--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -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>
+