacuum</glossterm>,
+ <glossterm linkend="glossary-stats-collector">Statistics Collector</glossterm>,
+ <glossterm linkend="glossary-logger">Logger</glossterm>, and more.
+ Later, the Postmaster starts
+ <glossterm linkend="glossary-backend">Backend</glossterm> processes
+ which communicate with clients and handle their requests.
+ <xref linkend="tutorial-ram-proc-file-figure"/> visualizes the processes
+ of an Instance and the main aspects of their collaboration.
+ </para>
+
+ <figure id="tutorial-ram-proc-file-figure">
+ <title>Architecture</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/ram-proc-file-raw.svg" format="SVG" width="900px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/ram-proc-file-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>
+ When a client application tries to connect to a
+ <glossterm linkend="glossary-database">database</glossterm>,
+ this request is handled initially by the Postmaster. He
+ starts a new Backend process and instructs the client
+ application to connect to it. All further client requests
+ go to this process and are handled by it.
+ </para>
+
+ <para>
+ Client requests like <command>SELECT</command> or
+ <command>UPDATE</command> usually lead to the
+ necessity to read or write some data. This is carried out
+ by the client's backend process. Reads involve a page-level
+ cache housed in Shared Memory (for details see:
+ <xref linkend="sysvipc"/>) for the benefit of all processes
+ in the instance. Writes also involve this cache, in additional
+ to a journal, called a write-ahead-log or WAL.
+ </para>
+
+ <para>
+ Shared Memory is limited in size. Thus, it becomes necessary
+ to evict pages. As long as the content of such pages hasn't
+ changed, this is not a problem. But in Shared Memory also
+ write actions take place. Modified pages are called dirty
+ pages or dirty buffers and before they can be evicted they
+ must be written back to disk. This happens regularly by the
+ Background Writer and the Checkpointer process to ensure
+ that the disk version of the pages are kept up-to-date.
+ The synchronisation from RAM to disk consists of two steps.
+ </para>
+
+<!-- DGJ: WAL should already be done well before page
+ eviction/overwriting comes into play.
+ This is good material but seems misplaced.
+ JuP: Really misplaced? Don't think so.
+-->
+
+ <para>
+ First, whenever the content of a page changes, a
+ <glossterm linkend="glossary-wal-record">WAL record</glossterm>
+ is created out of the delta-information (difference between the
+ old and the new content) and stored in another area of
+ Shared Memory. The parallel running WAL Writer process
+ reads them and appends them to the end of the current
+ <glossterm linkend="glossary-wal-record">WAL file</glossterm>.
+ Such sequential writes are much faster than writes to random
+ positions of heap and index files. All WAL records created
+ out of one dirty page must be transferred to disk before the
+ dirty page itself can be transferred to disk in the second step.
+ </para>
+
+ <para>
+ Second, the transfer of dirty buffers from Shared Memory to
+ files must take place. This is the primary task of the
+ Background Writer process. Because I/O activities can block
+ other processes significantly, it starts periodically and
+ acts only for a short period. Doing so, its extensive (and
+ expensive) I/O activities are spread over time, avoiding
+ debilitating I/O peaks. Also, the Checkpointer process
+ transfers dirty buffers to file.
+ </para>
+
+ <para>
+ The Checkpointer creates
+ <glossterm linkend="glossary-checkpoint">Checkpoints</glossterm>.
+ A Checkpoint is a point in time when all older dirty buffers,
+ all older WAL records, and finally a special Checkpoint record
+ have been written and flushed to disk. Heap and index files
+ on the one hand and WAL files on the other hand are in sync.
+ Previous WAL is no longer required. In other words,
+ a possibly occurring recovery, which integrates the delta
+ information of WAL into heap and index files, will happen
+ by replaying only WAL past the last recorded checkpoint
+ on top of the current heap and files. This speeds up recovery.
+ </para>
+
+ <para>
+ While the Checkpointer ensures that a running system can crash
+ and restart itself in a valid state, the administrator needs
+ to handle the case where the heap and files themselves become
+ corrupted (and possibly the locally written WAL, though that is
+ less common). The options and details are covered extensively
+ in the backup and restore section (<xref linkend="backup"/>).
+ For our purposes here, note just that the WAL Archiver process
+ can be enabled and configured to run a script on filled WAL
+ files — usually to copy them to a remote location.
+ </para>
+
+
+<!-- DGJ: after the heavy focus on client processes and data these two
+ seem to come out of left field. Skipping over for now.
+ JuP: Don't skip, we want to explain all elements of the graphic.
+-->
+
+ <para>
+ The Statistics Collector collects counters about accesses to
+ SQL objects like tables, rows, indexes, pages, and more. It
+ stores the obtained information in system tables.
+ </para>
+
+ <para>
+ The Logger writes text lines about serious and less serious
+ events which can happen during database access, e.g., wrong
+ password, no permission, long-running queries, etc.
+ </para>
+
+ </sect1>
+
+ <sect1 id="tutorial-cluster-db-schema">
+ <title>The logical Perspective: Cluster, Database, Schema</title>
+
+ <para>
+ A <glossterm linkend="glossary-server">server</glossterm> contains one or more
+ <glossterm linkend="glossary-db-cluster">database clusters</glossterm>
+ (<glossterm linkend="glossary-db-cluster">clusters</glossterm>
+ for short). Each cluster contains three or more
+ <glossterm linkend="glossary-database">databases</glossterm>.
+ Each database can contain many
+ <glossterm linkend="glossary-schema">schemas</glossterm>.
+ A schema can contain
+ <glossterm linkend="glossary-table">tables</glossterm>,
+ <glossterm linkend="glossary-view">views</glossterm>, and a lot
+ of other objects. Each table or view belongs to a single schema
+ only; they cannot belong to another schema as well. The same is
+ true for the schema/database and database/cluster relation.
+ <xref linkend="tutorial-cluster-db-schema-figure"/> visualizes
+ this hierarchy.
+ </para>
+
+ <figure id="tutorial-cluster-db-schema-figure">
+ <title>Cluster, Database, Schema</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/cluster-db-schema-raw.svg" format="SVG" width="900px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/cluster-db-schema-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>
+ A cluster is the outer container for a
+ collection of databases. Clusters are created by the command
+ <xref linkend="app-initdb"/>.
+ </para>
+
+ <para>
+ <literal>template0</literal> is the very first
+ database of any cluster. Database <literal>template0</literal>
+ is created during the initialization phase of the cluster.
+ In a second step, database <literal>template1</literal> is generated
+ as a copy of <literal>template0</literal>, and finally database
+ <literal>postgres</literal> is generated as a copy of
+ <literal>template1</literal>. Any
+ <glossterm linkend="app-createdb">new databases</glossterm>
+ of the cluster that a user might need,
+ such as <literal>my_db</literal>, will be copied from the
+ <literal>template1</literal> database. Due to the unique
+ role of <literal>template0</literal> as the pristine original
+ of all other databases, no client can connect to it.
+ </para>
+
+ <para>
+ Every database must contain at least one schema because all
+ <glossterm linkend="glossary-sql-object">SQL Objects</glossterm>
+ are contained in a schema.
+ Schemas are namespaces for their SQL objects and ensure
+ (with one exception) that within their scope names are used
+ only once across all types of SQL objects. E.g., it is not possible
+ to have a table <literal>employee</literal> and a view
+ <literal>employee</literal> within the same schema. But it is
+ possible to have two tables <literal>employee</literal> in
+ different schemas. In this case, the two tables
+ are separate objects and independent of each
+ other. The only exception to this cross-type uniqueness is that
+ <glossterm linkend="glossary-unique-constraint">unique constraints
+ </glossterm> and the according unique index
+ (<xref linkend="indexes-unique"/>) use the same name.
+ </para>
+
+ <para>
+ Some schemas are predefined. <literal>public</literal>
+ acts as the default schema and contains all SQL objects
+ which are created within <literal>public</literal> or
+ without using an explicit schema name. <literal>public</literal>
+ should not contain user-defined SQL objects. Instead, it is
+ recommended to create a separate schema that holds individual
+ objects like application-specific tables or views.
+ <literal>pg_catalog</literal> is a schema for all tables and views of the
+ <glossterm linkend="glossary-system-catalog">System Catalog</glossterm>.
+ <literal>information_schema</literal> is a schema for several
+ tables and views of the System Catalog in a way that conforms
+ to the SQL standard.
+ </para>
+
+ <para>
+ There are many different SQL object
+ types: <firstterm>database, schema, table, view, materialized
+ view, index, constraint, sequence, function, procedure,
+ trigger, role, data type, operator, tablespace, extension,
+ foreign data wrapper</firstterm>, and more. A few of them, the
+ <firstterm>Global SQL Objects</firstterm>, are outside of the
+ strict hierarchy: All <firstterm>database names</firstterm>,
+ all <firstterm>tablespace names</firstterm>, and all
+ <firstterm>role names</firstterm> are automatically known and
+ available throughout the cluster, independent from
+ the database or schema in which they where defined originally.
+ <xref linkend="tutorial-internal-objects-hierarchy-figure"/>
+ shows the relation between the object types.
+ </para>
+
+ <figure id="tutorial-internal-objects-hierarchy-figure">
+ <title>Hierarchy of Internal Objects</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/internal-objects-hierarchy-raw.svg" format="SVG" width="720px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/internal-objects-hierarchy-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ </sect1>
+
+ <sect1 id="tutorial-directories">
+ <title>The physical Perspective: Directories and Files</title>
+
+ <para>
+ <productname>PostgreSQL</productname> organizes long-lasting
+ data as well as volatile state information about transactions
+ or replication actions in the file system. Every
+ <xref linkend="glossary-db-cluster"/> has its root directory
+ somewhere in the file system. In many cases, the environment
+ variable <literal>PGDATA</literal> points to this directory.
+ The example shown in
+ <xref linkend="tutorial-directories-figure"/> uses
+ <literal>data</literal> as the name of this root directory.
+ </para>
+
+ <figure id="tutorial-directories-figure">
+ <title>Directory Structure</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/directories-raw.svg" format="SVG" width="900px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/directories-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>
+ <literal>data</literal> contains many subdirectories and
+ some files, all of which are necessary to store long-lasting
+ as well as temporary data. The following paragraphs
+ describe the files and subdirectories in
+ <literal>data</literal>.
+ </para>
+
+ <para>
+ <literal>base</literal> is a subdirectory in which one
+ subdirectory per database exists. The names of those
+ subdirectories consist of numbers. These are the internal
+ Object Identifiers (OID), which are numbers to identify
+ the database definition in the
+ <glossterm linkend="glossary-system-catalog">System Catalog</glossterm>.
+ </para>
+
+ <para>
+ Within the database-specific
+ subdirectories, there are many files: one or more for
+ every table and every index to store heap and index
+ data. Those files are accompanied by files for the
+ <link linkend="storage-fsm">Free Space Maps</link>
+ (extension <literal>_fsm</literal>) and
+ <link linkend="storage-vm">Visibility Maps</link>
+ (extension <literal>_vm</literal>), which contain optimization information.
+ </para>
+
+ <para>
+ Another subdirectory is <literal>global</literal>.
+ In analogy to the database-specific
+ subdirectories, there are files containing information about
+ <glossterm linkend="glossary-sql-object">Global SQL Objects</glossterm>.
+ One type of such Global SQL Objects are
+ <glossterm linkend="glossary-tablespace">tablespaces</glossterm>.
+ In <literal>global</literal> there is information about
+ the tablespaces, not the tablespaces themselves.
+ </para>
+
+ <para>
+ The subdirectory <literal>pg_wal</literal> contains the
+ <glossterm linkend="glossary-wal-file">WAL files</glossterm>.
+ They arise and grow parallel to data changes in the
+ cluster and remain alive as long as
+ they are required for recovery, archiving, or replication.
+ </para>
+
+ <para>
+ The subdirectory <literal>pg_xact</literal> contains
+ information about the status of each transaction:
+ <literal>in_progress</literal>, <literal>committed</literal>,
+ <literal>aborted</literal>, or <literal>sub_committed</literal>.
+ </para>
+
+ <para>
+ In <literal>pg_tblspc</literal>, there are symbolic links
+ that point to directories containing such SQL objects
+ that are created within tablespaces.
+ </para>
+
+ <para>
+ In the root directory <literal>data</literal>
+ there are also some files. In many cases, the configuration
+ files of the cluster are stored here. As long as the
+ instance is up and running, the file
+ <literal>postmaster.pid</literal> exists here
+ and contains the process ID (pid) of the
+ Postmaster which has started the instance.
+ </para>
+
+ <para>
+ For more details about the physical implementation
+ of database objects, see <xref linkend="storage"/>.
+ </para>
+
+ </sect1>
+
+ <sect1 id="tutorial-mvcc">
+ <title>MVCC — Multiversion Concurrency Control</title>
+
+ <para>
+ In most cases, <productname>PostgreSQL</productname> databases
+ support many clients at the same time. Therefore, it is necessary to
+ protect concurrently running requests from unwanted overwriting
+ of other's data as well as from reading inconsistent data. Imagine an
+ online shop offering the last copy of an article. Two clients have the
+ article displayed at their user interface. After a while, but at the same time,
+ both users decide to put it to their shopping cart or even to buy it.
+ Both have seen the article, but only one can be allowed to get it.
+ The database must bring the two requests in a row, permit the access
+ to one of them, block the other, and inform the blocked client
+ that the data was changed by a different process.
+ </para>
+
+ <para>
+ A first approach to implement protections against concurrent
+ accesses to the same data may be the locking of critical
+ rows. Two such techniques are:
+ <emphasis>Optimistic Concurrency Control</emphasis> (OCC)
+ and <emphasis>Two Phase Locking</emphasis> (2PL).
+ <productname>PostgreSQL</productname> implements a third, more
+ sophisticated technique: <firstterm>Multiversion Concurrency
+ Control</firstterm> (MVCC). The crucial advantage of MVCC
+ over other technologies gets evident in multiuser OLTP
+ environments with a massive number of concurrent write
+ actions. There, MVCC generally performs better than solutions
+ using locks. In a <productname>PostgreSQL</productname>
+ database reading never blocks writing and writing never
+ blocks reading, even in the strictest level of transaction
+ isolation.
+ </para>
+
+ <para>
+ Instead of locking rows, the <firstterm>MVCC</firstterm> technique creates
+ a new version of the row when a data-change takes place. To
+ distinguish between these two versions and to track the timeline
+ of the row, each of the versions contains, in addition to their user-defined
+ columns, two special system columns, which are not visible
+ for the usual <command>SELECT * FROM ...</command> command.
+ The column <literal>xmin</literal> contains the transaction ID (xid)
+ of the transaction, which created this version of the row. Accordingly,
+ <literal>xmax</literal> contains the xid of the transaction, which has
+ deleted this version, or zero, if the version is not
+ deleted. You can read both with the command
+ <command>SELECT xmin, xmax, * FROM ... </command>.
+ </para>
+
+ <para>
+ When we speak about transaction IDs, you need to know that xids are like
+ sequences. Every new transaction receives the next number as its ID.
+ Therefore, this flow of xids represents the flow of transaction
+ start events over time. But keep in mind that xids are independent of
+ any time measurement — in milliseconds or whatever. If you dive
+ deeper into <productname>PostgreSQL</productname>, you will recognize
+ parameters with names such as 'xxx_age'. Despite their names,
+ these '_age' parameters do not specify a period of time but represent
+ a certain number of transactions, e.g., 100 million.
+ </para>
+
+ <para>
+ The description in this chapter simplifies by omitting some details.
+ When many transactions are running simultaneously, things can
+ get complicated. Sometimes transactions get aborted via
+ <command>ROLLBACK</command> immediately or after a lot of other activities, sometimes
+ a single row is involved in more than one transaction, sometimes
+ a client crashes, sometimes the sequence of xids restarts
+ from zero, ... . Therefore, every version of a row contains more
+ system columns and flags, not only <literal>xmin</literal>
+ and <literal>xmax</literal>.
+ </para>
+
+ <para>
+ So, what's going on in detail when write accesses take place?
+ <xref linkend="tutorial-mvcc-figure"/> shows details concerning
+ <literal>xmin</literal>, <literal>xmax</literal>, and user data.
+ </para>
+
+ <figure id="tutorial-mvcc-figure">
+ <title>Multiversion Concurrency Control</title>
+ <mediaobject>
+ <imageobject role="html">
+ <imagedata fileref="images/mvcc-raw.svg" format="SVG" width="900px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/mvcc-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>
+ An <command>INSERT</command> command creates the first
+ version of a row. Besides its user data <literal>'x'</literal>,
+ this version contains the ID of the creating transaction
+ <literal>123</literal> in <literal>xmin</literal> and
+ <literal>0</literal> in <literal>xmax</literal>.
+ <literal>xmin</literal> indicates that the version
+ exists since transaction <literal>123</literal> and
+ <literal>xmax</literal> that it is currently not deleted.
+ </para>
+
+ <para>
+ Somewhat later, transaction <literal>135</literal>
+ executes an <command>UPDATE</command> of this row by
+ changing the user data from <literal>'x'</literal> to
+ <literal>'y'</literal>. According to the MVCC principles,
+ the data in the old version of the row does not change!
+ The value <literal>'x'</literal> remains as it was before.
+ Only <literal>xmax</literal> changes to <literal>135</literal>.
+ Now, this version is treated as valid exclusively for
+ transactions with xids from <literal>123</literal> to
+ <literal>134</literal>. As a substitute for the non-occurring
+ data change in the old version, the <command>UPDATE</command>
+ creates a new version of the row with its xid in
+ <literal>xmin</literal>, <literal>0</literal> in
+ <literal>xmax</literal>, and <literal>'y'</literal> in the
+ user data (plus all the other user data from the old version).
+ This version is now valid for all coming transactions.
+ </para>
+
+ <para>
+ All subsequent <command>UPDATE</command> commands behave
+ in the same way as the first one: they put their xid to
+ <literal>xmax</literal> of the current version, create
+ the next version with their xid in <literal>xmin</literal>,
+ <literal>0</literal> in <literal>xmax</literal>, and the
+ new user data.
+ </para>
+
+ <para>
+ Finally, a row may be deleted by a <command>DELETE</command>
+ command. Even in this case, all versions of the row remain as
+ before. Nothing is thrown away so far! Only <literal>xmax</literal>
+ of the last version changes to the xid of the <command>DELETE</command>
+ transaction, which indicates that it is only valid for
+ transactions with xids older than its own (from
+ <literal>142</literal> to <literal>820</literal> in this
+ example).
+ </para>
+
+ <para>
+ In summary, the MVCC technology creates more and more versions
+ of the same row in the table's heap file and leaves them there,
+ even after a <command>DELETE</command> command. Only the youngest
+ version is relevant for all future transactions. But the
+ system must also preserve some of the older ones for a
+ certain amount of time because the possibility exists that
+ they are or could become relevant for any pending
+ transactions. Over time, also the older ones get out of scope
+ for ALL transactions and therefore become unnecessary.
+ Nevertheless, they do exist physically on the disk and occupy
+ space.
+ </para>
+
+ <para>
+ Please keep in mind:
+ </para>
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ <literal>xmin</literal> and <literal>xmax</literal>
+ indicate the range from where to where
+ row versions are valid (visible) for transactions.
+ This range doesn't imply any direct temporal meaning;
+ the sequence of xids reflects only the sequence of
+ transaction begin events. As
+ xids grow, old row versions get out of scope over time.
+ If an old row version is no longer valid for ALL existing
+ transactions, it's called <firstterm>dead</firstterm>. The
+ space occupied by dead row versions is part of the
+ <glossterm linkend="glossary-bloat">bloat</glossterm>.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ Internally, an <command>UPDATE</command> command acts in the
+ same way as a <command>DELETE</command> command followed by
+ an <command>INSERT</command> command.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ Nothing gets wiped away — with the consequence that the database
+ occupies more and more disk space. It is obvious that
+ this behavior has to be corrected in some
+ way. The next chapter explains how <firstterm>autovacuum</firstterm>
+ fulfills this task.
+ </simpara>
+ </listitem>
+
+ </itemizedlist>
+
+ </sect1>
+
+ <sect1 id="tutorial-vacuum">
+ <title>Vacuum</title>
+
+ <para>
+ As we have seen in the previous chapter, the database
+ tends to occupy more and more disk space, the
+ <glossterm linkend="glossary-bloat">bloat</glossterm>.
+ This chapter explains how the SQL command
+ <command>VACUUM</command> and the automatically running
+ <firstterm>Autovacuum</firstterm> processes clean up
+ by eliminating bloat.
+ </para>
+
+ <note>
+ <para>
+ Autovacuum runs automatically by
+ default. Its default parameters as well as such for
+ <command>VACUUM</command> fit well for most standard
+ situations. Therefore a novice database manager can
+ easily skip the rest of this chapter which explains
+ a lot of details.
+ </para>
+ </note>
+
+ <para>
+ Client processes can issue the SQL command <command>VACUUM</command>
+ at arbitrary points in time. DBAs do this when they recognize
+ special situations, or they start it in batch jobs which run
+ periodically. Autovacuum processes run as part of the
+ <link linkend="glossary-instance">Instance</link> at the server.
+ There is a constantly running Autovacuum daemon. It permanently
+ controls the state of all databases based on values that are collected by the
+ <link linkend="glossary-stats-collector">Statistics Collector</link>
+ and starts Autovacuum processes whenever it detects
+ certain situations. Thus, it's a dynamic behavior of
+ <productname>PostgreSQL</productname> with the intention to tidy
+ up — whenever it is appropriate.
+ </para>
+
+ <para>
+ <command>VACUUM</command>, as well as Autovacuum, don't just eliminate
+ bloat. They perform additional tasks for minimizing future
+ I/O activities of themselves as well as of other processes.
