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Describe (auto-)analyze behavior for partitioned tables

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This explains the new behavior introduced by 0827e8af70 as well as
preexisting.

Author: Justin Pryzby <pryzby@telsasoft.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20210423180152.GA17270@telsasoft.com
This commit is contained in:
Alvaro Herrera
2021-05-14 13:10:52 -04:00
parent 5eb1b27d20
commit 1b5617eb84
4 changed files with 41 additions and 14 deletions
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doc/src/sgml/ref/analyze.sgml
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@@ -250,20 +250,38 @@ ANALYZE [ VERBOSE ] [ <replaceable class="parameter">table_and_columns</replacea
</para>
<para>
If the table being analyzed has one or more children,
<command>ANALYZE</command> will gather statistics twice: once on the
rows of the parent table only, and a second time on the rows of the
parent table with all of its children. This second set of statistics
is needed when planning queries that traverse the entire inheritance
tree. The autovacuum daemon, however, will only consider inserts or
updates on the parent table itself when deciding whether to trigger an
automatic analyze for that table. If that table is rarely inserted into
or updated, the inheritance statistics will not be up to date unless you
run <command>ANALYZE</command> manually.
If the table being analyzed is partitioned, <command>ANALYZE</command>
will gather statistics by sampling blocks randomly from its partitions;
in addition, it will recurse into each partition and update its statistics.
(However, in multi-level partitioning scenarios, each leaf partition
will only be analyzed once.)
By constrast, if the table being analyzed has inheritance children,
<command>ANALYZE</command> will gather statistics for it twice:
once on the rows of the parent table only, and a second time on the
rows of the parent table with all of its children. This second set of
statistics is needed when planning queries that traverse the entire
inheritance tree. The child tables themselves are not individually
analyzed in this case.
</para>
<para>
If any of the child tables are foreign tables whose foreign data wrappers
The autovacuum daemon counts inserts, updates and deletes in the
partitions to determine if auto-analyze is needed. However, adding
or removing partitions does not affect autovacuum daemon decisions,
so triggering a manual <command>ANALYZE</command> is recommended
when this occurs.
</para>
<para>
Tuples changed in inheritance children do not count towards analyze
on the parent table. If the parent table is empty or rarely modified,
it may never be processed by autovacuum. It's necessary to
periodically run a manual <command>ANALYZE</command> to keep the
statistics of the table hierarchy up to date.
</para>
<para>
If any of the child tables or partitions are foreign tables whose foreign data wrappers
do not support <command>ANALYZE</command>, those child tables are ignored while
gathering inheritance statistics.
</para>