From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: PoC/WIP: Extended statistics on expressions
Date: Sat, 16 Jan 2021 17:22:08 -0600
Message-ID: <20210116232208.GB8560@telsasoft.com> (raw)
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On Sat, Jan 16, 2021 at 05:48:43PM +0100, Tomas Vondra wrote:
> + <entry role="catalog_table_entry"><para role="column_definition">
> + <structfield>expr</structfield> <type>text</type>
> + </para>
> + <para>
> + Expression the extended statistics is defined on
> + </para></entry>
Expression the extended statistics ARE defined on
Or maybe say "on which the extended statistics are defined"
> + <para>
> + The <command>CREATE STATISTICS</command> command has two basic forms. The
> + simple variant allows to build statistics for a single expression, does
.. ALLOWS BUILDING statistics for a single expression, AND does (or BUT does)
> + Expression statistics are per-expression and are similar to creating an
> + index on the expression, except that they avoid the overhead of the index.
Maybe say "overhead of index maintenance"
> + All functions and operators used in a statistics definition must be
> + <quote>immutable</quote>, that is, their results must depend only on
> + their arguments and never on any outside influence (such as
> + the contents of another table or the current time). This restriction
say "outside factor" or "external factor"
> + results of those expression, and uses default estimates as illustrated
> + by the first query. The planner also does not realize the value of the
realize THAT
> + second column fully defines the value of the other column, because date
> + truncated to day still identifies the month. Then expression and
> + ndistinct statistics are built on those two columns:
I got an error doing this:
CREATE TABLE t AS SELECT generate_series(1,9) AS i;
CREATE STATISTICS s ON (i+1) ,(i+1+0) FROM t;
ANALYZE t;
SELECT i+1 FROM t GROUP BY 1;
ERROR: corrupt MVNDistinct entry
--
Justin
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To: pgsql-hackers@postgresql.org
Cc: pryzby@telsasoft.com, tomas.vondra@enterprisedb.com, dean.a.rasheed@gmail.com
Subject: Re: PoC/WIP: Extended statistics on expressions
In-Reply-To: <20210116232208.GB8560@telsasoft.com>
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