+ This extra work can be done in a very efficient way since in most
+ cases the expensive physical access to pages has taken place anyway
+ to eliminate bloat. The additional operations are:
+ </para>
+
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ <firstterm>Freeze</firstterm>: Mark the youngest row version
+ as frozen. This means that the version
+ is always treated as valid (visible) independent from
+ the <firstterm>wraparound problem</firstterm> (see below).
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ <firstterm>Visibility Map</firstterm> and
+ <firstterm>Free Space Map</firstterm>: Log information about
+ the state of the handled pages in two additional files, the
+ Visibility Map and the Free Space Map.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ <emphasis>Statistics</emphasis>: Collect statistics about the
+ number of rows per table, the distribution of values, and so on,
+ as the basis for decisions of the query planner.
+ </simpara>
+ </listitem>
+
+ </itemizedlist>
+
+ <para>
+ The eagerness — you can call it 'aggression' — of the
+ operations <emphasis>eliminating bloat</emphasis> and
+ <emphasis>freeze</emphasis> is controlled by configuration
+ parameters, runtime flags, and in extreme situations by
+ the processes themselves. Because vacuum operations typically are I/O
+ intensive, which can hinder other activities, Autovacuum
+ avoids performing many vacuum operations in bulk. Instead,
+ it carries out many small actions with time gaps in between.
+ The SQL command <command>VACUUM</command> runs immediately
+ and without any time gaps.
+ </para>
+
+ <bridgehead renderas="sect2">Eliminate Bloat</bridgehead>
+
+ <para>
+ To determine which of the row versions are superfluous, the
+ elimination operation must evaluate <literal>xmax</literal>
+ against several criteria which all must apply:
+ </para>
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ <literal>xmax</literal> must be different from zero because a
+ value of zero indicates that the row version is still valid.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ <literal>xmax</literal> must contain an xid which is older
+ than the oldest xid of all currently running transactions
+ <literal>(min(pg_stat_activity.backend_xmin))</literal>.
+ This criterion guarantees that no existing or upcoming transaction
+ will have read or write access to this row version.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ The transaction of <literal>xmax</literal> must be committed. If it was rollback-ed,
+ this row version is treated as valid.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ If there is the situation that the row version is part of
+ multiple transactions, special care and some more actions
+ must be taken, see: <xref linkend="vacuum-for-multixact-wraparound"/>.
+ </simpara>
+ </listitem>
+
+ </itemizedlist>
+
+ <para>
+ After the vacuum operation detects a superfluous row version, it
+ marks its space as free for future use of writing actions. Only
+ in rare situations (or in the case of <command>VACUUM FULL</command>),
+ this space is released to the operating system. In most cases,
+ it remains occupied by <productname>PostgreSQL</productname>
+ and will be used by future <command>INSERT</command> or
+ <command>UPDATE</command> commands concerning this row or a
+ completely different one.
+ </para>
+
+ <para>
+ Which actions start the elimination of bloat?
+
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ When a client issues the SQL command <command>VACUUM</command>
+ in its default format, i.e., without any option. To boost performance,
+ in this and the next case <command>VACUUM</command> does not
+ read and act on all pages of the heap.
+ The Visibility Map, which is very compact and therefore has a small
+ size, contains information about pages, where bloat-candidates might
+ be found. Only such pages are processed.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ When a client issues the SQL command <command>VACUUM</command>
+ with the option <command>FREEZE</command>. (In this case,
+ it undertakes much more actions, see
+ <link linkend="tutorial-freeze">Freeze Row Versions</link>.)
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ When a client issues the SQL command <command>VACUUM</command>
+ with the option <command>FULL</command>.
+ Also, in this mode, the bloat disappears, but the strategy used
+ is very different: In this case, the complete table is copied
+ to a different file skipping all outdated row versions. This
+ leads to a significant reduction of used disk space because
+ the new file contains only the actual data. The old file
+ is deleted.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ When an Autovacuum process acts. For optimization
+ purposes, it considers the Visibility Map in the same way as
+ <command>VACUUM</command>. Additionally, it ignores tables with few modifications;
+ see <xref linkend="guc-autovacuum-vacuum-threshold"/>,
+ which defaults to 50 rows and
+ <xref linkend="guc-autovacuum-vacuum-scale-factor"/>,
+ which defaults to 20%.
+ </simpara>
+ </listitem>
+
+ </itemizedlist>
+ </para>
+
+ <para>
+ This logic only applies to row versions of the heap. Index entries
+ don't use <literal>xmin/xmax</literal>. Nevertheless, such index
+ entries, which would lead to outdated row versions, are released
+ accordingly.
+ </para>
+
+ <para>
+ The above descriptions omit the fact that xids on a real computer
+ have a limited size. They count up in the same way as sequences, and after
+ a certain number of new transactions they are forced to restart
+ from the beginning, which is called <firstterm>wraparound</firstterm>.
+ Therefore the terms 'old transaction' / 'young transaction' does
+ not always correlate with low / high values of xids. Near to the
+ wraparound point, there are cases where <literal>xmin</literal> has
+ a higher value than <literal>xmax</literal>, although their meaning
+ is said to be older than <literal>xmax</literal>.
+ </para>
+
+ <figure id="tutorial-wraparound-figure">
+ <title>Cyclic usage of XIDs</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/wraparound-raw.svg" format="SVG" width="850px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/wraparound-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <bridgehead renderas="sect2" id="tutorial-freeze">Freeze Row Versions</bridgehead>
+
+ <para>
+ The use of a limited range of IDs for transactions leads
+ to the necessity to restart the sequence sooner or later.
+ This does not only have the rare consequence previously
+ described that sometimes <literal>xmin</literal> is
+ higher than <literal>xmax</literal>. The far
+ more critical problem is that whenever the system has
+ to evaluate a WHERE condition, it must decide which row
+ version is valid (visible) from the perspective of the
+ transaction of this query. If a wraparound couldn't happen,
+ this decision would be relatively easy: the xid
+ must be between <literal>xmin</literal> and <literal>xmax</literal>,
+ and the corresponding transactions of <literal>xmin</literal>
+ and <literal>xmax</literal> must be committed. However,
+ <productname>PostgreSQL</productname> has to consider the
+ possibility of wraparounds.
+ Therefore the decision becomes more complex. The general
+ idea of the solution is to use the 'between
+ <literal>xmin</literal> and <literal>xmax</literal>'
+ comparison only during the youngest period of the row
+ versions lifetime and afterward replace it with a
+ 'valid forever' flag in its header.
+ </para>
+
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ As a first step, <productname>PostgreSQL</productname>
+ divides the complete range of
+ possible xids into two halves with the two split-points
+ 'txid_current' and 'txid_current + 2^31'. The half behind
+ 'txid_current' is considered to represent xids of the
+ 'past' and the half ahead of 'txid_current' those of the
+ 'future'. Those of the 'past' are valid (visible) and those
+ of the 'future' not.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ With each newly created transaction the two split-points
+ move forward. When 'txid_current + 2^31' would reach a
+ row version with <literal>xmin</literal> equal to that value, it would
+ immediately jump from 'past' to 'future' and would be
+ no longer visible!
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ To avoid this unacceptable extinction of data, the vacuum
+ operation <firstterm>freeze</firstterm> clears the situation
+ long before the split-point is reached. It sets a flag
+ in the header of the row version, which completely eliminates
+ the future use of <literal>xmin/xmax</literal> and indicates
+ that the version is valid not only in the 'past'-half
+ but also in the 'future'-half as well as in all coming
+ <glossterm linkend="glossary-xid">epochs</glossterm>.
+ </simpara>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ Which row versions can be frozen by the vacuum operation?
+ Again, several criteria must be checked, and all must be met.
+
+ <itemizedlist>
+
+ <listitem>
+ <simpara>
+ <literal>xmax</literal> must be zero because only
+ non-deleted rows can be visible 'forever'.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ <literal>xmin</literal> must be older than all currently
+ existing transactions. This guarantees that no existing
+ transaction can modify or delete the version.
+ </simpara>
+ </listitem>
+
+ <listitem>
+ <simpara>
+ The transactions of <literal>xmin</literal> and
+ <literal>xmax</literal> must be committed.
+ </simpara>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ At what point in time does the freeze operation take place?
+
+ <itemizedlist>
+ <listitem>
+ <simpara>
+ When a client issues the SQL command <command>VACUUM</command>
+ with its <command>FREEZE</command> option. In this case, all
+ pages are processed that are marked in the Visibility Map
+ to potentially have unfrozen rows.
+ </simpara>
+ </listitem>
+ <listitem>
+ <simpara>
+ When a client issues the SQL command <command>VACUUM</command> without
+ any options but finds that there are xids older than
+ <xref linkend="guc-vacuum-freeze-table-age"/>
+ (default: 150 million) minus
+ <xref linkend="guc-vacuum-freeze-min-age"/>
+ (default: 50 million).
+ As before, all pages are processed that are
+ marked in the Visibility Map to potentially have unfrozen
+ rows.
+ </simpara>
+ </listitem>
+ <listitem>
+ <simpara>
+ When an Autovacuum process runs. Such a process acts in one
+ of two modes:
+ </simpara>
+
+ <itemizedlist>
+ <listitem>
+ <simpara>
+ In the <emphasis>normal mode</emphasis>, it skips
+ pages with row versions that are younger than
+ <xref linkend="guc-vacuum-freeze-min-age"/>
+ (default: 50 million) and works only on pages where
+ all xids are older. The skipping of young xids prevents
+ work on such pages, which are likely to be changed
+ by one of the future SQL commands.
+ </simpara>
+ </listitem>
+ <listitem>
+ <simpara>
+ The process switches
+ to an <emphasis>aggressive mode</emphasis> if it recognizes
+ that for the processed table their oldest xid exceeds
+ <xref linkend="guc-autovacuum-freeze-max-age"/>
+ (default: 200 million). The value of the oldest unfrozen
+ xid is stored per table in <literal>pg_class.relfrozenxid</literal>.
+ In this <emphasis>aggressive mode</emphasis> Autovacuum
+ processes all such pages of the selected table that are marked
+ in the Visibility Map to potentially have bloat or unfrozen rows.
+ </simpara>
+ </listitem>
+
+ </itemizedlist>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ In the first two cases and with Autovacuum in
+ <emphasis>aggressive mode</emphasis>, the system knows
+ to which value the oldest unfrozen xid has moved forward and
+ logs the value in <literal>pg_class.relfrozenxid</literal>.
+ The distance between this value and the 'txid_current' split
+ point becomes smaller, and the distance to 'txid_current + 2^31'
+ becomes larger than before.
+ </para>
+
+ <figure id="tutorial-freeze-figure">
+ <title>Freeze</title>
+ <mediaobject>
+ <imageobject role="html">
+ <!-- attribute 'width=..px' is necessary to keep font-size of SVG text
+ in correlation with font-size of surrounding HTML -->
+ <imagedata fileref="images/freeze-raw.svg" format="SVG" width="850px" />
+ </imageobject>
+ <imageobject role="fo">
+ <!-- For PDF attribute 'width=100%' is necessary to keep complete SVG visible
+ on the page, which has a fixed width. Font sizes will be adopted. -->
+ <imagedata fileref="images/freeze-raw.svg" format="SVG" width="100%" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <bridgehead renderas="sect2">Protection against Wraparound Failure</bridgehead>
+
+ <para>
+ The Autovacuum processes are initiated by the constantly running
+ Autovacuum daemon. If the daemon detects that for a table
+ <literal>autovacuum_freeze_max_age</literal> is exceeded, it
+ starts an Autovacuum process in <emphasis>aggressive mode</emphasis>
+ (see above) — even if Autovacuum is disabled.
+ </para>
+
+ <bridgehead renderas="sect2">Visibility Map and Free Space Map</bridgehead>
+
+ <para>
+ The <link linkend="glossary-vm">Visibility Map</link>
+ (VM) contains two flags — stored as
+ two bits — for each page of the heap. If the first bit
+ is set, that indicates that the associated page does not
+ contain any bloat. If the second one is set, that indicates
+ that the page contains only frozen rows.
+ </para>
+
+ <para>
+ Please consider two details. First, in most cases a page
+ contains many rows, some of them in many versions.
+ However, the flags are associated with the page,
+ not with a row or a row version. The flags are set
+ only under the condition that they are valid for ALL
+ row versions of the page. Second, since there
+ are only two bits per page, the VM is considerably
+ smaller than the heap. Therefore it is buffered
+ in RAM in almost all cases.
+ </para>
+
+ <para>
+ The setting of the flags is silently done by <command>VACUUM</command>
+ and Autovacuum during their bloat and freeze operations.
+ This is done to speed up future vacuum actions,
+ regular accesses to heap pages, and some accesses to
+ the index. Every data-modifying operation on any row
+ version of the page clears the flags.
+ </para>
+
+ <para>
+ The <link linkend="glossary-fsm">Free Space Map</link>
+ (FSM) tracks the amount of free space per page. It is
+ organized as a highly condensed b-tree of (rounded) sizes.
+ As long as <command>VACUUM</command> or Autovacuum change
+ the free space on any processed page, they log the new
+ values in the FSM in the same way as all other writing
+ processes.
+ </para>
+
+ <bridgehead renderas="sect2">Statistics</bridgehead>
+
+ <para>
+ Statistic information helps the <link
+ linkend="planner-stats">Query Planner</link> to make optimal
+ decisions for the generation of execution plans. This
+ information can be gathered with the SQL commands
+ <command>ANALYZE</command> or <command>VACUUM ANALYZE</command>.
+ But also Autovacuum processes gather
+ such information. Depending on the percentage of changed rows
+ per table <xref linkend="guc-autovacuum-analyze-scale-factor"/>,
+ the Autovacuum daemon starts Autovacuum processes to collect
+ statistics per table. This dynamic invocation of analyze
+ operations allows <productname>PostgreSQL</productname> to
+ adopt queries to changing circumstances.
+ </para>
+
+ <para>
+ For more details about vacuum operations, especially for its
+ numerous parameters, see <xref linkend="routine-vacuuming"/>.
+ </para>
+
+ </sect1>
+
+ <sect1 id="tutorial-transactions-mvcc">
+ <title>Transactions</title>
+ <para>
+ <link linkend="tutorial-transactions">Transactions</link>
+ are a fundamental concept of relational database systems.
+ Their essential point is that they bundle multiple
+ read- or write-operations into a single all-or-nothing
+ operation. Furthermore, they separate and protect concurrent
+ actions of different connections from each other. Thereby
+ they implement the ACID paradigm.
+ </para>
+
+ <para>
+ In <productname>PostgreSQL</productname> there are two ways
+ to establish a transaction. The explicit way uses the keywords
+ <link linkend="sql-begin">BEGIN</link> and
+ <link linkend="sql-commit">COMMIT</link> (respectively
+ <link linkend="sql-rollback">ROLLBACK</link>) before
+ and after a sequence of SQL statements. The keywords mark
+ the transaction's start- and end-point. On the other hand, you
+ can omit the keywords. This is the implicit way, where
+ every single SQL command automatically establishes a new
+ transaction.
+
+ <programlisting>
+BEGIN; -- establish a new transaction
+UPDATE accounts SET balance = balance - 100.00 WHERE name = 'Alice';
+UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
+COMMIT; -- finish the transaction
+
+-- this UPDATE runs as the only command of a separate transaction ...
+UPDATE accounts SET balance = balance - 100.00 WHERE name = 'Alice';
+
+-- ... and this one runs in another transaction
+UPDATE accounts SET balance = balance + 100.00 WHERE name = 'Bob';
+ </programlisting>
+ </para>
+
+ <para>
+ As mentioned, the primary property of a transaction is its
+ atomicity: either all or none of its operations succeed,
+ regardless of the fact that it may consist of a lot of
+ different write-operations, and each such operation may
+ affect thousands or millions of rows. As soon as one of the
+ operations fails, all previous operations fail also, which
+ means that all modified rows retain their values as of the
+ beginning of the transaction.
+ </para>
+
+ <para>
+ The atomicity also affects the visibility of changes. No
+ connection running simultaneously to a data modifying
+ transaction will ever see any change before the
+ transaction successfully executes a <command>COMMIT</command>
+ — even in the lowest
+ <link linkend="transaction-iso">isolation level</link>
+ of transactions. <productname>PostgreSQL</productname>
+ does never show uncommitted changes to other connections.
+ </para>
+
+ <para>
+ The situation regarding visibility is somewhat different
+ from the point of view of the modifying transaction.
+ <command>SELECT</command> commands issued inside a
+ transaction delivers all changes done so far by this
+ transaction.
+ </para>
+
+ <bridgehead renderas="sect2">How does it work?</bridgehead>
+
+ <para>
+ Every <command>INSERT</command>, <command>UPDATE</command>,
+ and <command>DELETE</command> command creates new row
+ versions — according to the MVCC rules. This
+ creates the risk that other transactions may see the
+ new row versions, and after a while and some more
+ activities of the modifying transaction they may see the
+ next row versions. Results would be a kind of 'moving
+ target' in absolute contrast to the all-or-nothing
+ principle.
+ </para>
+
+ <para>
+ <productname>PostgreSQL</productname> overcomes this
+ problem by showing only such row versions to other
+ transactions whose originating transaction is
+ successfully committed. It skips all row versions of
+ uncommitted transactions. And
+ <productname>PostgreSQL</productname> solves one more
+ problem. Even the single <command>COMMIT</command>
+ command needs a short time interval for its execution.
+ Therefore its critical 'dead-or-survival' phase
+ runs in a priviledged mode where it cannot be
+ interrupted by other processes.
+ </para>
+
+ <bridgehead renderas="sect2">What are the benefits?</bridgehead>
+
+ <para>
+ Transactions relieve applications from many standard
+ actions that must be implemented for nearly every use case.
+ </para>
+
+ <para>
+ Business logic often contains strong, but for a computer,
+ relative abstract requirements. The above example shows
+ the transfers of some money from one account to another.
+ It is obvious
+ that the decrease of the one and the increase of the
+ other must be indivisible. Nevertheless, there is no particular
+ need for an application to do something to ensure the
+ <glossterm linkend="glossary-atomicity">atomicity</glossterm>
+ of this behavior. It's enough to surround them with
+ <command>BEGIN</command> and <command>COMMIT</command>.
+ </para>
+
+ <para>
+ Applications often demand the feature of 'undoing'
+ previously taken actions under some application-specific
+ conditions. In such cases, the application simply issues a
+ <command>ROLLBACK</command> command instead of a
+ <command>COMMIT</command>. The <command>ROLLBACK</command>
+ cancels the transaction, and all changes made so far remain
+ invisible forever; it is as if they had never happened. There
+ is no need for the application to log its activities and
+ undo every step of the transaction separately.
+ </para>
+
+ <para>
+ Transactions ensure that the
+ <glossterm linkend="glossary-consistency">consistency</glossterm>
+ of the complete database always keeps valid. Declarative
+ rules like
+ <link linkend="ddl-constraints-primary-keys">primary</link>- or
+ <link linkend="ddl-constraints-fk">foreign keys</link>,
+ <link linkend="ddl-constraints-check-constraints">checks</link>,
+ other constraints, or
+ <link linkend="trigger-definition">triggers</link>
+ are part of the all-or-nothing nature of transactions.
+ </para>
+
+ <para>
+ Also, all self-evident — but possibly not obvious
+ — low-level demands on the database system are
+ ensured; e.g. index entries for rows must become
+ visible at the same moment as the rows themselves.
+ </para>
+
+ <para>
+ There is the additional feature
+ '<link linkend="transaction-iso">isolation level</link>',
+ which separates transactions from each other in certain ways.
+ It automatically prevents applications from some strange
+ situations.
+ </para>
+
+ <para>
+ Lastly, it is worth to notice that changes done by a
+ committed transaction will survive all future application,
+ instance, or hardware failures. The next chapter
+ explains this
+ <glossterm linkend="glossary-durability">durability</glossterm>.
+ </para>
+ </sect1>
+
+ <sect1 id="tutorial-reliability">
+ <title>Reliability</title>
+
+ <para>
+ Nothing is perfect and failures inevitably happen.
+ However, the most common types of failure are
+ well known and <productname>PostgreSQL</productname>
+ implements strategies to overcome them.
+ Such strategies use parts of the previously presented
+ techniques MVCC and transaction-rollback, plus additional
+ features.
+ </para>
+
+ <bridgehead renderas="sect2">Failures at the client side</bridgehead>
+ <para>
+ A <glossterm linkend="glossary-client">client</glossterm>
+ can fail in different ways. Its hardware can get damaged,
+ the power supply can fail, the network connection to the
+ server can break, or the client application may run into
+ a severe software error like a null pointer exception.
+ Because <productname>PostgreSQL</productname> uses a
+ client/server architecture, no direct problem for the
+ database will occur. In all of this cases, the
+ <glossterm linkend="glossary-backend">Backend process</glossterm>,
+ which is the client's counterpart at the server-side,
+ may recognize that the network connection is no longer
+ working, or it may run into a timeout after a while. It
+ terminates, and there is no harm to the database. As
+ usual, uncommitted data changes initiated by this client
+ are not visible to any other client.
+ </para>
+
+ <bridgehead renderas="sect2">Failures at the server-side</bridgehead>
+
+ <bridgehead renderas="sect3">Instance failure</bridgehead>
+ <para>
+ The instance may suddenly fail because of <emphasis>power off</emphasis>
+ or other problems. This will affect all running processes, the RAM,
+ and possibly the consistency of disk files.
+ </para>
+ <para>
+ After a restart, <productname>PostgreSQL</productname>
+ automatically recognizes that the last shutdown of the
+ instance did not happen as expected: files might not be
+ closed properly and the <literal>postmaster.pid</literal>
+ file exists. <productname>PostgreSQL</productname>
+ tries to clean up the situation. This is possible because
+ all changes in the database are stored twice. First,
+ the WAL files contain them as a chronology of
+ <glossterm linkend="glossary-wal-record">WAL records</glossterm>,
+ which include the new data values and information about commit
+ actions. The WAL records are written first. Second,
+ the data itself shall exist in the heap and index files.
