From: Bruce Momjian <bruce@momjian.us>
Date: Thu, 12 Nov 2020 20:00:43 +0000 (-0500)
Subject: docs: mention that expression indexes need analyze
X-Git-Tag: REL9_5_25~71
X-Git-Url: http://git.postgresql.org/gitweb/?a=commitdiff_plain;h=3284fe452112c9f15975ebccc73c168ed6892d4f;p=postgresql.git

docs: mention that expression indexes need analyze

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
---

diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index 2ef7d8929d5..94e45d1db63 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -568,6 +568,16 @@ Indexes:
    sort high</>, 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">.