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docs: mention that expression indexes need analyze

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Expression indexes can't benefit from pre-computed statistics on
columns.

Reported-by: Nikolay Samokhvalov

Discussion: https://postgr.es/m/CANNMO++5rw9RDA=p40iMVbMNPaW6O=S0AFzTU=KpYHRpCd1voA@mail.gmail.com

Author: Nikolay Samokhvalov, modified

Backpatch-through: 9.5
This commit is contained in:
Bruce Momjian 2020-11-12 15:00:44 -05:00
parent 66a8f09048
commit 3370207986
1 changed files with 10 additions and 0 deletions
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10
doc/src/sgml/ref/create_index.sgml
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@ -745,6 +745,16 @@ Indexes:
sort high</quote>, in queries that depend on indexes to avoid sorting steps.
</para>
<para>
The regularly system collects statistics on all of a table's
columns. Newly-created non-expression indexes can immediately
use these statistics to determine an index's usefulness.
For new expression indexes, it is necessary to run <link
linkend="sql-analyze"><command>ANALYZE</command></link> or wait for
the <link linkend="autovacuum">autovacuum daemon</link> to analyze
the table to generate statistics about new expression indexes.
</para>
<para>
For most index methods, the speed of creating an index is
dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.