+ In opposite to the WAL records, this part may or may
+ not have been transferred entirely from Shared Memory
+ to the files.
+ </para>
+ <para>
+ The automatic recovery searches within the WAL files for
+ the latest
+ <glossterm linkend="glossary-checkpoint">checkpoint</glossterm>.
+ This checkpoint signals that the database files are in
+ a consistent state, especially that all WAL records up to
+ this point were successfully stored in heap and index. Starting
+ here, the recovery process copies the following WAL records
+ to heap and index. As a result, the files contain all
+ changes and reach a consistent state. Changes of committed
+ transactions are visible; those of uncommited transactions
+ are also in the files, but - as usual - they are never seen
+ by any of the following transactions because uncommited
+ changes are never shown. Such recovery actions run
+ completely automatically, it is not necessary that a
+ database administrator configure or start anything by
+ himself.
+ </para>
+
+ <bridgehead renderas="sect3">Disk crash</bridgehead>
+ <para>
+ If a disk crashes, the course of action described previously
+ cannot work. It is likely that the WAL files and/or the
+ data and index files are no longer available. The
+ database administrator must take special actions to
+ overcome such situations.
+ </para>
+ <para>
+ He obviously needs a backup. How to take such a backup
+ and use it as a starting point for a recovery of the
+ cluster is explained in more detail in the next
+ <link linkend="tutorial-backup">chapter</link>.
+ </para>
+
+ <bridgehead renderas="sect3">Disk full</bridgehead>
+ <para>
+ It is conceivable that over time the disk gets full,
+ and there is no room for additional data. In this case,
+ <productname>PostgreSQL</productname> stops accepting
+ data-modifying commands or even terminates completely.
+ No data loss or data corruption will occur.
+ </para>
+ <para>
+ To come out of such a situation, the administrator should
+ remove unused files from this disk. But he should never
+ delete files from the
+ <glossterm linkend="glossary-data-directory">data directory</glossterm>.
+ Nearly all of them are necessary for the consistency
+ of the database.
+ </para>
+
+ <bridgehead renderas="sect2">High availability</bridgehead>
+ <para>
+ Database servers can work together to allow a second
+ server to quickly take over the workload if the
+ primary server fails for whatever reason
+ (<link linkend="high-availability">high availability</link>),
+ or to allow several computers to serve the same data
+ for the purpose of load balancing.
+ </para>
+
+ </sect1>
+
+ <sect1 id="tutorial-backup">
+ <title>Backup</title>
+
+ <para>
+ Taking backups is a basic task of database maintenance.
+ <productname>PostgreSQL</productname> supports
+ three different strategies; each has its own
+ strengths and weaknesses.
+ <itemizedlist>
+ <listitem>
+ <simpara>
+ File system level backup
+ </simpara>
+ </listitem>
+ <listitem>
+ <simpara>
+ Logical backup via <command>pg_dump</command>
+ </simpara>
+ </listitem>
+ <listitem>
+ <simpara>
+ Continuous archiving based on <command>pg_basebackup</command>
+ and WAL files
+ </simpara>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <bridgehead renderas="sect2">File system level backup</bridgehead>
+ <para>
+ You can use any appropriate OS tool to create a
+ <link linkend="backup-file">copy</link>
+ of the cluster's directory structure and files. In
+ case of severe problems such a copy can serve as
+ the source of recovery. But in order to get a
+ <emphasis>USABLE</emphasis> backup by this method,
+ the database server <emphasis>MUST</emphasis> be
+ shut down during the complete runtime of the copy
+ command!
+ </para>
+ <para>
+ The obvious disadvantage of this method is that there
+ is a downtime where no user interaction is possible.
+ The other two strategies run during regular operating
+ times.
+ </para>
+
+ <bridgehead renderas="sect2">Logical backup via pg_dump</bridgehead>
+ <para>
+ The tool <command>pg_dump</command> is able to take a
+ <link linkend="backup-dump">copy</link>
+ of the complete cluster or certain parts of it. It stores
+ the copy in the form of SQL <command>CREATE</command> and
+ <command>INSERT</command> commands. It runs in
+ parallel to other processes in its own transaction.
+ </para>
+ <para>
+ The output of <command>pg_dump</command> may be used as
+ input of <command>psql</command> to restore the data
+ (or to copy it to another database).
+ </para>
+ <para>
+ The main advantage over the other two methods is that it
+ can pick parts of the cluster, e.g., a single table or one
+ database. The other two methods work only at the level of
+ the complete cluster.
+ </para>
+
+ <bridgehead renderas="sect2">Continuous archiving based on pg_basebackup and WAL files</bridgehead>
+ <para>
+ <link linkend="continuous-archiving">This method</link>
+ is the most sophisticated and complex one. It
+ consists of two phases.
+ </para>
+ <para>
+ First, you need to create a so called
+ <firstterm>basebackup</firstterm> with the tool
+ <command>pg_basebackup</command>. The result is a
+ directory structure plus files which contains a
+ consistent copy of the original cluster.
+ <command>pg_basebackup</command> runs in
+ parallel to other processes in its own transaction.
+ </para>
+ <para>
+ The second step is recommended but not necessary. All
+ changes to the data are stored in WAL files. If you
+ continuously save such WAL files, you have the history
+ of the cluster. This history can be applied to a
+ basebackup in order to recreate
+ any state of the cluster between the time of
+ <command>pg_basebackup</command>'s start time and
+ any later point in time. This technique
+ is called 'Point-in-Time Recovery (PITR)'.
+ </para>
+ <para>
+ If configured, the
+ <glossterm linkend="glossary-wal-archiver">Archiver process</glossterm>
+ will automatically copy every single WAL file to a save location.
+ <link linkend="backup-archiving-wal">Its configuration</link>
+ consists mainly of a string, which contains a copy command
+ in the operating system's syntax. In order to protect your
+ data against a disk crash, the destination location
+ of a basebackup as well as of the
+ <firstterm>archived WAL files</firstterm> should be on a
+ disk which is different from the data disk.
+ </para>
+ <para>
+ If it gets necessary to restore the cluster, you have to
+ copy the basebackup and the
+ archived WAL files to
+ their original directories. The configuration of this
+ <link linkend="backup-pitr-recovery">recovery procedure</link>
+ contains a string with the reverse copy command: from
+ archive location to database location.
+ </para>
+
+ </sect1>
+
+<!-- ToDo: replication, index-types, extension mechanism, ...
+ <sect1 id="tutorial-replication">
+ <title>Replication</title>
+
+ <para>
+...
+ </para>
+
+ </sect1>
+-->
+
+ </chapter>
diff --git a/doc/src/sgml/filelist.sgml b/doc/src/sgml/filelist.sgml
index 38e8aa0bbf..7490d3c9c2 100644
--- a/doc/src/sgml/filelist.sgml
+++ b/doc/src/sgml/filelist.sgml
@@ -80,6 +80,7 @@
%allfiles;
<!-- developer's guide -->
+<!ENTITY architecture SYSTEM "architecture.sgml">
<!ENTITY arch-dev SYSTEM "arch-dev.sgml">
<!ENTITY bki SYSTEM "bki.sgml">
<!ENTITY catalogs SYSTEM "catalogs.sgml">
diff --git a/doc/src/sgml/images/cluster-db-schema-ink-svgo.svg b/doc/src/sgml/images/cluster-db-schema-ink-svgo.svg
new file mode 100644
index 0000000000..7e13753d48
--- /dev/null
+++ b/doc/src/sgml/images/cluster-db-schema-ink-svgo.svg
@@ -0,0 +1,160 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="900" height="685" viewBox="0 0 900 685">
+ <title>
+ Server (Hardware, Container, or VM)
+ </title>
+ <style>
+ .text_normal,.text_small{font-style:normal;font-weight:400;font-family:"Open Sans",sans-serif;fill:#000}.text_small{font-size:12px}.text_normal{font-size:16px}
+ </style>
+ <defs>
+ <symbol id="rectangle_special_0">
+ <rect width="225" height="155" rx="10" stroke="blue" fill="none"/>
+ <rect x="15" y="40" width="195" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2"/>
+ <text class="text_normal" x="20" y="60">
+ schema 'public'
+ </text>
+ <text class="text_small" x="20" y="80">
+ tables, views, ...
+ </text>
+ <rect x="15" y="105" width="195" height="30" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2"/>
+ <text class="text_small" x="20" y="125">
+ (more system schemas)
+ </text>
+ </symbol>
+ <symbol id="rectangle_special_1">
+ <rect width="245" height="225" rx="10" stroke="blue" fill="none"/>
+ <rect x="15" y="40" width="205" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2"/>
+ <text class="text_normal" x="20" y="60">
+ schema 'public'
+ </text>
+ <text class="text_small" x="20" y="80">
+ tables, views, ...
+ </text>
+ <rect x="15" y="105" width="205" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2"/>
+ <text class="text_normal" x="20" y="125">
+ 'my_schema' (optional)
+ </text>
+ <text class="text_small" x="20" y="145">
+ tables, views, ...
+ </text>
+ <rect x="15" y="170" width="205" height="30" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2"/>
+ <text class="text_small" x="20" y="190">
+ (more system schemas)
+ </text>
+ </symbol>
+ <symbol id="note" stroke="black" fill="lightyellow">
+ <title>
+ UML Note
+ </title>
+ <path d="M450 10v230H0V0h440v10h10L440 0"/>
+ </symbol>
+ <marker id="arrowhead_end" markerWidth="10" markerHeight="10" refX="6" refY="3" orient="auto">
+ <path d="M0 0l6 3-6 3" stroke="black" fill="none"/>
+ </marker>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="270" y="40" font-weight="400" font-size="24" font-family=""Open Sans",sans-serif">
+ Server (Hardware, Container, or VM)
+ </text>
+ <path stroke="blue" stroke-width="2" fill="none" d="M20 110h790v555H20z"/>
+ <text class="text_normal" x="180" y="25" transform="translate(20 110)">
+ cluster 'data' (default, managed by one instance)
+ </text>
+ <path d="M50 110V80h790v555h-30" stroke="blue" stroke-width="2" fill="none"/>
+ <text class="text_normal" x="190" y="-10" transform="translate(45 110)">
+ cluster 'cluster_2' (optional, managed by a different instance)
+ </text>
+ <g transform="translate(40 155)">
+ <use xlink:href="#rectangle_special_0"/>
+ <text class="text_normal" x="10" y="25">
+ database 'template0'
+ </text>
+ </g>
+ <g transform="translate(290 155)">
+ <use xlink:href="#rectangle_special_0"/>
+ <text class="text_normal" x="10" y="25">
+ database 'template1'
+ </text>
+ </g>
+ <g transform="translate(540 155)">
+ <use xlink:href="#rectangle_special_1"/>
+ <text class="text_normal" x="10" y="25">
+ database 'postgres'
+ </text>
+ </g>
+ <g transform="translate(40 350)">
+ <use xlink:href="#rectangle_special_1"/>
+ <text class="text_normal" x="10" y="25">
+ database 'my_db' (optional)
+ </text>
+ </g>
+ <g transform="translate(320 330)">
+ <rect width="180" height="45" rx="10" stroke="blue" fill="none" stroke-dasharray="10 4 4 4"/>
+ <text class="text_normal" x="15" y="27">
+ Global SQL objects
+ </text>
+ <path d="M0 5l-65-35" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M80 0v-30" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M180 40h50" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M0 40l-45 20" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ </g>
+ <g transform="translate(335 405)">
+ <use xlink:href="#note"/>
+ <text class="text_small" x="10" y="20">
+ 1)
+ </text>
+ <text class="text_small" x="30" y="20">
+ By default, you work in the cluster 'data', database 'postgres',
+ </text>
+ <text class="text_small" x="30" y="35">
+ schema 'public'.
+ </text>
+ <text class="text_small" x="10" y="55">
+ 2)
+ </text>
+ <text class="text_small" x="30" y="55">
+ More system schemas: pg_catalog, information_schema,
+ </text>
+ <text class="text_small" x="30" y="70">
+ pg_temp, pg_toast.
+ </text>
+ <text class="text_small" x="10" y="90">
+ 3)
+ </text>
+ <text class="text_small" x="30" y="90">
+ Global SQL objects: Some SQL objects are automatically active
+ </text>
+ <text class="text_small" x="30" y="105">
+ and known database- or even cluster-wide.
+ </text>
+ <text class="text_small" x="10" y="125">
+ 4)
+ </text>
+ <text class="text_small" x="30" y="125">
+ The command 'initdb' creates a new cluster with the three
+ </text>
+ <text class="text_small" x="30" y="140">
+ databases 'template0', 'template1', and 'postgres'. The command
+ </text>
+ <text class="text_small" x="30" y="155">
+ 'createdb' creates a new database.
+ </text>
+ <text class="text_small" x="10" y="175">
+ 5)
+ </text>
+ <text class="text_small" x="30" y="175">
+ If multiple clusters are active on one server at the same time,
+ </text>
+ <text class="text_small" x="30" y="190">
+ each one is managed by an individual instance. Each such instance
+ </text>
+ <text class="text_small" x="30" y="205">
+ uses a different port.
+ </text>
+ <text class="text_small" x="10" y="225">
+ 6)
+ </text>
+ <text class="text_small" x="30" y="225">
+ No client application is allowed to connect to 'template0'.
+ </text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/cluster-db-schema-ink.svg b/doc/src/sgml/images/cluster-db-schema-ink.svg
new file mode 100644
index 0000000000..1fffb9737a
--- /dev/null
+++ b/doc/src/sgml/images/cluster-db-schema-ink.svg
@@ -0,0 +1,482 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="900px"
+ height="685px"
+ viewBox="0 0 900 685"
+ id="svg147"
+ sodipodi:docname="cluster-db-schema-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata151">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>Server (Hardware, Container, or VM)</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="1129"
+ inkscape:window-height="922"
+ id="namedview149"
+ showgrid="false"
+ inkscape:zoom="0.68905109"
+ inkscape:cx="737.16805"
+ inkscape:cy="342.1375"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg147" />
+ <title
+ id="title2">Server (Hardware, Container, or VM)</title>
+ <!-- common text classes -->
+ <style
+ type="text/css"
+ id="style4">
+ .text_small {font-style:normal;
+ font-weight:normal;
+ font-size:12px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+ <defs
+ id="defs49">
+ <!-- a rectangle with rounded corners and inner definitions for schemas -->
+ <symbol
+ id="rectangle_special_0">
+ <!-- the database -->
+ <rect
+ width="225"
+ height="155"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ id="rect6" />
+ <!-- schemas -->
+ <rect
+ x="15"
+ y="40"
+ width="195"
+ height="50"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="4 2"
+ id="rect8" />
+ <text
+ class="text_normal"
+ x="20"
+ y="60"
+ id="text10">schema 'public'</text>
+ <text
+ class="text_small"
+ x="20"
+ y="80"
+ id="text12">tables, views, ...</text>
+ <rect
+ x="15"
+ y="105"
+ width="195"
+ height="30"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="4 2"
+ id="rect14" />
+ <text
+ class="text_small"
+ x="20"
+ y="125"
+ id="text16">(more system schemas)</text>
+ </symbol>
+ <!-- same as before, but one more schema -->
+ <symbol
+ id="rectangle_special_1">
+ <!-- the database -->
+ <rect
+ width="245"
+ height="225"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ id="rect19" />
+ <!-- schemas -->
+ <rect
+ x="15"
+ y="40"
+ width="205"
+ height="50"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="4 2"
+ id="rect21" />
+ <text
+ class="text_normal"
+ x="20"
+ y="60"
+ id="text23">schema 'public'</text>
+ <text
+ class="text_small"
+ x="20"
+ y="80"
+ id="text25">tables, views, ...</text>
+ <rect
+ x="15"
+ y="105"
+ width="205"
+ height="50"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="4 2"
+ id="rect27" />
+ <text
+ class="text_normal"
+ x="20"
+ y="125"
+ id="text29">'my_schema' (optional)</text>
+ <text
+ class="text_small"
+ x="20"
+ y="145"
+ id="text31">tables, views, ...</text>
+ <rect
+ x="15"
+ y="170"
+ width="205"
+ height="30"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="4 2"
+ id="rect33" />
+ <text
+ class="text_small"
+ x="20"
+ y="190"
+ id="text35">(more system schemas)</text>
+ </symbol>
+ <symbol
+ id="note"
+ stroke="black"
+ fill="lightyellow">
+ <title
+ id="title38">UML Note</title>
+ <path
+ d="M 450,10 v 230 h -450 v -240 h 440 v 10 h 10 l -10,-10"
+ id="path40" />
+ </symbol>
+ <!-- marker start/end -->
+ <marker
+ id="arrowhead_start"
+ markerWidth="10"
+ markerHeight="10"
+ refX="0"
+ refY="3"
+ orient="auto">
+ <path
+ d="M 6,0 l -6,3 l 6,3"
+ stroke="black"
+ fill="none"
+ id="path43" />
+ </marker>
+ <marker
+ id="arrowhead_end"
+ markerWidth="10"
+ markerHeight="10"
+ refX="6"
+ refY="3"
+ orient="auto">
+ <path
+ d="M 0,0 l 6,3 l -6,3"
+ stroke="black"
+ fill="none"
+ id="path46" />
+ </marker>
+ </defs>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect51" />
+ <text
+ class="text_big"
+ x="270"
+ y="40"
+ id="text53">Server (Hardware, Container, or VM)</text>
+ <!-- two clusters -->
+ <g
+ transform="translate(20 110)"
+ id="g59">
+ <rect
+ x="0"
+ y="0"
+ width="790"
+ height="555"
+ stroke="blue"
+ stroke-width="2px"
+ fill="none"
+ id="rect55" />
+ <text
+ class="text_normal"
+ x="180"
+ y="25"
+ id="text57">cluster 'data' (default, managed by one instance)</text>
+ </g>
+ <g
+ transform="translate(45 110)"
+ id="g65">
+ <path
+ d="M 5,0 v -30 h 790 v 555 h -30"
+ stroke="blue"
+ stroke-width="2px"
+ fill="none"
+ id="path61" />
+ <text
+ class="text_normal"
+ x="190"
+ y="-10"
+ id="text63">cluster 'cluster_2' (optional, managed by a different instance)</text>
+ </g>
+ <!-- database template 0 -->
+ <g
+ transform="translate(40 155)"
+ id="g71">
+ <use
+ xlink:href="#rectangle_special_0"
+ id="use67" />
+ <text
+ class="text_normal"
+ x="10"
+ y="25"
+ id="text69">database 'template0'</text>
+ </g>
+ <!-- database template 1 -->
+ <g
+ transform="translate(290 155)"
+ id="g77">
+ <use
+ xlink:href="#rectangle_special_0"
+ id="use73" />
+ <text
+ class="text_normal"
+ x="10"
+ y="25"
+ id="text75">database 'template1'</text>
+ </g>
+ <!-- database postgres -->
+ <g
+ transform="translate(540 155)"
+ id="g83">
+ <use
+ xlink:href="#rectangle_special_1"
+ id="use79" />
+ <text
+ class="text_normal"
+ x="10"
+ y="25"
+ id="text81">database 'postgres'</text>
+ </g>
+ <!-- database my_db -->
+ <g
+ transform="translate(40 350)"
+ id="g89">
+ <use
+ xlink:href="#rectangle_special_1"
+ id="use85" />
+ <text
+ class="text_normal"
+ x="10"
+ y="25"
+ id="text87">database 'my_db' (optional)</text>
+ </g>
+ <!-- global objects -->
+ <g
+ transform="translate(320 330)"
+ id="g103">
+ <rect
+ x="0"
+ y="0"
+ width="180"
+ height="45"
+ rx="10"
+ stroke="blue"
+ fill="none"
+ stroke-dasharray="10 4 4 4"
+ id="rect91" />
+ <text
+ class="text_normal"
+ x="15"
+ y="27"
+ id="text93">Global SQL objects</text>
+ <path
+ d="M 0,5 l-65,-35"
+ stroke="black"
+ fill="none"
+ marker-end="url(#arrowhead_end)"
+ id="path95" />
+ <path
+ d="M 80,0 v-30"
+ stroke="black"
+ fill="none"
+ marker-end="url(#arrowhead_end)"
+ id="path97" />
+ <path
+ d="M 180,40 h50"
+ stroke="black"
+ fill="none"
+ marker-end="url(#arrowhead_end)"
+ id="path99" />
+ <path
+ d="M 0,40 l-45,20"
+ stroke="black"
+ fill="none"
+ marker-end="url(#arrowhead_end)"
+ id="path101" />
+ </g>
+ <!-- Some comments -->
+ <g
+ transform="translate(335 405)"
+ id="g145">
+ <use
+ xlink:href="#note"
+ x="0"
+ y="0"
+ id="use105" />
+ <text
+ class="text_small"
+ x="10"
+ y="20"
+ id="text107">1)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="20"
+ id="text109">By default, you work in the cluster 'data', database 'postgres',</text>
+ <text
+ class="text_small"
+ x="30"
+ y="35"
+ id="text111">schema 'public'.</text>
+ <text
+ class="text_small"
+ x="10"
+ y="55"
+ id="text113">2)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="55"
+ id="text115">More system schemas: pg_catalog, information_schema,</text>
+ <text
+ class="text_small"
+ x="30"
+ y="70"
+ id="text117">pg_temp, pg_toast.</text>
+ <text
+ class="text_small"
+ x="10"
+ y="90"
+ id="text119">3)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="90"
+ id="text121">Global SQL objects: Some SQL objects are automatically active</text>
+ <text
+ class="text_small"
+ x="30"
+ y="105"
+ id="text123">and known database- or even cluster-wide.</text>
+ <text
+ class="text_small"
+ x="10"
+ y="125"
+ id="text125">4)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="125"
+ id="text127">The command 'initdb' creates a new cluster with the three</text>
+ <text
+ class="text_small"
+ x="30"
+ y="140"
+ id="text129">databases 'template0', 'template1', and 'postgres'. The command</text>
+ <text
+ class="text_small"
+ x="30"
+ y="155"
+ id="text131">'createdb' creates a new database.</text>
+ <text
+ class="text_small"
+ x="10"
+ y="175"
+ id="text133">5)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="175"
+ id="text135">If multiple clusters are active on one server at the same time,</text>
+ <text
+ class="text_small"
+ x="30"
+ y="190"
+ id="text137">each one is managed by an individual instance. Each such instance</text>
+ <text
+ class="text_small"
+ x="30"
+ y="205"
+ id="text139">uses a different port.</text>
+ <text
+ class="text_small"
+ x="10"
+ y="225"
+ id="text141">6)</text>
+ <text
+ class="text_small"
+ x="30"
+ y="225"
+ id="text143">No client application is allowed to connect to 'template0'.</text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/cluster-db-schema-raw.svg b/doc/src/sgml/images/cluster-db-schema-raw.svg
new file mode 100644
index 0000000000..af50c07330
--- /dev/null
+++ b/doc/src/sgml/images/cluster-db-schema-raw.svg
@@ -0,0 +1,173 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="900px" height="685px"
+ viewBox="0 0 900 685" >
+
+ <title>Server (Hardware, Container, or VM)</title>
+
+ <!-- common text classes -->
+ <style type="text/css">
+ .text_small {font-style:normal;
+ font-weight:normal;
+ font-size:12px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+ <!-- a rectangle with rounded corners and inner definitions for schemas -->
+ <symbol id="rectangle_special_0">
+
+ <!-- the database -->
+ <rect width="225" height="155" rx="10" stroke="blue" fill="none"/>
+
+ <!-- schemas -->
+ <rect x="15" y="40" width="195" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2" />
+ <text class="text_normal" x="20" y="60">schema 'public'</text>
+ <text class="text_small" x="20" y="80">tables, views, ...</text>
+
+ <rect x="15" y="105" width="195" height="30" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2" />
+ <text class="text_small" x="20" y="125">(more system schemas)</text>
+ </symbol>
+
+ <!-- same as before, but one more schema -->
+ <symbol id="rectangle_special_1">
+ <!-- the database -->
+ <rect width="245" height="225" rx="10" stroke="blue" fill="none"/>
+
+ <!-- schemas -->
+ <rect x="15" y="40" width="205" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2" />
+ <text class="text_normal" x="20" y="60">schema 'public'</text>
+ <text class="text_small" x="20" y="80">tables, views, ...</text>
+
+ <rect x="15" y="105" width="205" height="50" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2" />
+ <text class="text_normal" x="20" y="125">'my_schema' (optional)</text>
+ <text class="text_small" x="20" y="145">tables, views, ...</text>
+
+ <rect x="15" y="170" width="205" height="30" rx="10" stroke="blue" fill="none" stroke-dasharray="4 2" />
