Add some comments about why function parameter default expressions are

restricted.

diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 91b65b3a58e87b3cc5c309540311a3dfff8f65de..a813fd9a028cb30f506ef018c6552357fce7fd6a 100644 (file)
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/commands/functioncmds.c,v 1.106 2009/01/01 17:23:38 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/commands/functioncmds.c,v 1.107 2009/01/06 02:01:27 tgl Exp $
  *
  * DESCRIPTION
  *   These routines take the parse tree and pick out the
@@ -311,7 +311,15 @@ examine_parameter_list(List *parameters, Oid languageOid,
                         errmsg("cannot use table references in parameter default value")));
 
            /*
+            * It can't return a set either --- but coerce_to_specific_type
+            * already checked that for us.
+            *
             * No subplans or aggregates, either...
+            *
+            * Note: the point of these restrictions is to ensure that an
+            * expression that, on its face, hasn't got subplans, aggregates,
+            * etc cannot suddenly have them after function default arguments
+            * are inserted.
             */
            if (pstate->p_hasSubLinks)
                ereport(ERROR,