doc: mention GROUP BY columns can reference target col numbers

Reported-by: hape <[email protected]>
Discussion: https://postgr.es/m/168871536004.379168.9352636188330923805@wrigleys.postgresql.org

Backpatch-through: 11

diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 1eb8cc042813a3e0e6c58bdf4f60cf23b2c6e46d..474ee23bf25884602c49ff36d3dce3f3fb56fb69 100644 (file)
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -130,6 +130,9 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
       eliminates groups that do not satisfy the given condition.  (See
       <xref linkend="sql-groupby" endterm="sql-groupby-title"/> and
       <xref linkend="sql-having" endterm="sql-having-title"/> below.)
+      Although query output columns are nominally computed in the next
+      step, they can also be referenced (by name or ordinal number)
+      in the <literal>GROUP BY</literal> clause.
      </para>
     </listitem>