+ <text class="text_small" x="20" y="190">(more system schemas)</text>
+ </symbol>
+
+ <symbol id="note" stroke="black" fill="lightyellow">
+ <title>UML Note</title>
+ <path d="M 450,10 v 230 h -450 v -240 h 440 v 10 h 10 l -10,-10" />
+ </symbol>
+
+ <!-- marker start/end -->
+ <marker id="arrowhead_start"
+ markerWidth="10"
+ markerHeight="10"
+ refX="0"
+ refY="3"
+ orient="auto">
+ <path d="M 6,0 l -6,3 l 6,3" stroke="black" fill="none" />
+ </marker>
+ <marker id="arrowhead_end"
+ markerWidth="10"
+ markerHeight="10"
+ refX="6"
+ refY="3"
+ orient="auto">
+ <path d="M 0,0 l 6,3 l -6,3" stroke="black" fill="none" />
+ </marker>
+
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+ <text class="text_big" x="270" y="40">Server (Hardware, Container, or VM)</text>
+
+ <!-- two clusters -->
+ <g transform="translate(20 110)">
+ <rect x="0" y="0" width="790" height="555" stroke="blue" stroke-width="2px" fill="none" />
+ <text class="text_normal" x="180" y="25">cluster 'data' (default, managed by one instance)</text>
+ </g>
+ <g transform="translate(45 110)">
+ <path d="M 5,0 v -30 h 790 v 555 h -30" stroke="blue" stroke-width="2px" fill="none" />
+ <text class="text_normal" x="190" y="-10">cluster 'cluster_2' (optional, managed by a different instance)</text>
+ </g>
+
+
+ <!-- database template 0 -->
+ <g transform="translate(40 155)">
+ <use xlink:href="#rectangle_special_0" />
+ <text class="text_normal" x="10" y="25">database 'template0'</text>
+ </g>
+
+ <!-- database template 1 -->
+ <g transform="translate(290 155)">
+ <use xlink:href="#rectangle_special_0" />
+ <text class="text_normal" x="10" y="25">database 'template1'</text>
+ </g>
+
+ <!-- database postgres -->
+ <g transform="translate(540 155)">
+ <use xlink:href="#rectangle_special_1" />
+ <text class="text_normal" x="10" y="25">database 'postgres'</text>
+ </g>
+
+ <!-- database my_db -->
+ <g transform="translate(40 350)">
+ <use xlink:href="#rectangle_special_1" />
+ <text class="text_normal" x="10" y="25">database 'my_db' (optional)</text>
+ </g>
+
+ <!-- global objects -->
+ <g transform="translate(320 330)">
+ <rect x="0" y="0" width="180" height="45" rx="10" stroke="blue" fill="none" stroke-dasharray="10 4 4 4" />
+ <text class="text_normal" x="15" y="27">Global SQL objects</text>
+ <path d="M 0,5 l-65,-35" stroke="black" fill="none" marker-end="url(#arrowhead_end)" />
+ <path d="M 80,0 v-30" stroke="black" fill="none" marker-end="url(#arrowhead_end)" />
+ <path d="M 180,40 h50" stroke="black" fill="none" marker-end="url(#arrowhead_end)" />
+ <path d="M 0,40 l-45,20" stroke="black" fill="none" marker-end="url(#arrowhead_end)" />
+ </g>
+
+ <!-- Some comments -->
+ <g transform="translate(335 405)">
+ <use xlink:href="#note" x="0" y="0" />
+
+ <text class="text_small" x="10" y="20">1)</text>
+ <text class="text_small" x="30" y="20">By default, you work in the cluster 'data', database 'postgres',</text>
+ <text class="text_small" x="30" y="35">schema 'public'.</text>
+
+ <text class="text_small" x="10" y="55">2)</text>
+ <text class="text_small" x="30" y="55">More system schemas: pg_catalog, information_schema,</text>
+ <text class="text_small" x="30" y="70">pg_temp, pg_toast.</text>
+
+ <text class="text_small" x="10" y="90">3)</text>
+ <text class="text_small" x="30" y="90">Global SQL objects: Some SQL objects are automatically active</text>
+ <text class="text_small" x="30" y="105">and known database- or even cluster-wide.</text>
+
+ <text class="text_small" x="10" y="125">4)</text>
+ <text class="text_small" x="30" y="125">The command 'initdb' creates a new cluster with the three</text>
+ <text class="text_small" x="30" y="140">databases 'template0', 'template1', and 'postgres'. The command</text>
+ <text class="text_small" x="30" y="155">'createdb' creates a new database.</text>
+
+ <text class="text_small" x="10" y="175">5)</text>
+ <text class="text_small" x="30" y="175">If multiple clusters are active on one server at the same time,</text>
+ <text class="text_small" x="30" y="190">each one is managed by an individual instance. Each such instance</text>
+ <text class="text_small" x="30" y="205">uses a different port.</text>
+
+ <text class="text_small" x="10" y="225">6)</text>
+ <text class="text_small" x="30" y="225">No client application is allowed to connect to 'template0'.</text>
+
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/directories-ink-svgo.svg b/doc/src/sgml/images/directories-ink-svgo.svg
new file mode 100644
index 0000000000..95fa76b9c6
--- /dev/null
+++ b/doc/src/sgml/images/directories-ink-svgo.svg
@@ -0,0 +1,164 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="900" height="640" viewBox="0 0 900 640">
+ <title>
+ Directory structure of a cluster
+ </title>
+ <style>
+ .text_normal{font-style:normal;font-weight:400;font-size:16px;font-family:"Open Sans",sans-serif;fill:#000}
+ </style>
+ <defs>
+ <symbol id="directory" stroke="blue" stroke-width=".3" fill="aqua">
+ <title>
+ Directory
+ </title>
+ <path d="M0 10h110v20H0z"/>
+ <path d="M0 10V8l3-3h35l3 3v2"/>
+ </symbol>
+ <symbol id="file" stroke="black" fill="none">
+ <title>
+ File
+ </title>
+ <path stroke="blue" d="M0 0h40v50H0z"/>
+ <path d="M5 10h20" stroke-dasharray="4 2"/>
+ <path d="M5 17h15" stroke-dasharray="6 2"/>
+ <path d="M5 24h25" stroke-dasharray="4 2"/>
+ <path d="M5 31h20" stroke-dasharray="5 2"/>
+ </symbol>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="200" y="50" font-weight="400" font-size="24" font-family=""Open Sans",sans-serif">
+ Directory Structure
+ </text>
+ <g transform="translate(20 100)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ ... /pg/
+ </text>
+ <text x="300" y="26" class="text_normal">
+ An arbitrary directory
+ </text>
+ </g>
+ <g transform="translate(70 130)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ data/
+ </text>
+ <text x="250" y="26" class="text_normal">
+ Root of cluster 'data' (see: PGDATA)
+ </text>
+ </g>
+ <g transform="translate(120 160)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ base/
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Subdirectory containing per-database subdirectories
+ </text>
+ </g>
+ <g transform="translate(170 190)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ 1/
+ </text>
+ <text x="150" y="26" class="text_normal">
+ Subdirectory for data of first database 'template0'
+ </text>
+ </g>
+ <g transform="translate(170 220)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ 12992/
+ </text>
+ <text x="150" y="26" class="text_normal">
+ Subdirectory for data of second database 'template1'
+ </text>
+ </g>
+ <g transform="translate(170 250)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ 12999/
+ </text>
+ <text x="150" y="26" class="text_normal">
+ Subdirectory for data of third database 'postgres'
+ </text>
+ </g>
+ <g transform="translate(170 280)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ nnnnn/
+ </text>
+ <text x="150" y="26" class="text_normal">
+ Optional: more subdirectories for databases, e.g. 'my_db'
+ </text>
+ </g>
+ <g transform="translate(120 310)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ global/
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Subdirectory with information about Global SQL Objects
+ </text>
+ </g>
+ <g transform="translate(120 340)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ pg_wal/
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Subdirectory for Write Ahead Log files ('pg_xlog' before version 10)
+ </text>
+ </g>
+ <g transform="translate(120 370)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ pg_xact/
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Subdirectory for transaction commit status ('pg_clog' before version 10)
+ </text>
+ </g>
+ <g transform="translate(120 400)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ pg_tblspc/
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Subdirectory containing symbolic links to tablespaces
+ </text>
+ </g>
+ <g transform="translate(120 430)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ pg_... /
+ </text>
+ <text x="200" y="26" class="text_normal">
+ Some more subdirectories
+ </text>
+ </g>
+ <g transform="translate(120 465)">
+ <use xlink:href="#file"/>
+ <use xlink:href="#file" x="50"/>
+ <text x="200" y="26" class="text_normal">
+ 'postmaster.pid' and other files with cluster-wide relevance
+ </text>
+ </g>
+ <g transform="translate(20 540)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ ... /xyz/
+ </text>
+ <text x="300" y="26" class="text_normal">
+ Same or another arbitrary directory
+ </text>
+ </g>
+ <g transform="translate(70 570)">
+ <use xlink:href="#directory"/>
+ <text x="10" y="26" class="text_normal">
+ cluster_2/
+ </text>
+ <text x="250" y="26" class="text_normal">
+ Root of another cluster 'cluster_2'
+ </text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/directories-ink.svg b/doc/src/sgml/images/directories-ink.svg
new file mode 100644
index 0000000000..8151cf583a
--- /dev/null
+++ b/doc/src/sgml/images/directories-ink.svg
@@ -0,0 +1,397 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="900px"
+ height="640px"
+ viewBox="0 0 900 640"
+ id="svg303"
+ sodipodi:docname="directories-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata307">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>Directory structure of a cluster</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="1083"
+ inkscape:window-height="808"
+ id="namedview305"
+ showgrid="false"
+ inkscape:zoom="0.36875"
+ inkscape:cx="450"
+ inkscape:cy="320"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg303" />
+ <title
+ id="title153">Directory structure of a cluster</title>
+ <style
+ type="text/css"
+ id="style155">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+ <defs
+ id="defs177">
+ <!-- Directory -->
+ <symbol
+ id="directory"
+ stroke="blue"
+ stroke-width="0.3px"
+ fill="aqua">
+ <title
+ id="title157">Directory</title>
+ <rect
+ x="0"
+ y="10"
+ width="110"
+ height="20"
+ id="rect159" />
+ <path
+ d="M 0,10 l 0,-2 3,-3 35,0, 3,3 0,2"
+ id="path161" />
+ </symbol>
+ <!-- File -->
+ <symbol
+ id="file"
+ stroke="black"
+ fill="none">
+ <title
+ id="title164">File</title>
+ <rect
+ x="0"
+ y="0"
+ width="40"
+ height="50"
+ stroke="blue"
+ id="rect166" />
+ <path
+ d="M 5,10 h 20"
+ stroke-dasharray="4 2"
+ id="path168" />
+ <path
+ d="M 5,17 h 15"
+ stroke-dasharray="6 2"
+ id="path170" />
+ <path
+ d="M 5,24 h 25"
+ stroke-dasharray="4 2"
+ id="path172" />
+ <path
+ d="M 5,31 h 20"
+ stroke-dasharray="5 2"
+ id="path174" />
+ </symbol>
+ </defs>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect179" />
+ <!-- caption -->
+ <text
+ x="200"
+ y="50"
+ class="text_big"
+ id="text181">Directory Structure</text>
+ <!-- the directories -->
+ <g
+ transform="translate(20, 100)"
+ id="g189">
+ <use
+ xlink:href="#directory"
+ id="use183" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text185">... /pg/</text>
+ <text
+ x="300"
+ y="26"
+ class="text_normal"
+ id="text187">An arbitrary directory</text>
+ </g>
+ <g
+ transform="translate(70, 130)"
+ id="g197">
+ <use
+ xlink:href="#directory"
+ id="use191" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text193">data/</text>
+ <text
+ x="250"
+ y="26"
+ class="text_normal"
+ id="text195">Root of cluster 'data' (see: PGDATA)</text>
+ </g>
+ <g
+ transform="translate(120, 160)"
+ id="g205">
+ <use
+ xlink:href="#directory"
+ id="use199" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text201">base/</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text203">Subdirectory containing per-database subdirectories</text>
+ </g>
+ <!-- -->
+ <g
+ transform="translate(170, 190)"
+ id="g213">
+ <use
+ xlink:href="#directory"
+ id="use207" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text209">1/</text>
+ <text
+ x="150"
+ y="26"
+ class="text_normal"
+ id="text211">Subdirectory for data of first database 'template0'</text>
+ </g>
+ <g
+ transform="translate(170, 220)"
+ id="g221">
+ <use
+ xlink:href="#directory"
+ id="use215" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text217">12992/</text>
+ <text
+ x="150"
+ y="26"
+ class="text_normal"
+ id="text219">Subdirectory for data of second database 'template1'</text>
+ </g>
+ <g
+ transform="translate(170, 250)"
+ id="g229">
+ <use
+ xlink:href="#directory"
+ id="use223" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text225">12999/</text>
+ <text
+ x="150"
+ y="26"
+ class="text_normal"
+ id="text227">Subdirectory for data of third database 'postgres'</text>
+ </g>
+ <g
+ transform="translate(170, 280)"
+ id="g237">
+ <use
+ xlink:href="#directory"
+ id="use231" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text233">nnnnn/</text>
+ <text
+ x="150"
+ y="26"
+ class="text_normal"
+ id="text235">Optional: more subdirectories for databases, e.g. 'my_db'</text>
+ </g>
+ <g
+ transform="translate(120, 310)"
+ id="g245">
+ <use
+ xlink:href="#directory"
+ id="use239" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text241">global/</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text243">Subdirectory with information about Global SQL Objects</text>
+ </g>
+ <g
+ transform="translate(120, 340)"
+ id="g253">
+ <use
+ xlink:href="#directory"
+ id="use247" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text249">pg_wal/</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text251">Subdirectory for Write Ahead Log files ('pg_xlog' before version 10)</text>
+ </g>
+ <g
+ transform="translate(120, 370)"
+ id="g261">
+ <use
+ xlink:href="#directory"
+ id="use255" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text257">pg_xact/</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text259">Subdirectory for transaction commit status ('pg_clog' before version 10)</text>
+ </g>
+ <g
+ transform="translate(120, 400)"
+ id="g269">
+ <use
+ xlink:href="#directory"
+ id="use263" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text265">pg_tblspc/</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text267">Subdirectory containing symbolic links to tablespaces</text>
+ </g>
+ <g
+ transform="translate(120, 430)"
+ id="g277">
+ <use
+ xlink:href="#directory"
+ id="use271" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text273">pg_... /</text>
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text275">Some more subdirectories</text>
+ </g>
+ <g
+ transform="translate(120, 465)"
+ id="g285">
+ <use
+ xlink:href="#file"
+ x="0"
+ y="0"
+ id="use279" />
+ <use
+ xlink:href="#file"
+ x="50"
+ y="0"
+ id="use281" />
+ <text
+ x="200"
+ y="26"
+ class="text_normal"
+ id="text283">'postmaster.pid' and other files with cluster-wide relevance</text>
+ </g>
+ <!-- next cluster -->
+ <g
+ transform="translate(20, 540)"
+ id="g293">
+ <use
+ xlink:href="#directory"
+ id="use287" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text289">... /xyz/</text>
+ <text
+ x="300"
+ y="26"
+ class="text_normal"
+ id="text291">Same or another arbitrary directory</text>
+ </g>
+ <g
+ transform="translate(70, 570)"
+ id="g301">
+ <use
+ xlink:href="#directory"
+ id="use295" />
+ <text
+ x="10"
+ y="26"
+ class="text_normal"
+ id="text297">cluster_2/</text>
+ <text
+ x="250"
+ y="26"
+ class="text_normal"
+ id="text299">Root of another cluster 'cluster_2'</text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/directories-raw.svg b/doc/src/sgml/images/directories-raw.svg
new file mode 100644
index 0000000000..6d16a03169
--- /dev/null
+++ b/doc/src/sgml/images/directories-raw.svg
@@ -0,0 +1,144 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="900px" height="640px"
+ viewBox="0 0 900 640">
+
+ <title>Directory structure of a cluster</title>
+
+ <style type="text/css">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+
+ <!-- Directory -->
+ <symbol id="directory" stroke="blue" stroke-width="0.3px" fill="aqua">
+ <title>Directory</title>
+ <rect x="0" y="10" width="110" height="20" />
+ <path d="M 0,10 l 0,-2 3,-3 35,0, 3,3 0,2" />
+ </symbol>
+
+ <!-- File -->
+ <symbol id="file" stroke="black" fill="none" >
+ <title>File</title>
+ <rect x="0" y="0" width="40" height="50" stroke="blue" />
+ <path d="M 5,10 h 20" stroke-dasharray="4 2" />
+ <path d="M 5,17 h 15" stroke-dasharray="6 2" />
+ <path d="M 5,24 h 25" stroke-dasharray="4 2" />
+ <path d="M 5,31 h 20" stroke-dasharray="5 2" />
+ </symbol>
+
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+
+ <!-- caption -->
+ <text x="200" y="50" class="text_big">Directory Structure</text>
+
+ <!-- the directories -->
+ <g transform="translate(20, 100)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">... /pg/</text>
+ <text x="300" y="26" class="text_normal">An arbitrary directory</text>
+ </g>
+
+ <g transform="translate(70, 130)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">data/</text>
+ <text x="250" y="26" class="text_normal">Root of cluster 'data' (see: PGDATA)</text>
+ </g>
+
+ <g transform="translate(120, 160)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">base/</text>
+ <text x="200" y="26" class="text_normal">Subdirectory containing per-database subdirectories</text>
+ </g>
+
+ <!-- -->
+ <g transform="translate(170, 190)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">1/</text>
+ <text x="150" y="26" class="text_normal">Subdirectory for data of first database 'template0'</text>
+ </g>
+ <g transform="translate(170, 220)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">12992/</text>
+ <text x="150" y="26" class="text_normal">Subdirectory for data of second database 'template1'</text>
+ </g>
+ <g transform="translate(170, 250)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">12999/</text>
+ <text x="150" y="26" class="text_normal">Subdirectory for data of third database 'postgres'</text>
+ </g>
+ <g transform="translate(170, 280)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">nnnnn/</text>
+ <text x="150" y="26" class="text_normal">Optional: more subdirectories for databases, e.g. 'my_db'</text>
+ </g>
+
+ <g transform="translate(120, 310)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">global/</text>
+ <text x="200" y="26" class="text_normal">Subdirectory with information about Global SQL Objects</text>
+ </g>
+
+ <g transform="translate(120, 340)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">pg_wal/</text>
+ <text x="200" y="26" class="text_normal">Subdirectory for Write Ahead Log files ('pg_xlog' before version 10)</text>
+ </g>
+
+ <g transform="translate(120, 370)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">pg_xact/</text>
+ <text x="200" y="26" class="text_normal">Subdirectory for transaction commit status ('pg_clog' before version 10)</text>
+ </g>
+
+ <g transform="translate(120, 400)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">pg_tblspc/</text>
+ <text x="200" y="26" class="text_normal">Subdirectory containing symbolic links to tablespaces</text>
+ </g>
+
+ <g transform="translate(120, 430)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">pg_... /</text>
+ <text x="200" y="26" class="text_normal">Some more subdirectories</text>
+ </g>
+
+ <g transform="translate(120, 465)">
+ <use xlink:href="#file" x="0" y="0" />
+ <use xlink:href="#file" x="50" y="0" />
+ <text x="200" y="26" class="text_normal">'postmaster.pid' and other files with cluster-wide relevance</text>
+ </g>
+
+ <!-- next cluster -->
+ <g transform="translate(20, 540)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">... /xyz/</text>
+ <text x="300" y="26" class="text_normal">Same or another arbitrary directory</text>
+ </g>
+
+ <g transform="translate(70, 570)">
+ <use xlink:href="#directory" />
+ <text x="10" y="26" class="text_normal">cluster_2/</text>
+ <text x="250" y="26" class="text_normal">Root of another cluster 'cluster_2'</text>
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/freeze-ink-svgo.svg b/doc/src/sgml/images/freeze-ink-svgo.svg
new file mode 100644
index 0000000000..6fedfb7633
--- /dev/null
+++ b/doc/src/sgml/images/freeze-ink-svgo.svg
@@ -0,0 +1,84 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="850" height="320" viewBox="0 0 850 320">
+ <title>
+ Freeze
+ </title>
+ <style>
+ .text_normal{font-style:normal;font-weight:400;font-size:16px;font-family:"Open Sans",sans-serif;fill:#000}
+ </style>
+ <defs>
+ <path id="Path_080" d="M-80 0A80 80 0 1180 0 80 80 0 11-80 0"/>
+ <path id="Path_095" d="M-95 0A95 95 0 1195 0 95 95 0 11-95 0"/>
+ <path id="Path_120" d="M-120 0a120 120 0 11240 0 120 120 0 11-240 0"/>
+ <circle id="frozenPoint" r="3" stroke="green" fill="green" transform="translate(-100)"/>
+ <circle id="unfrozenPoint" r="3" stroke="green" fill="none" transform="translate(-100)"/>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="70" y="40" font-weight="400" font-size="24" font-family=""Open Sans",sans-serif,Helvetica">
+ <tspan font-style="italic" font-weight="700">Freeze</tspan> to keep visible
+ </text>
+ <g fill="none" transform="translate(170 190)">
+ <circle r="100" stroke="blue"/>
+ <text class="text_normal">
+ <textPath xlink:href="#Path_095" startOffset=".5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="0%">(0)</textPath> <textPath xlink:href="#Path_080" startOffset="1%" stroke="blue" stroke-width=".5"> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath> <textPath xlink:href="#Path_120" startOffset="11.2%">(1)</textPath> <textPath xlink:href="#Path_120" startOffset="61.2%">(5)</textPath> <textPath xlink:href="#Path_095" stroke="red" stroke-width="3" startOffset="34%">|</textPath> <textPath xlink:href="#Path_120" startOffset="34%">(2)</textPath> <textPath xlink:href="#Path_095" stroke="black" stroke-width="3" startOffset="45%">|</textPath> <textPath xlink:href="#Path_120" startOffset="45%">(3)</textPath> <textPath xlink:href="#Path_095" stroke="green" stroke-width="3" startOffset="55%">|</textPath> <textPath xlink:href="#Path_120" startOffset="55%">(4)</textPath>
+ </text>
+ <path d="M-80-80L80 80" stroke="black"/>
+ <text class="text_normal" letter-spacing="12" transform="rotate(45 40.78 -38.1)">
+ PAST
+ </text>
+ <text class="text_normal" letter-spacing="12" transform="rotate(-135 27.43 23.79)">
+ FUTURE
+ </text>
+ <use xlink:href="#frozenPoint" transform="rotate(10)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(25)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(40)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(55)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(70)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(85)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(100)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(115)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(130)"/>
+ <use xlink:href="#unfrozenPoint" transform="rotate(145)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(160)"/>
+ <use xlink:href="#frozenPoint" transform="rotate(175)"/>
+ <use xlink:href="#unfrozenPoint" transform="rotate(190)"/>
+ <use xlink:href="#unfrozenPoint" transform="rotate(205)"/>
+ <use xlink:href="#unfrozenPoint" transform="rotate(220)"/>
+ </g>
+ <g class="text_normal" transform="translate(380 30)">
+ <text>
+ 0: 0 .. 2 <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan> - 1
+ </text>
+ <text y="25">
+ 1: txid_current + 2 ^ 31 (split-point)
+ </text>
+ <text y="50">
+ 2: autovacuum_freeze_max_age (200 mio.)
