From 64ac51a1791fa2059c843e5e5533fb7c70bda699 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Thu, 12 Nov 2020 15:00:44 -0500
Subject: [PATCH] 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
---
 doc/src/sgml/ref/create_index.sgml | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index dc4b645452a..fb1ecb9ff6a 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -565,6 +565,16 @@ 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">.