Doc: mention that extended stats aren't used for joins

Statistics defined by the CREATE STATISTICS command are only used to
assist with the selectivity estimations of base relations, never for
joins.  Here we mention this fact in the notes section of the CREATE
STATISTICS command.

Discussion: https://postgr.es/m/CAApHDvrMuVgDOrmg_EtFDZ=AOovq6EsJNnHH1ddyZ8EqL4yzMw@mail.gmail.com
Backpatch-through: 11

diff --git a/doc/src/sgml/ref/create_statistics.sgml b/doc/src/sgml/ref/create_statistics.sgml
index 8770470cee145bf4a346adbfa6e9147afd0efadf..330bdc6494ab4e2609903d18768355e854e78da3 100644 (file)
--- a/doc/src/sgml/ref/create_statistics.sgml
+++ b/doc/src/sgml/ref/create_statistics.sgml
@@ -125,6 +125,12 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_na
    reading it.  Once created, however, the ownership of the statistics
    object is independent of the underlying table(s).
   </para>
+
+  <para>
+   Extended statistics are not currently used by the planner for selectivity
+   estimations made for table joins.  This limitation will likely be removed
+   in a future version of <productname>PostgreSQL</productname>.
+  </para>
  </refsect1>
 
  <refsect1 id="sql-createstatistics-examples">