+ </text>
+ <text y="75">
+ 3: vacuum_freeze_table_age (150 mio.)
+ </text>
+ <text y="100">
+ 4: vacuum_freeze_min_age (50 mio.)
+ </text>
+ <text y="125">
+ 5: txid_current (split-point, jungest xid)
+ </text>
+ <text y="155">
+ per table: pg_class.relfrozenxid <tspan font-weight="700">must</tspan> be between (1) and (5);
+ </text>
+ <text y="175" x="75">
+ normally it is between (3) and (4)
+ </text>
+ <use xlink:href="#unfrozenPoint" transform="translate(106 200)"/>
+ <text y="205" x="20">
+ Unfrozen xid
+ </text>
+ <use xlink:href="#frozenPoint" transform="translate(106 224)"/>
+ <text y="230" x="20">
+ Frozen xid
+ </text>
+ <text y="260">
+ (figure is out of scale)
+ </text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/freeze-ink.svg b/doc/src/sgml/images/freeze-ink.svg
new file mode 100644
index 0000000000..009cfe4b41
--- /dev/null
+++ b/doc/src/sgml/images/freeze-ink.svg
@@ -0,0 +1,365 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="850px"
+ height="320px"
+ viewBox="0 0 850 320"
+ id="svg454"
+ sodipodi:docname="freeze-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata458">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>Freeze</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="979"
+ inkscape:window-height="857"
+ id="namedview456"
+ showgrid="false"
+ inkscape:zoom="0.44"
+ inkscape:cx="425"
+ inkscape:cy="160"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg454" />
+ <title
+ id="title309">Freeze</title>
+ <!-- common text classes -->
+ <style
+ type="text/css"
+ id="style311">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif, Helvetica;
+ fill:black;
+ }
+ </style>
+ <defs
+ id="defs318">
+ <!-- SVG 1.x supports <texPath> only in combination with <path>. SVG 2 allows it with
+ every shape element, especially with <circle>. As of 2020 we must restrict ourselves
+ to SVG 1. Therefore we simulate a circle by defining two arcs, each of them is
+ a half-circle.
+ -->
+ <path
+ id="Path_080"
+ d="m -80 0 a 80 80 0 1 1 160 0 a 80 80 0 1 1 -160 0" />
+ <path
+ id="Path_095"
+ d="m -95 0 a 95 95 0 1 1 190 0 a 95 95 0 1 1 -190 0" />
+ <path
+ id="Path_120"
+ d="m -120 0 a 120 120 0 1 1 240 0 a 120 120 0 1 1 -240 0" />
+ <!-- split transform="translate(x y) rotate(arc)" into two steps for clarity -->
+ <circle
+ id="frozenPoint"
+ r="3"
+ stroke="green"
+ fill="green"
+ transform="translate(-100, 0)" />
+ <circle
+ id="unfrozenPoint"
+ r="3"
+ stroke="green"
+ fill="none"
+ transform="translate(-100, 0)" />
+ </defs>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect320" />
+ <text
+ class="text_big"
+ x="70"
+ y="40"
+ id="text324"><tspan
+ style="font-style:italic;font-weight:bold"
+ id="tspan322">
+ Freeze</tspan>
+ to keep visible</text>
+ <!-- set default values -->
+ <g
+ fill="none"
+ transform="translate(170 190)"
+ id="g418">
+ <circle
+ r="100"
+ stroke="blue"
+ id="circle326" />
+ <text
+ class="text_normal"
+ id="text350">
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="0.5%"
+ id="textPath328">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="00%"
+ id="textPath330">(0)</textPath>
+ <textPath
+ xlink:href="#Path_080"
+ startOffset="01%"
+ stroke="blue"
+ stroke-width="0.5px"
+ id="textPath332">
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="11.2%"
+ id="textPath334">(1)</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="61.2%"
+ id="textPath336">(5)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ stroke="red"
+ stroke-width="3"
+ startOffset="34%"
+ id="textPath338">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="34%"
+ id="textPath340">(2)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ stroke="black"
+ stroke-width="3"
+ startOffset="45%"
+ id="textPath342">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="45%"
+ id="textPath344">(3)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ stroke="green"
+ stroke-width="3"
+ startOffset="55%"
+ id="textPath346">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="55%"
+ id="textPath348">(4)</textPath>
+ </text>
+ <!-- split it into 'past' and 'future' -->
+ <path
+ d="M -80 -80 l 160 160"
+ stroke="black"
+ id="path352" />
+ <text
+ class="text_normal"
+ letter-spacing="12"
+ transform="translate(-15, -40) rotate(45)"
+ id="text354">PAST</text>
+ <text
+ class="text_normal"
+ letter-spacing="12"
+ transform="translate(30, 60) rotate(225)"
+ id="text356">FUTURE</text>
+ <g
+ transform="rotate(10)"
+ id="g360">
+ <use
+ xlink:href="#frozenPoint"
+ id="use358" />
+ </g>
+ <g
+ transform="rotate(25)"
+ id="g364">
+ <use
+ xlink:href="#frozenPoint"
+ id="use362" />
+ </g>
+ <g
+ transform="rotate(40)"
+ id="g368">
+ <use
+ xlink:href="#frozenPoint"
+ id="use366" />
+ </g>
+ <g
+ transform="rotate(55)"
+ id="g372">
+ <use
+ xlink:href="#frozenPoint"
+ id="use370" />
+ </g>
+ <g
+ transform="rotate(70)"
+ id="g376">
+ <use
+ xlink:href="#frozenPoint"
+ id="use374" />
+ </g>
+ <g
+ transform="rotate(85)"
+ id="g380">
+ <use
+ xlink:href="#frozenPoint"
+ id="use378" />
+ </g>
+ <g
+ transform="rotate(100)"
+ id="g384">
+ <use
+ xlink:href="#frozenPoint"
+ id="use382" />
+ </g>
+ <g
+ transform="rotate(115)"
+ id="g388">
+ <use
+ xlink:href="#frozenPoint"
+ id="use386" />
+ </g>
+ <g
+ transform="rotate(130)"
+ id="g392">
+ <use
+ xlink:href="#frozenPoint"
+ id="use390" />
+ </g>
+ <g
+ transform="rotate(145)"
+ id="g396">
+ <use
+ xlink:href="#unfrozenPoint"
+ id="use394" />
+ </g>
+ <g
+ transform="rotate(160)"
+ id="g400">
+ <use
+ xlink:href="#frozenPoint"
+ id="use398" />
+ </g>
+ <g
+ transform="rotate(175)"
+ id="g404">
+ <use
+ xlink:href="#frozenPoint"
+ id="use402" />
+ </g>
+ <g
+ transform="rotate(190)"
+ id="g408">
+ <use
+ xlink:href="#unfrozenPoint"
+ id="use406" />
+ </g>
+ <g
+ transform="rotate(205)"
+ id="g412">
+ <use
+ xlink:href="#unfrozenPoint"
+ id="use410" />
+ </g>
+ <g
+ transform="rotate(220)"
+ id="g416">
+ <use
+ xlink:href="#unfrozenPoint"
+ id="use414" />
+ </g>
+ </g>
+ <!-- legend -->
+ <g
+ class="text_normal"
+ transform="translate(380 30)"
+ id="g452">
+ <text
+ id="text424">0: 0 .. 2 <tspan
+ dy="-5"
+ id="tspan420">^</tspan>
+<tspan
+ dy="5"
+ id="tspan422">32</tspan>
+ - 1</text>
+ <text
+ y="25"
+ id="text426">1: txid_current + 2 ^ 31 (split-point)</text>
+ <text
+ y="50"
+ id="text428">2: autovacuum_freeze_max_age (200 mio.)</text>
+ <text
+ y="75"
+ id="text430">3: vacuum_freeze_table_age (150 mio.)</text>
+ <text
+ y="100"
+ id="text432">4: vacuum_freeze_min_age (50 mio.)</text>
+ <text
+ y="125"
+ id="text434">5: txid_current (split-point, jungest xid)</text>
+ <text
+ y="155"
+ id="text438">per table: pg_class.relfrozenxid
+ <tspan
+ style="font-weight:bold"
+ id="tspan436">must</tspan>
+ be between (1) and (5);</text>
+ <text
+ y="175"
+ x="75"
+ id="text440">normally it is between (3) and (4)</text>
+ <use
+ xlink:href="#unfrozenPoint"
+ transform="translate(106, 200)"
+ id="use442" />
+ <text
+ y="205"
+ x="20"
+ id="text444">Unfrozen xid</text>
+ <use
+ xlink:href="#frozenPoint"
+ transform="translate(106, 224)"
+ id="use446" />
+ <text
+ y="230"
+ x="20"
+ id="text448">Frozen xid</text>
+ <text
+ y="260"
+ id="text450">(figure is out of scale)</text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/freeze-raw.svg b/doc/src/sgml/images/freeze-raw.svg
new file mode 100644
index 0000000000..2d1d256184
--- /dev/null
+++ b/doc/src/sgml/images/freeze-raw.svg
@@ -0,0 +1,123 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="850px" height="320px"
+ viewBox="0 0 850 320" >
+
+ <title>Freeze</title>
+
+ <!-- common text classes -->
+ <style type="text/css">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif, Helvetica;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+ <!-- SVG 1.x supports <texPath> only in combination with <path>. SVG 2 allows it with
+ every shape element, especially with <circle>. As of 2020 we must restrict ourselves
+ to SVG 1. Therefore we simulate a circle by defining two arcs, each of them is
+ a half-circle.
+ -->
+ <path id="Path_080" d="m -80 0
+ a 80 80 0 1 1 160 0
+ a 80 80 0 1 1 -160 0"/>
+ <path id="Path_095" d="m -95 0
+ a 95 95 0 1 1 190 0
+ a 95 95 0 1 1 -190 0"/>
+ <path id="Path_120" d="m -120 0
+ a 120 120 0 1 1 240 0
+ a 120 120 0 1 1 -240 0"/>
+
+ <!-- split transform="translate(x y) rotate(arc)" into two steps for clarity -->
+ <circle id="frozenPoint" r="3" stroke="green" fill="green" transform="translate(-100, 0)"/>
+ <circle id="unfrozenPoint" r="3" stroke="green" fill="none" transform="translate(-100, 0)"/>
+
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+ <text class="text_big" x="70" y="40"><tspan style="font-style:italic;font-weight:bold">
+ Freeze</tspan> to keep visible</text>
+
+ <!-- set default values -->
+ <g fill="none" transform="translate(170 190)">
+
+ <circle r="100" stroke="blue" />
+
+ <text class="text_normal">
+ <textPath xlink:href="#Path_095" startOffset="0.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="00%" >(0)</textPath>
+ <textPath xlink:href="#Path_080" startOffset="01%" stroke="blue" stroke-width="0.5px" >
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath>
+ <textPath xlink:href="#Path_120" startOffset="11.2%" >(1)</textPath>
+ <textPath xlink:href="#Path_120" startOffset="61.2%" >(5)</textPath>
+
+ <textPath xlink:href="#Path_095" stroke="red" stroke-width="3" startOffset="34%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="34%" >(2)</textPath>
+
+ <textPath xlink:href="#Path_095" stroke="black" stroke-width="3" startOffset="45%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="45%" >(3)</textPath>
+
+ <textPath xlink:href="#Path_095" stroke="green" stroke-width="3" startOffset="55%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="55%" >(4)</textPath>
+
+ </text>
+
+ <!-- split it into 'past' and 'future' -->
+ <path d="M -80 -80 l 160 160" stroke="black" />
+ <text class="text_normal" letter-spacing="12" transform="translate(-15, -40) rotate(45)">PAST</text>
+ <text class="text_normal" letter-spacing="12" transform="translate(30, 60) rotate(225)">FUTURE</text>
+
+ <g transform="rotate(10)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(25)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(40)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(55)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(70)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(85)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(100)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(115)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(130)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(145)"><use xlink:href="#unfrozenPoint"/></g>
+ <g transform="rotate(160)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(175)"><use xlink:href="#frozenPoint"/></g>
+ <g transform="rotate(190)"><use xlink:href="#unfrozenPoint"/></g>
+ <g transform="rotate(205)"><use xlink:href="#unfrozenPoint"/></g>
+ <g transform="rotate(220)"><use xlink:href="#unfrozenPoint"/></g>
+
+ </g>
+
+ <!-- legend -->
+ <g class="text_normal" transform="translate(380 30)">
+ <text>0: 0 .. 2 <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan> - 1</text>
+ <text y="25">1: txid_current + 2 ^ 31 (split-point)</text>
+ <text y="50">2: autovacuum_freeze_max_age (200 mio.)</text>
+ <text y="75">3: vacuum_freeze_table_age (150 mio.)</text>
+ <text y="100">4: vacuum_freeze_min_age (50 mio.)</text>
+ <text y="125">5: txid_current (split-point, jungest xid)</text>
+ <text y="155">per table: pg_class.relfrozenxid
+ <tspan style="font-weight:bold">must</tspan> be between (1) and (5);</text>
+ <text y="175" x="75">normally it is between (3) and (4)</text>
+
+ <use xlink:href="#unfrozenPoint" transform="translate(106, 200)"/>
+ <text y="205" x="20">Unfrozen xid</text>
+
+ <use xlink:href="#frozenPoint" transform="translate(106, 224)"/>
+ <text y="230" x="20">Frozen xid</text>
+
+ <text y="260">(figure is out of scale)</text>
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/internal-objects-hierarchy-ink-svgo.svg b/doc/src/sgml/images/internal-objects-hierarchy-ink-svgo.svg
new file mode 100644
index 0000000000..26bce6176d
--- /dev/null
+++ b/doc/src/sgml/images/internal-objects-hierarchy-ink-svgo.svg
@@ -0,0 +1,83 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="720" height="430" viewBox="0 0 720 430">
+ <title>
+ Hierarchy of Internal Objects
+ </title>
+ <style>
+ .text_normal{font-style:normal;font-weight:400;font-size:16px;font-family:"Open Sans",sans-serif;fill:#000}
+ </style>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="200" y="40" font-weight="400" font-size="24" font-family=""Open Sans",sans-serif">
+ Hierarchy of internal Objects
+ </text>
+ <g fill="none">
+ <g transform="translate(350 240)">
+ <ellipse rx="320" ry="170" stroke="blue"/>
+ <text class="text_normal" x="-140" y="-130">
+ Cluster
+ </text>
+ <g transform="translate(40 -125)">
+ <ellipse rx="80" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-60" y="5">
+ Database Names
+ </text>
+ </g>
+ <g transform="translate(180 -70)">
+ <ellipse rx="60" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-40" y="5">
+ Tablespace
+ </text>
+ </g>
+ <g transform="translate(230 -5)">
+ <ellipse rx="80" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-70" y="5">
+ Replication Origins
+ </text>
+ </g>
+ <g transform="translate(200 70)">
+ <ellipse rx="78" ry="27" stroke="blue"/>
+ <text class="text_normal" x="-60" y="-3">
+ Subscription for
+ </text>
+ <text class="text_normal" x="-68" y="10">
+ Logical Replication
+ </text>
+ </g>
+ <g transform="translate(100 120)">
+ <ellipse rx="40" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-15" y="5">
+ Role
+ </text>
+ </g>
+ </g>
+ <g transform="translate(270 250)">
+ <ellipse rx="220" ry="110" stroke="blue" stroke-width="2"/>
+ <text class="text_normal" x="-60" y="-80">
+ Database
+ </text>
+ <g transform="translate(-150 -30)">
+ <ellipse rx="50" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-35" y="5">
+ Extension
+ </text>
+ </g>
+ <g transform="translate(-155 35)">
+ <ellipse rx="40" ry="20" stroke="blue"/>
+ <text class="text_normal" x="-35" y="5">
+ Collation
+ </text>
+ </g>
+ <g transform="translate(30 20)">
+ <ellipse rx="140" ry="70" stroke="blue"/>
+ <text class="text_normal" x="-80" y="-35">
+ Schema
+ </text>
+ <g transform="translate(20 10)">
+ <ellipse rx="90" ry="30" stroke="blue"/>
+ <text class="text_normal" x="-50" y="5">
+ Table, View, ...
