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Document autoanalyze limitations for partitioned tables

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When dealing with partitioned tables, counters for partitioned tables
are not updated when modifying child tables. This means autoanalyze may
not update optimizer statistics for the parent relations, which can
result in poor plans for some queries.

It's worth documenting this limitation, so that people are aware of it
and can take steps to mitigate it (e.g. by setting up a script executing
ANALYZE regularly).

Backpatch to v10. Older branches are affected too, of couse, but we no
longer maintain those.

Author: Justin Pryzby
Reviewed-by: Zhihong Yu, Tomas Vondra
Backpatch-through: 10
Discussion: https://postgr.es/m/20210913035409.GA10647%40telsasoft.com
This commit is contained in:
Tomas Vondra
2022-03-28 14:27:36 +02:00
parent e26114c817
commit 61fa6ca79b
2 changed files with 58 additions and 3 deletions
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doc/src/sgml/ref/analyze.sgml
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@@ -263,9 +263,35 @@ ANALYZE [ VERBOSE ] [ <replaceable class="parameter">table_and_columns</replacea
</para>
<para>
If any of the child tables are foreign tables whose foreign data wrappers
do not support <command>ANALYZE</command>, those child tables are ignored while
gathering inheritance statistics.
For partitioned tables, <command>ANALYZE</command> gathers statistics by
sampling rows from all partitions; in addition, it will recurse into each
partition and update its statistics. Each leaf partition is analyzed only
once, even with multi-level partitioning. No statistics are collected for
only the parent table (without data from its partitions), because with
partitioning it's guaranteed to be empty.
</para>
<para>
By constrast, if the table being analyzed has inheritance children,
<command>ANALYZE</command> gathers two sets of statistics: one on the rows
of the parent table only, and a second including rows of both the parent
table and all of its children. This second set of statistics is needed when
planning queries that process the inheritance tree as a whole. The child
tables themselves are not individually analyzed in this case.
</para>
<para>
The autovacuum daemon does not process partitioned tables, nor does it
process inheritance parents if only the children are ever modified.
It is usually necessary to periodically run a manual
<command>ANALYZE</command> to keep the statistics of the table hierarchy
up to date.
</para>
<para>
If any child tables or partitions are foreign tables whose foreign
data wrappers do not support <command>ANALYZE</command>, those tables are
ignored while gathering inheritance statistics.
</para>
<para>