docs: mention that expression indexes need analyze

bmomjian · bmomjian · commit 05a719ceb0aa · 2020-11-12T15:00:44.000-05:00
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
@@ -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">.