+ </text>
+ </g>
+ </g>
+ </g>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/internal-objects-hierarchy-ink.svg b/doc/src/sgml/images/internal-objects-hierarchy-ink.svg
new file mode 100644
index 0000000000..e5745818d9
--- /dev/null
+++ b/doc/src/sgml/images/internal-objects-hierarchy-ink.svg
@@ -0,0 +1,255 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="720px"
+ height="430px"
+ viewBox="0 0 720 430"
+ id="svg540"
+ sodipodi:docname="internal-objects-hierarchy-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata546">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>Hierarchy of Internal Objects</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <defs
+ id="defs544" />
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="1050"
+ inkscape:window-height="838"
+ id="namedview542"
+ showgrid="false"
+ inkscape:zoom="0.51944444"
+ inkscape:cx="360"
+ inkscape:cy="215"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg540" />
+ <title
+ id="title460">Hierarchy of Internal Objects</title>
+ <!-- common text classes -->
+ <style
+ type="text/css"
+ id="style462">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect464" />
+ <text
+ class="text_big"
+ x="200"
+ y="40"
+ id="text466">Hierarchy of internal Objects</text>
+ <!-- set centre of figure and default values -->
+ <g
+ transform="translate(350 240)"
+ fill="none"
+ id="g538">
+ <g
+ id="g504">
+ <ellipse
+ rx="320"
+ ry="170"
+ stroke="blue"
+ id="ellipse468" />
+ <text
+ class="text_normal"
+ x="-140"
+ y="-130"
+ id="text470">Cluster</text>
+ <g
+ transform="translate(40 -125)"
+ id="g476">
+ <ellipse
+ rx="80"
+ ry="20"
+ stroke="blue"
+ id="ellipse472" />
+ <text
+ class="text_normal"
+ x="-60"
+ y="5"
+ id="text474">Database Names</text>
+ </g>
+ <g
+ transform="translate(180 -70)"
+ id="g482">
+ <ellipse
+ rx="60"
+ ry="20"
+ stroke="blue"
+ id="ellipse478" />
+ <text
+ class="text_normal"
+ x="-40"
+ y="5"
+ id="text480">Tablespace</text>
+ </g>
+ <g
+ transform="translate(230 -5)"
+ id="g488">
+ <ellipse
+ rx="80"
+ ry="20"
+ stroke="blue"
+ id="ellipse484" />
+ <text
+ class="text_normal"
+ x="-70"
+ y="5"
+ id="text486">Replication Origins</text>
+ </g>
+ <g
+ transform="translate(200 70)"
+ id="g496">
+ <ellipse
+ rx="78"
+ ry="27"
+ stroke="blue"
+ id="ellipse490" />
+ <text
+ class="text_normal"
+ x="-60"
+ y="-3"
+ id="text492">Subscription for</text>
+ <text
+ class="text_normal"
+ x="-68"
+ y="10"
+ id="text494">Logical Replication</text>
+ </g>
+ <g
+ transform="translate(100 120)"
+ id="g502">
+ <ellipse
+ rx="40"
+ ry="20"
+ stroke="blue"
+ id="ellipse498" />
+ <text
+ class="text_normal"
+ x="-15"
+ y="5"
+ id="text500">Role</text>
+ </g>
+ </g>
+ <g
+ transform="translate(-80 10)"
+ id="g536">
+ <ellipse
+ rx="220"
+ ry="110"
+ stroke="blue"
+ stroke-width="2px"
+ id="ellipse506" />
+ <text
+ class="text_normal"
+ x="-60"
+ y="-80"
+ id="text508">Database</text>
+ <g
+ transform="translate(-120 -50)"
+ id="g534">
+ <g
+ transform="translate(-30 20)"
+ id="g514">
+ <ellipse
+ rx="50"
+ ry="20"
+ stroke="blue"
+ id="ellipse510" />
+ <text
+ class="text_normal"
+ x="-35"
+ y="5"
+ id="text512">Extension</text>
+ </g>
+ <g
+ transform="translate(-35 85)"
+ id="g520">
+ <ellipse
+ rx="40"
+ ry="20"
+ stroke="blue"
+ id="ellipse516" />
+ <text
+ class="text_normal"
+ x="-35"
+ y="5"
+ id="text518">Collation</text>
+ </g>
+ <g
+ transform="translate(150 70)"
+ id="g532">
+ <ellipse
+ rx="140"
+ ry="70"
+ stroke="blue"
+ id="ellipse522" />
+ <text
+ class="text_normal"
+ x="-80"
+ y="-35"
+ id="text524">Schema</text>
+ <g
+ transform="translate(20 10)"
+ id="g530">
+ <ellipse
+ rx="90"
+ ry="30"
+ stroke="blue"
+ id="ellipse526" />
+ <text
+ class="text_normal"
+ x="-50"
+ y="5"
+ id="text528">Table, View, ...</text>
+ </g>
+ </g>
+ </g>
+ </g>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/internal-objects-hierarchy-raw.svg b/doc/src/sgml/images/internal-objects-hierarchy-raw.svg
new file mode 100644
index 0000000000..f0dc890f6b
--- /dev/null
+++ b/doc/src/sgml/images/internal-objects-hierarchy-raw.svg
@@ -0,0 +1,95 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="720px" height="430px"
+ viewBox="0 0 720 430" >
+
+ <title>Hierarchy of Internal Objects</title>
+
+ <!-- common text classes -->
+ <style type="text/css">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+ <text class="text_big" x="200" y="40">Hierarchy of internal Objects</text>
+
+
+ <!-- set centre of figure and default values -->
+ <g transform="translate(350 240)" fill="none">
+
+ <g>
+ <ellipse rx="320" ry="170" stroke="blue" />
+ <text class="text_normal" x="-140" y="-130">Cluster</text>
+
+ <g transform="translate(40 -125)">
+ <ellipse rx="80" ry="20" stroke="blue" />
+ <text class="text_normal" x="-60" y="5">Database Names</text>
+ </g>
+
+ <g transform="translate(180 -70)">
+ <ellipse rx="60" ry="20" stroke="blue" />
+ <text class="text_normal" x="-40" y="5">Tablespace</text>
+ </g>
+
+ <g transform="translate(230 -5)">
+ <ellipse rx="80" ry="20" stroke="blue" />
+ <text class="text_normal" x="-70" y="5">Replication Origins</text>
+ </g>
+
+ <g transform="translate(200 70)">
+ <ellipse rx="78" ry="27" stroke="blue" />
+ <text class="text_normal" x="-60" y="-3">Subscription for</text>
+ <text class="text_normal" x="-68" y="10">Logical Replication</text>
+ </g>
+
+ <g transform="translate(100 120)">
+ <ellipse rx="40" ry="20" stroke="blue" />
+ <text class="text_normal" x="-15" y="5">Role</text>
+ </g>
+
+ </g>
+
+ <g transform="translate(-80 10)">
+ <ellipse rx="220" ry="110" stroke="blue" stroke-width="2px" />
+ <text class="text_normal" x="-60" y="-80">Database</text>
+
+ <g transform="translate(-120 -50)">
+ <g transform="translate(-30 20)">
+ <ellipse rx="50" ry="20" stroke="blue" />
+ <text class="text_normal" x="-35" y="5">Extension</text>
+ </g>
+
+ <g transform="translate(-35 85)">
+ <ellipse rx="40" ry="20" stroke="blue" />
+ <text class="text_normal" x="-35" y="5">Collation</text>
+ </g>
+
+ <g transform="translate(150 70)">
+ <ellipse rx="140" ry="70" stroke="blue" />
+ <text class="text_normal" x="-80" y="-35">Schema</text>
+
+ <g transform="translate(20 10)">
+ <ellipse rx="90" ry="30" stroke="blue" />
+ <text class="text_normal" x="-50" y="5">Table, View, ...</text>
+ </g>
+ </g>
+ </g>
+ </g>
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/mvcc-ink-svgo.svg b/doc/src/sgml/images/mvcc-ink-svgo.svg
new file mode 100644
index 0000000000..8e67da93d1
--- /dev/null
+++ b/doc/src/sgml/images/mvcc-ink-svgo.svg
@@ -0,0 +1,151 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="850" height="150" viewBox="0 0 850 150">
+ <title>
+ MVCC
+ </title>
+ <style>
+ .text_small{font-style:normal;font-weight:400;font-size:10px;font-family:"Open Sans",sans-serif;fill:#000}
+ </style>
+ <defs>
+ <marker id="triangle_1" markerWidth="10" markerHeight="10" refX="5" refY="5" orient="auto" stroke="black" fill="none">
+ <path d="M0 0l10 5-10 5"/>
+ </marker>
+ <path id="arrow" d="M0 0q20-15 40 0" stroke="black" marker-end="url(#triangle_1)"/>
+ <g id="tuple" stroke="black">
+ <path d="M80 0H0v20h80M29 0v20M60 0v20"/>
+ <path d="M80 0h15M80 20h15" stroke-dasharray="2 1"/>
+ </g>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <g fill="none">
+ <g transform="translate(20 40)">
+ <text class="text_small" transform="translate(0 -18)">
+ T <tspan dx="-3" dy="5" font-size="8">123</tspan> <tspan dx="-2" dy="-5">: INSERT</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5)"/>
+ <use xlink:href="#tuple" transform="translate(60 70)"/>
+ <text class="text_small" transform="translate(65 83)" fill="#00f">
+ 123
+ </text>
+ <text class="text_small" transform="translate(95 83)" fill="#00f">
+ 0
+ </text>
+ <text class="text_small" transform="translate(125 83)" fill="#00f">
+ 'x'
+ </text>
+ </g>
+ <g transform="translate(185 40)">
+ <text class="text_small" transform="translate(0 -18)">
+ T <tspan dx="-3" dy="5" font-size="8">135</tspan> <tspan dx="-2" dy="-5">: UPDATE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5)"/>
+ <use xlink:href="#tuple" transform="translate(60 40)"/>
+ <text class="text_small" transform="translate(65 53)" fill="#00f">
+ 135
+ </text>
+ <text class="text_small" transform="translate(95 53)" fill="#00f">
+ 0
+ </text>
+ <text class="text_small" transform="translate(125 53)" fill="#00f">
+ 'y'
+ </text>
+ <use xlink:href="#tuple" transform="translate(60 70)"/>
+ <text class="text_small" transform="translate(65 83)">
+ 123
+ </text>
+ <text class="text_small" transform="translate(95 83)" fill="#00f">
+ 135
+ </text>
+ <text class="text_small" transform="translate(125 83)">
+ 'x'
+ </text>
+ </g>
+ <g transform="translate(350 40)">
+ <text class="text_small" transform="translate(0 -18)">
+ T <tspan dx="-3" dy="5" font-size="8">142</tspan> <tspan dx="-2" dy="-5">: UPDATE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5)"/>
+ <use xlink:href="#tuple" transform="translate(60 10)"/>
+ <text class="text_small" transform="translate(65 23)" fill="#00f">
+ 142
+ </text>
+ <text class="text_small" transform="translate(95 23)" fill="#00f">
+ 0
+ </text>
+ <text class="text_small" transform="translate(125 23)" fill="#00f">
+ 'z'
+ </text>
+ <use xlink:href="#tuple" transform="translate(60 40)"/>
+ <text class="text_small" transform="translate(65 53)">
+ 135
+ </text>
+ <text class="text_small" transform="translate(95 53)" fill="#00f">
+ 142
+ </text>
+ <text class="text_small" transform="translate(125 53)">
+ 'y'
+ </text>
+ <use xlink:href="#tuple" transform="translate(60 70)"/>
+ <text class="text_small" transform="translate(65 83)">
+ 123
+ </text>
+ <text class="text_small" transform="translate(95 83)">
+ 135
+ </text>
+ <text class="text_small" transform="translate(125 83)">
+ 'x'
+ </text>
+ </g>
+ <g transform="translate(515 40)">
+ <text class="text_small" transform="translate(0 -18)">
+ T <tspan dx="-3" dy="5" font-size="8">821</tspan> <tspan dx="-2" dy="-5">: DELTE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5)"/>
+ <use xlink:href="#tuple" transform="translate(60 10)"/>
+ <text class="text_small" transform="translate(65 23)">
+ 142
+ </text>
+ <text class="text_small" transform="translate(95 23)" fill="#00f">
+ 821
+ </text>
+ <text class="text_small" transform="translate(125 23)">
+ 'z'
+ </text>
+ <use xlink:href="#tuple" transform="translate(60 40)"/>
+ <text class="text_small" transform="translate(65 53)">
+ 135
+ </text>
+ <text class="text_small" transform="translate(95 53)">
+ 142
+ </text>
+ <text class="text_small" transform="translate(125 53)">
+ 'y'
+ </text>
+ <use xlink:href="#tuple" transform="translate(60 70)"/>
+ <text class="text_small" transform="translate(65 83)">
+ 123
+ </text>
+ <text class="text_small" transform="translate(95 83)">
+ 135
+ </text>
+ <text class="text_small" transform="translate(125 83)">
+ 'x'
+ </text>
+ </g>
+ <g transform="translate(740 110)">
+ <path d="M102-20H-18v60" stroke="black"/>
+ <text font-weight="400" font-size="16" font-family=""Open Sans",sans-serif" fill="#000">
+ Legend
+ </text>
+ <use xlink:href="#tuple" transform="translate(0 10)"/>
+ <text class="text_small" transform="translate(3 23)">
+ xmin
+ </text>
+ <text class="text_small" transform="translate(32 23)">
+ xmax
+ </text>
+ <text class="text_small" transform="translate(65 23)">
+ data
+ </text>
+ </g>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/mvcc-ink.svg b/doc/src/sgml/images/mvcc-ink.svg
new file mode 100644
index 0000000000..f4161b3e79
--- /dev/null
+++ b/doc/src/sgml/images/mvcc-ink.svg
@@ -0,0 +1,398 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="850px"
+ height="150px"
+ viewBox="0 0 850 150"
+ id="svg147"
+ sodipodi:docname="mvcc-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata151">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>MVCC</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="1256"
+ inkscape:window-height="801"
+ id="namedview149"
+ showgrid="false"
+ inkscape:zoom="0.44"
+ inkscape:cx="425"
+ inkscape:cy="75"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg147" />
+ <title
+ id="title2">MVCC</title>
+ <!-- common text classes -->
+ <style
+ type="text/css"
+ id="style4">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_small {font-style:normal;
+ font-weight:normal;
+ font-size:10px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+ <defs
+ id="defs15">
+ <marker
+ id="triangle_1"
+ markerWidth="10"
+ markerHeight="10"
+ refX="5"
+ refY="5"
+ orient="auto"
+ stroke="black"
+ fill="none">
+ <path
+ d="M 0,0 L 10,5 L 0,10"
+ id="path6" />
+ </marker>
+ <g
+ id="tuple">
+ <!-- an rectangle, open at the right side -->
+ <path
+ d="M 80 0 h -80 v 20 h 80 M 29 0 v 20 M 60 0 v 20"
+ stroke="black"
+ id="path9" />
+ <!-- prolong the rectangle -->
+ <path
+ d="M 80 0 h 15 M 80 20 h 15"
+ stroke="black"
+ stroke-dasharray="2 1"
+ id="path11" />
+ </g>
+ <!-- an arrow in the form of a quadratic bezier curve -->
+ <path
+ id="arrow"
+ d="M 0 0 Q 20 -15 40 0"
+ stroke="black"
+ marker-end="url(#triangle_1)" />
+ </defs>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect17" />
+ <!-- set default values -->
+ <g
+ fill="none"
+ id="g145">
+ <!-- start plus first INSERT -->
+ <g
+ transform="translate(20, 40)"
+ id="g35">
+ <text
+ class="text_small"
+ transform="translate(0, -18)"
+ id="text23">T
+ <tspan
+ style="font-size:8px"
+ dx="-3"
+ dy="5"
+ id="tspan19">123</tspan>
+<tspan
+ dx="-2"
+ dy="-5"
+ id="tspan21">: INSERT</tspan>
+</text>
+ <use
+ xlink:href="#arrow"
+ transform="translate(5, 0)"
+ id="use25" />
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 70)"
+ id="use27" />
+ <text
+ class="text_small"
+ transform="translate(65, 83)"
+ style="fill:blue"
+ id="text29">123</text>
+ <text
+ class="text_small"
+ transform="translate(95, 83)"
+ style="fill:blue"
+ id="text31">0</text>
+ <text
+ class="text_small"
+ transform="translate(125, 83)"
+ style="fill:blue"
+ id="text33">'x'</text>
+ </g>
+ <!-- first UPDATE -->
+ <g
+ transform="translate(185, 40)"
+ id="g61">
+ <text
+ class="text_small"
+ transform="translate(0, -18)"
+ id="text41">T
+ <tspan
+ style="font-size:8px"
+ dx="-3"
+ dy="5"
+ id="tspan37">135</tspan>
+<tspan
+ dx="-2"
+ dy="-5"
+ id="tspan39">: UPDATE</tspan>
+</text>
+ <use
+ xlink:href="#arrow"
+ transform="translate(5, 0)"
+ id="use43" />
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 40)"
+ id="use45" />
+ <text
+ class="text_small"
+ transform="translate(65, 53)"
+ style="fill:blue"
+ id="text47">135</text>
+ <text
+ class="text_small"
+ transform="translate(95, 53)"
+ style="fill:blue"
+ id="text49">0</text>
+ <text
+ class="text_small"
+ transform="translate(125, 53)"
+ style="fill:blue"
+ id="text51">'y'</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 70)"
+ id="use53" />
+ <text
+ class="text_small"
+ transform="translate(65, 83)"
+ id="text55">123</text>
+ <text
+ class="text_small"
+ transform="translate(95, 83)"
+ style="fill:blue"
+ id="text57">135</text>
+ <text
+ class="text_small"
+ transform="translate(125, 83)"
+ id="text59">'x'</text>
+ </g>
+ <!-- next UPDATE -->
+ <g
+ transform="translate(350, 40)"
+ id="g95">
+ <text
+ class="text_small"
+ transform="translate(0, -18)"
+ id="text67">T
+ <tspan
+ style="font-size:8px"
+ dx="-3"
+ dy="5"
+ id="tspan63">142</tspan>
+<tspan
+ dx="-2"
+ dy="-5"
+ id="tspan65">: UPDATE</tspan>
+</text>
+ <use
+ xlink:href="#arrow"
+ transform="translate(5, 0)"
+ id="use69" />
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 10)"
+ id="use71" />
+ <text
+ class="text_small"
+ transform="translate(65, 23)"
+ style="fill:blue"
+ id="text73">142</text>
+ <text
+ class="text_small"
+ transform="translate(95, 23)"
+ style="fill:blue"
+ id="text75">0</text>
+ <text
+ class="text_small"
+ transform="translate(125, 23)"
+ style="fill:blue"
+ id="text77">'z'</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 40)"
+ id="use79" />
+ <text
+ class="text_small"
+ transform="translate(65, 53)"
+ id="text81">135</text>
+ <text
+ class="text_small"
+ transform="translate(95, 53)"
+ style="fill:blue"
+ id="text83">142</text>
+ <text
+ class="text_small"
+ transform="translate(125, 53)"
+ id="text85">'y'</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 70)"
+ id="use87" />
+ <text
+ class="text_small"
+ transform="translate(65, 83)"
+ id="text89">123</text>
+ <text
+ class="text_small"
+ transform="translate(95, 83)"
+ id="text91">135</text>
+ <text
+ class="text_small"
+ transform="translate(125, 83)"
+ id="text93">'x'</text>
+ </g>
+ <!-- DELETE -->
+ <g
+ transform="translate(515, 40)"
+ id="g129">
+ <text
+ class="text_small"
+ transform="translate(0, -18)"
+ id="text101">T
+ <tspan
+ style="font-size:8px"
+ dx="-3"
+ dy="5"
+ id="tspan97">821</tspan>
+<tspan
+ dx="-2"
+ dy="-5"
+ id="tspan99">: DELTE</tspan>
+</text>
+ <use
+ xlink:href="#arrow"
+ transform="translate(5, 0)"
+ id="use103" />
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 10)"
+ id="use105" />
+ <text
+ class="text_small"
+ transform="translate(65, 23)"
+ id="text107">142</text>
+ <text
+ class="text_small"
+ transform="translate(95, 23)"
+ style="fill:blue"
+ id="text109">821</text>
+ <text
+ class="text_small"
+ transform="translate(125, 23)"
+ id="text111">'z'</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 40)"
+ id="use113" />
+ <text
+ class="text_small"
+ transform="translate(65, 53)"
+ id="text115">135</text>
+ <text
+ class="text_small"
+ transform="translate(95, 53)"
+ id="text117">142</text>
+ <text
+ class="text_small"
+ transform="translate(125, 53)"
+ id="text119">'y'</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(60, 70)"
+ id="use121" />
+ <text
+ class="text_small"
+ transform="translate(65, 83)"
+ id="text123">123</text>
+ <text
+ class="text_small"
+ transform="translate(95, 83)"
+ id="text125">135</text>
+ <text
+ class="text_small"
+ transform="translate(125, 83)"
+ id="text127">'x'</text>
+ </g>
+ <!-- LEGEND -->
+ <g
+ transform="translate(740, 110)"
+ id="g143">
+ <path
+ d="M 102 -20 h -120 v 60"
+ stroke="black"
+ id="path131" />
+ <text
+ class="text_normal"
+ transform="translate(0, 0)"
+ id="text133">Legend</text>
+ <use
+ xlink:href="#tuple"
+ transform="translate(0, 10)"
+ id="use135" />
+ <text
+ class="text_small"
+ transform="translate(3, 23)"
+ id="text137">xmin</text>
+ <text
+ class="text_small"
+ transform="translate(32, 23)"
+ id="text139">xmax</text>
+ <text
+ class="text_small"
+ transform="translate(65, 23)"
+ id="text141">data</text>
+ </g>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/mvcc-raw.svg b/doc/src/sgml/images/mvcc-raw.svg
new file mode 100644
index 0000000000..0481c4c938
--- /dev/null
+++ b/doc/src/sgml/images/mvcc-raw.svg
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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="850px" height="150px"
+ viewBox="0 0 850 150" >
+
+ <title>MVCC</title>
+
+ <!-- common text classes -->
+ <style type="text/css">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_small {font-style:normal;
+ font-weight:normal;
+ font-size:10px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+ <marker id="triangle_1"
+ markerWidth="10"
+ markerHeight="10"
+ refX="5"
+ refY="5"
+ orient="auto" stroke="black" fill="none">
+ <path d="M 0,0 L 10,5 L 0,10" />
+ </marker>
+
+ <g id="tuple">
+ <!-- an rectangle, open at the right side -->
+ <path d="M 80 0 h -80 v 20 h 80 M 29 0 v 20 M 60 0 v 20" stroke="black"/>
+ <!-- prolong the rectangle -->
+ <path d="M 80 0 h 15 M 80 20 h 15" stroke="black" stroke-dasharray="2 1"/>
+ </g>
+
+ <!-- an arrow in the form of a quadratic bezier curve -->
+ <path id="arrow" d="M 0 0 Q 20 -15 40 0" stroke="black" marker-end="url(#triangle_1)"/>
+
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+
+ <!-- set default values -->
+ <g fill="none">
+
+ <!-- start plus first INSERT -->
+ <g transform="translate(20, 40)">
+
+ <text class="text_small" transform="translate(0, -18)">T
+ <tspan style="font-size:8px" dx="-3" dy="5">123</tspan>
+ <tspan dx="-2" dy="-5">: INSERT</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5, 0)"/>
+
+ <use xlink:href="#tuple" transform="translate(60, 70)"/>
+ <text class="text_small" transform="translate(65, 83)" style="fill:blue">123</text>
+ <text class="text_small" transform="translate(95, 83)" style="fill:blue">0</text>
