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 749db2845e8c6fdcf58901aca167c1671a26f1f5..844f00904be94fc95f163d437d0490805f61a64c 100644 (file)
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -745,6 +745,16 @@ Indexes:
    sort high, in queries that depend on indexes to avoid sorting steps.
   
 
+  
+   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 
+   linkend="sql-analyze">ANALYZE or wait for
+   the autovacuum daemon to analyze
+   the table to generate statistics about new expression indexes.
+  
+
   
    For most index methods, the speed of creating an index is
    dependent on the setting of .