doc: mention GROUP BY columns can reference target col numbers

Reported-by: hape 
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 181edf32741b0c9d6b147495babf358a292ab7d0..b4a3f029b79a232281f66433aba35ed27292fd99 100644 (file)
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -131,6 +131,9 @@ TABLE [ ONLY ] table_name [ * ]
       eliminates groups that do not satisfy the given condition.  (See
        and
        below.)
+      Although query output columns are nominally computed in the next
+      step, they can also be referenced (by name or ordinal number)
+      in the GROUP BY clause.