+ <text class="text_small" transform="translate(125, 83)" style="fill:blue">'x'</text>
+ </g>
+
+ <!-- first UPDATE -->
+ <g transform="translate(185, 40)">
+ <text class="text_small" transform="translate(0, -18)">T
+ <tspan style="font-size:8px" dx="-3" dy="5">135</tspan>
+ <tspan dx="-2" dy="-5">: UPDATE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5, 0)"/>
+
+ <use xlink:href="#tuple" transform="translate(60, 40)"/>
+ <text class="text_small" transform="translate(65, 53)" style="fill:blue">135</text>
+ <text class="text_small" transform="translate(95, 53)" style="fill:blue">0</text>
+ <text class="text_small" transform="translate(125, 53)" style="fill:blue">'y'</text>
+ <use xlink:href="#tuple" transform="translate(60, 70)"/>
+ <text class="text_small" transform="translate(65, 83)">123</text>
+ <text class="text_small" transform="translate(95, 83)" style="fill:blue">135</text>
+ <text class="text_small" transform="translate(125, 83)">'x'</text>
+ </g>
+
+ <!-- next UPDATE -->
+ <g transform="translate(350, 40)">
+ <text class="text_small" transform="translate(0, -18)">T
+ <tspan style="font-size:8px" dx="-3" dy="5">142</tspan>
+ <tspan dx="-2" dy="-5">: UPDATE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5, 0)"/>
+
+ <use xlink:href="#tuple" transform="translate(60, 10)"/>
+ <text class="text_small" transform="translate(65, 23)" style="fill:blue">142</text>
+ <text class="text_small" transform="translate(95, 23)" style="fill:blue">0</text>
+ <text class="text_small" transform="translate(125, 23)" style="fill:blue">'z'</text>
+ <use xlink:href="#tuple" transform="translate(60, 40)"/>
+ <text class="text_small" transform="translate(65, 53)">135</text>
+ <text class="text_small" transform="translate(95, 53)" style="fill:blue">142</text>
+ <text class="text_small" transform="translate(125, 53)">'y'</text>
+ <use xlink:href="#tuple" transform="translate(60, 70)"/>
+ <text class="text_small" transform="translate(65, 83)">123</text>
+ <text class="text_small" transform="translate(95, 83)">135</text>
+ <text class="text_small" transform="translate(125, 83)">'x'</text>
+ </g>
+
+ <!-- DELETE -->
+ <g transform="translate(515, 40)">
+ <text class="text_small" transform="translate(0, -18)">T
+ <tspan style="font-size:8px" dx="-3" dy="5">821</tspan>
+ <tspan dx="-2" dy="-5">: DELTE</tspan>
+ </text>
+ <use xlink:href="#arrow" transform="translate(5, 0)"/>
+
+ <use xlink:href="#tuple" transform="translate(60, 10)"/>
+ <text class="text_small" transform="translate(65, 23)">142</text>
+ <text class="text_small" transform="translate(95, 23)" style="fill:blue">821</text>
+ <text class="text_small" transform="translate(125, 23)">'z'</text>
+ <use xlink:href="#tuple" transform="translate(60, 40)"/>
+ <text class="text_small" transform="translate(65, 53)">135</text>
+ <text class="text_small" transform="translate(95, 53)">142</text>
+ <text class="text_small" transform="translate(125, 53)">'y'</text>
+ <use xlink:href="#tuple" transform="translate(60, 70)"/>
+ <text class="text_small" transform="translate(65, 83)">123</text>
+ <text class="text_small" transform="translate(95, 83)">135</text>
+ <text class="text_small" transform="translate(125, 83)">'x'</text>
+ </g>
+
+ <!-- LEGEND -->
+ <g transform="translate(740, 110)">
+ <path d="M 102 -20 h -120 v 60" stroke="black"/>
+ <text class="text_normal" transform="translate(0, 0)">Legend</text>
+ <use xlink:href="#tuple" transform="translate(0, 10)"/>
+ <text class="text_small" transform="translate(3, 23)">xmin</text>
+ <text class="text_small" transform="translate(32, 23)">xmax</text>
+ <text class="text_small" transform="translate(65, 23)">data</text>
+ </g>
+
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/ram-proc-file-ink-svgo.svg b/doc/src/sgml/images/ram-proc-file-ink-svgo.svg
new file mode 100644
index 0000000000..723d67bd8d
--- /dev/null
+++ b/doc/src/sgml/images/ram-proc-file-ink-svgo.svg
@@ -0,0 +1,285 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="900" height="600" viewBox="0 0 900 600">
+ <title>
+ PG Overall Server Architecture
+ </title>
+ <style>
+ .text_big{font-style:normal}.text_big,.text_comment,.text_normal,.text_small{font-weight:400;font-family:"Open Sans",sans-serif;fill:#000}.text_normal,.text_small{font-style:normal}.text_small{font-size:12px}.text_normal{font-size:16px}.text_big{font-size:24px}.text_comment{font-style:italic;font-size:16px}
+ </style>
+ <defs>
+ <symbol id="note_200x20" stroke="black" fill="lightyellow">
+ <title>
+ UML Note (200 x 20 px)
+ </title>
+ <path d="M200 10v10H0V0h190v10h10L190 0"/>
+ </symbol>
+ <symbol id="note_250x20" stroke="black" fill="lightyellow">
+ <title>
+ UML Note (250 x 20 px)
+ </title>
+ <path d="M250 10v10H0V0h240v10h10L240 0"/>
+ </symbol>
+ <symbol id="note_100x35" stroke="black" fill="lightyellow">
+ <title>
+ UML Note (100 x 35 px)
+ </title>
+ <path d="M100 10v25H0V0h90v10h10L90 0"/>
+ </symbol>
+ <symbol id="note_170x50" stroke="black" fill="lightyellow">
+ <title>
+ UML Note (170 x 50 px)
+ </title>
+ <path d="M170 10v40H0V0h160v10h10L160 0"/>
+ </symbol>
+ <symbol id="state_300x120">
+ <title>
+ UML State (300x120)
+ </title>
+ <rect width="300" height="120" rx="10" stroke="blue" fill="none"/>
+ </symbol>
+ <symbol id="state_350x120">
+ <title>
+ UML State (350x120)
+ </title>
+ <rect width="350" height="120" rx="10" stroke="blue" fill="none"/>
+ </symbol>
+ <symbol id="disc" stroke="blue" fill="none">
+ <title>
+ Disc
+ </title>
+ <ellipse cx="51" cy="13" rx="50" ry="12"/>
+ <path d="M1 13v60"/>
+ <path d="M101 13v60"/>
+ <path d="M1 73a50 12 0 00100 0"/>
+ </symbol>
+ <symbol id="laptop" stroke="black" fill="none">
+ <title>
+ Laptop
+ </title>
+ <path d="M20 40V0h54v40l15 15H5l15-15h54"/>
+ <path d="M23 3h48v34H23z"/>
+ <path d="M30 10h20"/>
+ <path d="M30 15h25"/>
+ <path d="M30 20h10"/>
+ <path d="M30 30h20"/>
+ <path d="M25 50h45l2 2H22z"/>
+ </symbol>
+ <marker id="arrowhead_start" markerWidth="10" markerHeight="10" refX="0" refY="3" orient="auto">
+ <path d="M6 0L0 3l6 3" stroke="black" fill="none"/>
+ </marker>
+ <marker id="arrowhead_end" markerWidth="10" markerHeight="10" refX="6" refY="3" orient="auto">
+ <path d="M0 0l6 3-6 3" stroke="black" fill="none"/>
+ </marker>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="15" y="40" class="text_big">
+ Client
+ </text>
+ <text x="140" y="40" class="text_big">
+ Server
+ </text>
+ <use xlink:href="#laptop" x="5" y="210"/>
+ <g transform="translate(130 70)">
+ <use xlink:href="#state_350x120"/>
+ <text x="5" y="20" class="text_normal">
+ maintenance_work_mem (per connection)
+ </text>
+ <text x="5" y="45" class="text_normal">
+ work_mem (per query operation)
+ </text>
+ <text x="5" y="70" class="text_normal">
+ autovacuum_work_mem (per worker process)
+ </text>
+ <text x="5" y="95" class="text_normal">
+ temp_buffer (per connection)
+ </text>
+ <text x="5" y="110" class="text_normal">
+ ...
+ </text>
+ <use xlink:href="#note_200x20" x="140" y="-15"/>
+ <text x="150" class="text_comment">
+ Individual Memory
+ </text>
+ </g>
+ <g transform="translate(520 70)">
+ <use xlink:href="#state_300x120"/>
+ <text x="10" y="30" class="text_normal">
+ shared_buffers (heap and index)
+ </text>
+ <text x="10" y="70" class="text_normal">
+ wal_buffers (WAL records)
+ </text>
+ <text x="10" y="100" class="text_normal">
+ ...
+ </text>
+ <use xlink:href="#note_250x20" x="40" y="-15"/>
+ <text x="50" class="text_comment">
+ Shared Memory (per Instance)
+ </text>
+ </g>
+ <path stroke="blue" fill="none" d="M180 215h250v30H180z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(180 215)">
+ Postmaster
+ </text>
+ <path d="M90 230h75" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(140 230)">
+ <circle r="8" stroke="black" fill="lightyellow"/>
+ <text x="-4" y="4" class="text_small">
+ 1
+ </text>
+ </g>
+ <path stroke="blue" fill="none" d="M150 315h370v30H150z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(150 315)">
+ Backend processes (one per connection)
+ </text>
+ <path d="M155 315v-5h370v30h-5" stroke="blue" fill="none"/>
+ <path d="M160 310v-5h370v30h-5" stroke="blue" fill="none"/>
+ <path d="M90 240l63 63" stroke="black" fill="none" marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(140 290)">
+ <circle r="8" stroke="black" fill="lightyellow"/>
+ <text x="-4" y="4" class="text_small">
+ 3
+ </text>
+ </g>
+ <path d="M360 250v50" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(180 255)">
+ <use xlink:href="#note_250x20"/>
+ <text x="10" y="15" class="text_comment">
+ Creates backend processes
+ </text>
+ </g>
+ <g transform="translate(360 281)">
+ <circle r="8" stroke="black" fill="lightyellow"/>
+ <text x="-4" y="4" class="text_small">
+ 2
+ </text>
+ </g>
+ <path d="M460 300V200" stroke="black" fill="none" marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <path d="M498 300V95h30" stroke="black" fill="none" marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <path d="M508 300V135h20" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path stroke="blue" fill="none" d="M550 220h120v30H550z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(550 220)">
+ WAL Writer
+ </text>
+ <path d="M590 150v65" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M590 255v230" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path stroke="blue" fill="none" d="M610 340h140v30H610z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(610 340)">
+ Checkpointer
+ </text>
+ <path d="M740 110v220" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M605 355H475v130" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M700 330V150" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(570 330)">
+ <use xlink:href="#note_100x35" x="50" y="-50"/>
+ <text x="60" y="-35" class="text_comment">
+ Checkpoint
+ </text>
+ <text x="60" y="-20" class="text_comment">
+ Record
+ </text>
+ </g>
+ <path stroke="blue" fill="none" d="M610 380h180v30H610z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(610 380)">
+ Background Writer
+ </text>
+ <path d="M770 110v260" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M605 395H485v90" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path stroke="blue" fill="none" d="M610 420h180v30H610z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(610 420)">
+ WAL Archiver
+ </text>
+ <path d="M620 485l30-30" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M690 455l30 30" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path stroke="blue" fill="none" d="M135 380h120v30H135z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(135 380)">
+ Autovacuum
+ </text>
+ <path d="M140 380v-5h120v30h-5" stroke="blue" fill="none"/>
+ <path d="M145 375v-5h120v30h-5" stroke="blue" fill="none"/>
+ <path stroke="blue" fill="none" d="M135 430h120v30H135z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(135 430)">
+ Logger
+ </text>
+ <path stroke="blue" fill="none" d="M290 370h140v30H290z"/>
+ <text x="10" y="20" class="text_normal" transform="translate(290 370)">
+ Stats Collector
+ </text>
+ <g transform="translate(145 490)">
+ <use xlink:href="#disc"/>
+ <text x="35" y="45" class="text_normal">
+ Log
+ </text>
+ <text x="20" y="60" class="text_small">
+ text lines,
+ </text>
+ <text x="20" y="75" class="text_small">
+ sequential
+ </text>
+ </g>
+ <path d="M195 465v20" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(410 490)">
+ <use xlink:href="#disc"/>
+ <text x="10" y="40" class="text_normal">
+ Heap and
+ </text>
+ <text x="25" y="55" class="text_normal">
+ Index
+ </text>
+ <text x="15" y="70" class="text_small">
+ binary blocks,
+ </text>
+ <text x="30" y="80" class="text_small">
+ random
+ </text>
+ </g>
+ <path d="M450 485V350" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(295 420)">
+ <use xlink:href="#note_170x50"/>
+ <text x="5" y="15" class="text_comment">
+ Read heap and index
+ </text>
+ <text x="5" y="30" class="text_comment">
+ pages and transfer
+ </text>
+ <text x="5" y="45" class="text_comment">
+ them to shared_buffers
+ </text>
+ </g>
+ <g transform="translate(550 490)">
+ <use xlink:href="#disc"/>
+ <text x="30" y="45" class="text_normal">
+ WAL
+ </text>
+ <text x="10" y="60" class="text_small">
+ binary records,
+ </text>
+ <text x="20" y="75" class="text_small">
+ sequential
+ </text>
+ </g>
+ <g transform="translate(690 490)">
+ <use xlink:href="#disc"/>
+ <text x="16" y="45" class="text_normal">
+ Archived
+ </text>
+ <text x="36" y="60" class="text_normal">
+ WAL
+ </text>
+ </g>
+ <path d="M110 20v550" stroke="black" fill="none"/>
+ <g transform="rotate(90 -33.5 156.5)">
+ <use xlink:href="#note_200x20"/>
+ <text class="text_comment" x="10" y="15">
+ Via TCP/IP or socket
+ </text>
+ </g>
+ <text class="text_big" x="95" transform="rotate(90 425 425)">
+ RAM
+ </text>
+ <text class="text_big" x="250" transform="rotate(90 425 425)">
+ PROCESSES
+ </text>
+ <text class="text_big" x="500" transform="rotate(90 425 425)">
+ FILES
+ </text>
+</svg>
diff --git a/doc/src/sgml/images/ram-proc-file-ink.svg b/doc/src/sgml/images/ram-proc-file-ink.svg
new file mode 100644
index 0000000000..4490bf51e1
--- /dev/null
+++ b/doc/src/sgml/images/ram-proc-file-ink.svg
@@ -0,0 +1,841 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
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+ x="5"
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+ <!-- individual memory -->
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+ y="0"
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+ id="text82">maintenance_work_mem (per connection)</text>
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+ class="text_normal"
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+ <text
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+ <text
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+ stroke="black"
+ fill="none"
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+ id="path220" />
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+ <!-- -->
+ <!-- files -->
+ <!-- -->
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+ transform="translate(145, 490)"
+ id="g244">
+ <use
+ xlink:href="#disc"
+ id="use236" />
+ <text
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+ y="45"
+ class="text_normal"
+ id="text238">Log</text>
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+ y="15"
+ class="text_comment"
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+ <text
+ x="5"
+ y="30"
+ class="text_comment"
+ id="text266">pages and transfer</text>
+ <text
+ x="5"
+ y="45"
+ class="text_comment"
+ id="text268">them to shared_buffers</text>
+ </g>
+ <g
+ transform="translate(550, 490)"
+ id="g280">
+ <use
+ xlink:href="#disc"
+ id="use272" />
+ <text
+ x="30"
+ y="45"
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+ <text
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+ y="60"
+ class="text_small"
+ id="text276">binary records,</text>
+ <text
+ x="20"
+ y="75"
+ class="text_small"
+ id="text278">sequential</text>
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+ <g
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+ id="g288">
+ <use
+ xlink:href="#disc"
+ id="use282" />
+ <text
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+ y="45"
+ class="text_normal"
+ id="text284">Archived</text>
+ <text
+ x="36"
+ y="60"
+ class="text_normal"
+ id="text286">WAL</text>
+ </g>
+ <!-- boarder between client and server side -->
+ <path
+ d="M 110,20 v 550"
+ stroke="black"
+ fill="none"
+ id="path290" />
+ <g
+ transform="translate(123, 190) rotate(90)"
+ id="g296">
+ <use
+ xlink:href="#note_200x20"
+ id="use292" />
+ <text
+ class="text_comment"
+ x="10"
+ y="15"
+ id="text294">Via TCP/IP or socket</text>
+ </g>
+ <!-- right side -->
+ <g
+ transform="translate(850, 0) rotate(90)"
+ id="g304">
+ <text
+ class="text_big"
+ x="95"
+ id="text298">RAM</text>
+ <text
+ class="text_big"
+ x="250"
+ id="text300">PROCESSES</text>
+ <text
+ class="text_big"
+ x="500"
+ id="text302">FILES</text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/ram-proc-file-raw.svg b/doc/src/sgml/images/ram-proc-file-raw.svg
new file mode 100644
index 0000000000..aec5811c54
--- /dev/null
+++ b/doc/src/sgml/images/ram-proc-file-raw.svg
@@ -0,0 +1,301 @@
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+ xmlns:xlink="http://www.w3.org/1999/xlink"
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+ width="900px" height="600px"
+ viewBox="0 0 900 600">
+
+ <title>PG Overall Server Architecture</title>
+
+ <style type="text/css">
+ .text_small {font-style:normal;
+ font-weight:normal;
+ font-size:12px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_comment {font-style:italic;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+
+ <!-- Some notes in different sizes -->
+ <symbol id="note_200x20" stroke="black" fill="lightyellow">
+ <title>UML Note (200 x 20 px)</title>
+ <path d="M 200,10 v 10 h -200 v -20 h 190 v 10 h 10 l -10,-10" />
+ </symbol>
+ <symbol id="note_250x20" stroke="black" fill="lightyellow">
+ <title>UML Note (250 x 20 px)</title>
+ <path d="M 250,10 v 10 h -250 v -20 h 240 v 10 h 10 l -10,-10" />
+ </symbol>
+ <symbol id="note_100x35" stroke="black" fill="lightyellow">
+ <title>UML Note (100 x 35 px)</title>
+ <path d="M 100,10 v 25 h -100 v -35 h 90 v 10 h 10 l -10,-10" />
+ </symbol>
+ <symbol id="note_170x50" stroke="black" fill="lightyellow">
+ <title>UML Note (170 x 50 px)</title>
+ <path d="M 170,10 v 40 h -170 v -50 h 160 v 10 h 10 l -10,-10" />
+ </symbol>
+
+ <!-- UML states (used for buffers) -->
+ <symbol id="state_300x120">
+ <title>UML State (300x120)</title>
+ <rect x="0" y="0" width="300" height="120" rx="10" stroke="blue" fill="none"/>
+ </symbol>
+ <symbol id="state_350x120">
+ <title>UML State (350x120)</title>
+ <rect x="0" y="0" width="350" height="120" rx="10" stroke="blue" fill="none"/>
+ </symbol>
+
+ <!-- Discs -->
+ <symbol id="disc" stroke="blue" fill="none" >
+ <title>Disc</title>
+ <ellipse cx="51" cy="13" rx="50" ry="12" /> <!-- top -->
+ <path d="M 1,13 v 60" /> <!-- left -->
+ <path d="M 101,13 v 60" /> <!-- right -->
+ <path d="M 1,73 A 50, 12, 0, 0, 0, 101,73" /> <!-- bottom -->
+ </symbol>
+
+ <!-- Laptop -->
+ <symbol id="laptop" stroke="black" fill="none" >
+ <title>Laptop</title>
+ <path d="M 20,40 v -40 h 54 v 40 l 15,15 h -84 l 15,-15 h 54" />
+ <rect x="23" y="3" width="48" height="34" />
+ <!-- symbolize some lines -->
+ <path d="M 30,10 h 20" />
+ <path d="M 30,15 h 25" />
+ <path d="M 30,20 h 10" />
+ <path d="M 30,30 h 20" />
+ <!-- symbolize keyboard -->
+ <path d="M 25,50 h 45 l 2,2 h -50 z " />
+ </symbol>
+
+ <!-- marker start/end -->
+ <marker id="arrowhead_start"
+ markerWidth="10"
+ markerHeight="10"
+ refX="0"
+ refY="3"
+ orient="auto">
+ <path d="M 6,0 l -6,3 l 6,3" stroke="black" fill="none" />
+ </marker>
+ <marker id="arrowhead_end"
+ markerWidth="10"
+ markerHeight="10"
+ refX="6"
+ refY="3"
+ orient="auto">
+ <path d="M 0,0 l 6,3 l -6,3" stroke="black" fill="none" />
+ </marker>
+
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+
+ <!-- caption, client side -->
+ <text x="15" y="40" class="text_big">Client</text>
+ <text x="140" y="40" class="text_big">Server</text>
+ <use xlink:href="#laptop" x="5" y="210" />
+
+
+ <!-- individual memory -->
+ <g transform="translate(130, 70)">
+ <use xlink:href="#state_350x120" x="0" y="0" />
+ <text x="5" y="20" class="text_normal">maintenance_work_mem (per connection)</text>
+ <text x="5" y="45" class="text_normal">work_mem (per query operation)</text>
+ <text x="5" y="70" class="text_normal">autovacuum_work_mem (per worker process)</text>
+ <text x="5" y="95" class="text_normal">temp_buffer (per connection)</text>
+ <text x="5" y="110" class="text_normal">...</text>
+ <use xlink:href="#note_200x20" x="140" y="-15" />
+ <text x="150" y="0" class="text_comment">Individual Memory</text>
+ </g>
+
+ <!-- shared memory -->
+ <g transform="translate(520, 70)">
+ <use xlink:href="#state_300x120" x="0" y="0" />
+ <text x="10" y="30" class="text_normal">shared_buffers (heap and index)</text>
+ <text x="10" y="70" class="text_normal">wal_buffers (WAL records)</text>
+ <text x="10" y="100" class="text_normal">...</text>
+ <use xlink:href="#note_250x20" x="40" y="-15" />
+ <text x="50" y="0" class="text_comment">Shared Memory (per Instance)</text>
+ </g>
+
+ <!-- postmaster -->
