Add OLD/NEW support to RETURNING in DML queries.

This allows the RETURNING list of INSERT/UPDATE/DELETE/MERGE queries
to explicitly return old and new values by using the special aliases
"old" and "new", which are automatically added to the query (if not
already defined) while parsing its RETURNING list, allowing things
like:

  RETURNING old.colname, new.colname, ...

  RETURNING old.*, new.*

Additionally, a new syntax is supported, allowing the names "old" and
"new" to be changed to user-supplied alias names, e.g.:

  RETURNING WITH (OLD AS o, NEW AS n) o.colname, n.colname, ...

This is useful when the names "old" and "new" are already defined,
such as inside trigger functions, allowing backwards compatibility to
be maintained -- the interpretation of any existing queries that
happen to already refer to relations called "old" or "new", or use
those as aliases for other relations, is not changed.

For an INSERT, old values will generally be NULL, and for a DELETE,
new values will generally be NULL, but that may change for an INSERT
with an ON CONFLICT ... DO UPDATE clause, or if a query rewrite rule
changes the command type. Therefore, we put no restrictions on the use
of old and new in any DML queries.

Dean Rasheed, reviewed by Jian He and Jeff Davis.

Discussion: https://postgr.es/m/CAEZATCWx0J0-v=Qjc6gXzR=KtsdvAE7Ow=D=mu50AgOe+pvisQ@mail.gmail.com

1 files changed, 14 insertions, 4 deletions

diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index 285a5c88d58..bad1df732ea 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -2619,6 +2619,13 @@ transformWholeRowRef(ParseState *pstate, ParseNamespaceItem *nsitem,
 	 * point, there seems no harm in expanding it now rather than during
 	 * planning.
 	 *
+	 * Note that if the nsitem is an OLD/NEW alias for the target RTE (as can
+	 * appear in a RETURNING list), its alias won't match the target RTE's
+	 * alias, but we still want to make a whole-row Var here rather than a
+	 * RowExpr, for consistency with direct references to the target RTE, and
+	 * so that any dropped columns are handled correctly.  Thus we also check
+	 * p_returning_type here.
+	 *
 	 * Note that if the RTE is a function returning scalar, we create just a
 	 * plain reference to the function value, not a composite containing a
 	 * single column.  This is pretty inconsistent at first sight, but it's
@@ -2626,13 +2633,17 @@ transformWholeRowRef(ParseState *pstate, ParseNamespaceItem *nsitem,
 	 * "rel.*" mean the same thing for composite relations, so why not for
 	 * scalar functions...
 	 */
-	if (nsitem->p_names == nsitem->p_rte->eref)
+	if (nsitem->p_names == nsitem->p_rte->eref ||
+		nsitem->p_returning_type != VAR_RETURNING_DEFAULT)
 	{
 		Var		   *result;
 
 		result = makeWholeRowVar(nsitem->p_rte, nsitem->p_rtindex,
 								 sublevels_up, true);
 
+		/* mark Var for RETURNING OLD/NEW, as necessary */
+		result->varreturningtype = nsitem->p_returning_type;
+
 		/* location is not filled in by makeWholeRowVar */
 		result->location = location;
 
@@ -2655,9 +2666,8 @@ transformWholeRowRef(ParseState *pstate, ParseNamespaceItem *nsitem,
 		 * are in the RTE.  We needn't worry about marking the RTE for SELECT
 		 * access, as the common columns are surely so marked already.
 		 */
-		expandRTE(nsitem->p_rte, nsitem->p_rtindex,
-				  sublevels_up, location, false,
-				  NULL, &fields);
+		expandRTE(nsitem->p_rte, nsitem->p_rtindex, sublevels_up,
+				  nsitem->p_returning_type, location, false, NULL, &fields);
 		rowexpr = makeNode(RowExpr);
 		rowexpr->args = list_truncate(fields,
 									  list_length(nsitem->p_names->colnames));