+ <g transform="translate(180, 215)">
+ <rect width="250" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Postmaster</text>
+ </g>
+ <path d="M 90,230 h 75" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(140, 230)">
+ <circle r="8" stroke="black" fill="lightyellow" />
+ <text x="-4" y="4" class="text_small">1</text>
+ </g>
+
+ <!-- backend processes -->
+ <g transform="translate(150, 315)">
+ <rect width="370" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Backend processes (one per connection)</text>
+ <path d="M 5,0 v -5 h 370 v 30 h -5" stroke="blue" fill="none" />
+ <path d="M 10,-5 v -5 h 370 v 30 h -5" stroke="blue" fill="none" />
+ </g>
+
+ <path d="M 90,240 153,303" stroke="black" fill="none"
+ marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(140, 290)">
+ <circle r="8" stroke="black" fill="lightyellow" />
+ <text x="-4" y="4" class="text_small">3</text>
+ </g>
+
+ <!-- connection between postmaster and backend processes -->
+ <path d="M 360,250 v 50" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(180, 255)">
+ <use xlink:href="#note_250x20" />
+ <text x="10" y="15" class="text_comment">Creates backend processes</text>
+ </g>
+ <g transform="translate(360, 281)">
+ <circle r="8" stroke="black" fill="lightyellow" />
+ <text x="-4" y="4" class="text_small">2</text>
+ </g>
+
+ <!-- backend process' access to individual memory -->
+ <path d="M 460,300 v -100" stroke="black" fill="none"
+ marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <!-- its access to shared buffers and WAL buffers -->
+ <path d="M 498,300 v -205 h 30" stroke="black" fill="none"
+ marker-start="url(#arrowhead_start)" marker-end="url(#arrowhead_end)"/>
+ <path d="M 508,300 v -165 h 20" stroke="black" fill="none"
+ marker-end="url(#arrowhead_end)"/>
+
+ <!-- WAL writer -->
+ <g transform="translate(550, 220)">
+ <rect width="120" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">WAL Writer</text>
+ </g>
+ <path d="M 590,150 v 65" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M 590,255 v 230" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+
+ <!-- Checkpoiner -->
+ <g transform="translate(610, 340)">
+ <rect width="140" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Checkpointer</text>
+ </g>
+ <path d="M 740,110 v 220" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M 605,355 h -130 v 130" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M 700,330 v -180" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <g transform="translate(570, 330)">
+ <use xlink:href="#note_100x35" x="50" y="-50" />
+ <text x="60" y="-35" class="text_comment">Checkpoint</text>
+ <text x="60" y="-20" class="text_comment">Record</text>
+ </g>
+
+ <!-- BG writer -->
+ <g transform="translate(610, 380)">
+ <rect width="180" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Background Writer</text>
+ </g>
+ <path d="M 770,110 v 260" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M 605,395 h -120 v 90" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+
+ <!-- Archiver -->
+ <g transform="translate(610, 420)">
+ <rect width="180" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">WAL Archiver</text>
+ </g>
+ <path d="M 620,485 l 30,-30" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+ <path d="M 690,455 l 30, 30" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+
+ <!-- Vacuum -->
+ <g transform="translate(135, 380)">
+ <rect width="120" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Autovacuum</text>
+ <path d="M 5,0 v -5 h 120 v 30 h -5" stroke="blue" fill="none" />
+ <path d="M 10,-5 v -5 h 120 v 30 h -5" stroke="blue" fill="none" />
+ </g>
+
+ <!-- Log Writer -->
+ <g transform="translate(135, 430)">
+ <rect width="120" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Logger</text>
+ </g>
+
+ <!-- Stats Collector -->
+ <g transform="translate(290, 370)">
+ <rect width="140" height="30" stroke="blue" fill="none" />
+ <text x="10" y="20" class="text_normal">Stats Collector</text>
+ </g>
+
+ <!-- -->
+ <!-- files -->
+ <!-- -->
+ <g transform="translate(145, 490)">
+ <use xlink:href="#disc" />
+ <text x="35" y="45" class="text_normal">Log</text>
+ <text x="20" y="60" class="text_small">text lines,</text>
+ <text x="20" y="75" class="text_small">sequential</text>
+ </g>
+ <path d="M 195,465 v 20" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+
+ <g transform="translate(410, 490)">
+ <use xlink:href="#disc" />
+ <text x="10" y="40" class="text_normal">Heap and</text>
+ <text x="25" y="55" class="text_normal">Index</text>
+ <text x="15" y="70" class="text_small">binary blocks,</text>
+ <text x="30" y="80" class="text_small">random</text>
+ </g>
+ <path d="M 450,485 v -135" stroke="black" fill="none" marker-end="url(#arrowhead_end)"/>
+
+ <g transform="translate(295, 420)">
+ <use xlink:href="#note_170x50" />
+ <text x="5" y="15" class="text_comment">Read heap and index</text>
+ <text x="5" y="30" class="text_comment">pages and transfer</text>
+ <text x="5" y="45" class="text_comment">them to shared_buffers</text>
+ </g>
+
+ <g transform="translate(550, 490)">
+ <use xlink:href="#disc" />
+ <text x="30" y="45" class="text_normal">WAL</text>
+ <text x="10" y="60" class="text_small">binary records,</text>
+ <text x="20" y="75" class="text_small">sequential</text>
+ </g>
+
+ <g transform="translate(690, 490)">
+ <use xlink:href="#disc" />
+ <text x="16" y="45" class="text_normal">Archived</text>
+ <text x="36" y="60" class="text_normal">WAL</text>
+ </g>
+
+ <!-- boarder between client and server side -->
+ <path d="M 110,20 v 550" stroke="black" fill="none" />
+ <g transform="translate(123, 190) rotate(90)">
+ <use xlink:href="#note_200x20" />
+ <text class="text_comment" x="10" y ="15">Via TCP/IP or socket</text>
+ </g>
+
+ <!-- right side -->
+ <g transform="translate(850, 0) rotate(90)">
+ <text class="text_big" x="95">RAM</text>
+ <text class="text_big" x="250">PROCESSES</text>
+ <text class="text_big" x="500">FILES</text>
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/images/wraparound-ink-svgo.svg b/doc/src/sgml/images/wraparound-ink-svgo.svg
new file mode 100644
index 0000000000..9882d2be23
--- /dev/null
+++ b/doc/src/sgml/images/wraparound-ink-svgo.svg
@@ -0,0 +1,40 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="850" height="280" viewBox="0 0 850 280">
+ <title>
+ Cyclic usage of XIDs
+ </title>
+ <style>
+ .text_normal{font-style:normal;font-weight:400;font-size:16px;font-family:"Open Sans",sans-serif;fill:#000}
+ </style>
+ <defs>
+ <path id="Path_080" d="M-80 0A80 80 0 1180 0 80 80 0 11-80 0"/>
+ <path id="Path_095" d="M-95 0A95 95 0 1195 0 95 95 0 11-95 0"/>
+ <path id="Path_120" d="M-120 0a120 120 0 11240 0 120 120 0 11-240 0"/>
+ </defs>
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none"/>
+ <text x="370" y="40" font-weight="400" font-size="24" font-family=""Open Sans",sans-serif,Helvetica">
+ Cyclic usage of XIDs modulo 2 <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan>
+ </text>
+ <g fill="none" transform="translate(170 150)">
+ <circle r="100" stroke="blue"/>
+ <text class="text_normal">
+ <textPath xlink:href="#Path_095" startOffset=".5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="0%">(0)</textPath> <textPath xlink:href="#Path_080" startOffset="1%" stroke="blue" stroke-width=".5"> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath> <textPath xlink:href="#Path_095" startOffset="30.5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="30%">(1)</textPath> <textPath xlink:href="#Path_095" startOffset="45.5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="45%">(2)</textPath> <textPath xlink:href="#Path_095" startOffset="50.5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="50%">(3)</textPath> <textPath xlink:href="#Path_095" startOffset="62.5%">|</textPath> <textPath xlink:href="#Path_120" startOffset="62%">(4)</textPath>
+ </text>
+ </g>
+ <g class="text_normal">
+ <text transform="translate(400 130)">
+ 0: 0 .. 2 <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan> - 1
+ </text>
+ <text y="25" transform="translate(400 130)">
+ 1: oldest <tspan font-weight="700">active</tspan> xid (pg_stat_activity.backend_xmin)
+ </text>
+ <text y="50" transform="translate(400 130)">
+ 2: xmin of one row version
+ </text>
+ <text y="75" transform="translate(400 130)">
+ 3: xmax of the same row version
+ </text>
+ <text y="100" transform="translate(400 130)">
+ 4: jungest xid (txid_current)
+ </text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/wraparound-ink.svg b/doc/src/sgml/images/wraparound-ink.svg
new file mode 100644
index 0000000000..a9c51f4e43
--- /dev/null
+++ b/doc/src/sgml/images/wraparound-ink.svg
@@ -0,0 +1,198 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+ xmlns:dc="http://purl.org/dc/elements/1.1/"
+ xmlns:cc="http://creativecommons.org/ns#"
+ xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+ xmlns:svg="http://www.w3.org/2000/svg"
+ xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+ xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+ version="1.1"
+ width="850px"
+ height="280px"
+ viewBox="0 0 850 280"
+ id="svg1072"
+ sodipodi:docname="wraparound-ink.svg"
+ inkscape:version="0.92.3 (2405546, 2018-03-11)">
+ <metadata
+ id="metadata1076">
+ <rdf:RDF>
+ <cc:Work
+ rdf:about="">
+ <dc:format>image/svg+xml</dc:format>
+ <dc:type
+ rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+ <dc:title>Cyclic usage of XIDs</dc:title>
+ </cc:Work>
+ </rdf:RDF>
+ </metadata>
+ <sodipodi:namedview
+ pagecolor="#ffffff"
+ bordercolor="#666666"
+ borderopacity="1"
+ objecttolerance="10"
+ gridtolerance="10"
+ guidetolerance="10"
+ inkscape:pageopacity="0"
+ inkscape:pageshadow="2"
+ inkscape:window-width="1193"
+ inkscape:window-height="788"
+ id="namedview1074"
+ showgrid="false"
+ inkscape:zoom="0.44"
+ inkscape:cx="425"
+ inkscape:cy="140"
+ inkscape:window-x="61"
+ inkscape:window-y="27"
+ inkscape:window-maximized="0"
+ inkscape:current-layer="svg1072" />
+ <title
+ id="title1009">Cyclic usage of XIDs</title>
+ <!-- common text classes -->
+ <style
+ type="text/css"
+ id="style1011">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif, Helvetica;
+ fill:black;
+ }
+ </style>
+ <defs
+ id="defs1016">
+ <!-- SVG 1.x supports <texPath> only in combination with <path>. SVG 2 allows the
+ combination with every shape element, especially with <circle>. As of 2020
+ we must restrict ourselves to SVG 1. Therefore: we simulate a circle by
+ defining two arcs, each of them is a half-circle.
+ -->
+ <path
+ id="Path_080"
+ d="m -80 0 a 80 80 0 1 1 160 0 a 80 80 0 1 1 -160 0" />
+ <path
+ id="Path_095"
+ d="m -95 0 a 95 95 0 1 1 190 0 a 95 95 0 1 1 -190 0" />
+ <path
+ id="Path_120"
+ d="m -120 0 a 120 120 0 1 1 240 0 a 120 120 0 1 1 -240 0" />
+ </defs>
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect
+ x="1"
+ y="1"
+ rx="5"
+ width="99%"
+ height="99%"
+ stroke="black"
+ fill="none"
+ id="rect1018" />
+ <text
+ class="text_big"
+ x="370"
+ y="40"
+ id="text1024">Cyclic usage of XIDs modulo 2
+ <tspan
+ dy="-5"
+ id="tspan1020">^</tspan>
+<tspan
+ dy="5"
+ id="tspan1022">32</tspan>
+</text>
+ <!-- set default values -->
+ <g
+ fill="none"
+ transform="translate(170 150)"
+ id="g1052">
+ <circle
+ r="100"
+ stroke="blue"
+ id="circle1026" />
+ <text
+ class="text_normal"
+ id="text1050">
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="0.5%"
+ id="textPath1028">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="00%"
+ id="textPath1030">(0)</textPath>
+ <textPath
+ xlink:href="#Path_080"
+ startOffset="01%"
+ stroke="blue"
+ stroke-width="0.5px"
+ id="textPath1032">
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath>
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="30.5%"
+ id="textPath1034">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="30%"
+ id="textPath1036">(1)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="45.5%"
+ id="textPath1038">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="45%"
+ id="textPath1040">(2)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="50.5%"
+ id="textPath1042">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="50%"
+ id="textPath1044">(3)</textPath>
+ <textPath
+ xlink:href="#Path_095"
+ startOffset="62.5%"
+ id="textPath1046">|</textPath>
+ <textPath
+ xlink:href="#Path_120"
+ startOffset="62%"
+ id="textPath1048">(4)</textPath>
+ </text>
+ </g>
+ <g
+ class="text_normal"
+ transform="translate(400 130)"
+ id="g1070">
+ <text
+ id="text1058">0: 0 .. 2 <tspan
+ dy="-5"
+ id="tspan1054">^</tspan>
+<tspan
+ dy="5"
+ id="tspan1056">32</tspan>
+ - 1</text>
+ <text
+ y="25"
+ id="text1062">1: oldest <tspan
+ style="font-weight:bold"
+ id="tspan1060">active</tspan>
+ xid (pg_stat_activity.backend_xmin)</text>
+ <text
+ y="50"
+ id="text1064">2: xmin of one row version</text>
+ <text
+ y="75"
+ id="text1066">3: xmax of the same row version</text>
+ <text
+ y="100"
+ id="text1068">4: jungest xid (txid_current)</text>
+ </g>
+</svg>
diff --git a/doc/src/sgml/images/wraparound-raw.svg b/doc/src/sgml/images/wraparound-raw.svg
new file mode 100644
index 0000000000..9406f52970
--- /dev/null
+++ b/doc/src/sgml/images/wraparound-raw.svg
@@ -0,0 +1,79 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+ xmlns:xlink="http://www.w3.org/1999/xlink"
+ version="1.1"
+ width="850px" height="280px"
+ viewBox="0 0 850 280" >
+
+ <title>Cyclic usage of XIDs</title>
+
+ <!-- common text classes -->
+ <style type="text/css">
+ .text_normal {font-style:normal;
+ font-weight:normal;
+ font-size:16px;
+ font-family:"Open Sans", sans-serif;
+ fill:black;
+ }
+ .text_big {font-style:normal;
+ font-weight:normal;
+ font-size:24px;
+ font-family:"Open Sans", sans-serif, Helvetica;
+ fill:black;
+ }
+ </style>
+
+ <defs>
+ <!-- SVG 1.x supports <texPath> only in combination with <path>. SVG 2 allows the
+ combination with every shape element, especially with <circle>. As of 2020
+ we must restrict ourselves to SVG 1. Therefore: we simulate a circle by
+ defining two arcs, each of them is a half-circle.
+ -->
+ <path id="Path_080" d="m -80 0
+ a 80 80 0 1 1 160 0
+ a 80 80 0 1 1 -160 0"/>
+ <path id="Path_095" d="m -95 0
+ a 95 95 0 1 1 190 0
+ a 95 95 0 1 1 -190 0"/>
+ <path id="Path_120" d="m -120 0
+ a 120 120 0 1 1 240 0
+ a 120 120 0 1 1 -240 0"/>
+ </defs>
+
+
+ <!-- start of rendering area -->
+ <!-- enclosing rectangle -->
+ <rect x="1" y="1" rx="5" width="99%" height="99%" stroke="black" fill="none" />
+ <text class="text_big" x="370" y="40">Cyclic usage of XIDs modulo 2
+ <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan></text>
+
+ <!-- set default values -->
+ <g fill="none" transform="translate(170 150)">
+
+ <circle r="100" stroke="blue" />
+
+ <text class="text_normal">
+ <textPath xlink:href="#Path_095" startOffset="0.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="00%" >(0)</textPath>
+ <textPath xlink:href="#Path_080" startOffset="01%" stroke="blue" stroke-width="0.5px" >
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ></textPath>
+ <textPath xlink:href="#Path_095" startOffset="30.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="30%" >(1)</textPath>
+ <textPath xlink:href="#Path_095" startOffset="45.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="45%" >(2)</textPath>
+ <textPath xlink:href="#Path_095" startOffset="50.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="50%" >(3)</textPath>
+ <textPath xlink:href="#Path_095" startOffset="62.5%" >|</textPath>
+ <textPath xlink:href="#Path_120" startOffset="62%" >(4)</textPath>
+ </text>
+ </g>
+
+ <g class="text_normal" transform="translate(400 130)">
+ <text>0: 0 .. 2 <tspan dy="-5">^</tspan> <tspan dy="5">32</tspan> - 1</text>
+ <text y="25">1: oldest <tspan style="font-weight:bold">active</tspan> xid (pg_stat_activity.backend_xmin)</text>
+ <text y="50">2: xmin of one row version</text>
+ <text y="75">3: xmax of the same row version</text>
+ <text y="100">4: jungest xid (txid_current)</text>
+ </g>
+
+</svg>
diff --git a/doc/src/sgml/postgres.sgml b/doc/src/sgml/postgres.sgml
index 730d5fdc34..e9e9f9495f 100644
--- a/doc/src/sgml/postgres.sgml
+++ b/doc/src/sgml/postgres.sgml
@@ -248,6 +248,7 @@ break is not needed in a wider output rendering.
</para>
</partintro>
+ &architecture;
&arch-dev;
&catalogs;
&protocol;
diff --git a/doc/src/sgml/start.sgml b/doc/src/sgml/start.sgml
index 9bb5c1a6d5..abb61445f2 100644
--- a/doc/src/sgml/start.sgml
+++ b/doc/src/sgml/start.sgml
@@ -53,7 +53,7 @@
<sect1 id="tutorial-arch">
- <title>Architectural Fundamentals</title>
+ <title>Client/Server Model</title>
<para>
Before we proceed, you should understand the basic
@@ -68,34 +68,52 @@
client/server model. A <productname>PostgreSQL</productname>
session consists of the following cooperating processes
(programs):
+ </para>
- <itemizedlist>
- <listitem>
- <para>
- A server process, which manages the database files, accepts
- connections to the database from client applications, and
- performs database actions on behalf of the clients. The
- database server program is called
- <filename>postgres</filename>.
- <indexterm><primary>postgres</primary></indexterm>
- </para>
- </listitem>
+ <itemizedlist>
+ <listitem>
+ <para>
+ A process at the server site with the name
+ <glossterm linkend="glossary-postmaster">Postmaster</glossterm>.
+ <indexterm><primary>postgres</primary></indexterm>
+ <indexterm><primary>postmaster</primary></indexterm>
+ It accepts connection requests from client applications, starts
+ (<quote>forks</quote>) a new <glossterm linkend="glossary-backend">
+ Backend</glossterm> process for each of them, and passes
+ the connection to it. From that point on, the client and the new
+ Backend process communicate directly without intervention by the original
+ Postmaster process. Thus, the Postmaster process is always running,
+ waiting for new client connections, whereas clients and associated
+ Backend processes come and go. (All of this is of course invisible
+ to the user. We only mention it here for completeness.)
+ </para>
+ </listitem>
- <listitem>
- <para>
- The user's client (frontend) application that wants to perform
- database operations. Client applications can be very diverse
- in nature: a client could be a text-oriented tool, a graphical
- application, a web server that accesses the database to
- display web pages, or a specialized database maintenance tool.
- Some client applications are supplied with the
- <productname>PostgreSQL</productname> distribution; most are
- developed by users.
- </para>
- </listitem>
+ <listitem>
+ <para>
+ A group of processes at the server site, the <glossterm
+ linkend="glossary-instance">Instance</glossterm>, to which also
+ the Postmaster process belongs. Their duties are handling of
+ central, common database activities like file access, transaction
+ handling, vacuum, checkpoints, replication, and more. The mentioned
+ Backend processes delegate those actions to the instance.
+ </para>
+ </listitem>
- </itemizedlist>
- </para>
+ <listitem>
+ <para>
+ The user's client (frontend) application that wants to perform
+ database operations. Client applications can be very diverse
+ in nature: a client could be a text-oriented tool, a graphical
+ application, a web server that accesses the database to
+ display web pages, or a specialized database maintenance tool.
+ Some client applications are supplied with the
+ <productname>PostgreSQL</productname> distribution; most are
+ developed by users.
+ </para>
+ </listitem>
+
+ </itemizedlist>
<para>
As is typical of client/server applications, the client and the
@@ -106,18 +124,6 @@
file name) on the database server machine.
</para>
- <para>
- The <productname>PostgreSQL</productname> server can handle
- multiple concurrent connections from clients. To achieve this it
- starts (<quote>forks</quote>) a new process for each connection.
- From that point on, the client and the new server process
- communicate without intervention by the original
- <filename>postgres</filename> process. Thus, the
- supervisor server process is always running, waiting for
- client connections, whereas client and associated server processes
- come and go. (All of this is of course invisible to the user. We
- only mention it here for completeness.)
- </para>
</sect1>
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