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, 19 insertions, 2 deletions

diff --git a/src/backend/optimizer/util/appendinfo.c b/src/backend/optimizer/util/appendinfo.c
index cece3a5be75..5b3dc0d8653 100644
--- a/src/backend/optimizer/util/appendinfo.c
+++ b/src/backend/optimizer/util/appendinfo.c
@@ -253,6 +253,13 @@ adjust_appendrel_attrs_mutator(Node *node,
 		 * all non-Var outputs of such subqueries, and then we could look up
 		 * the pre-existing PHV here.  Or perhaps just wrap the translations
 		 * that way to begin with?
+		 *
+		 * If var->varreturningtype is not VAR_RETURNING_DEFAULT, then that
+		 * also needs to be copied to the translated Var.  That too would fail
+		 * if the translation wasn't a Var, but that should never happen since
+		 * a non-default var->varreturningtype is only used for Vars referring
+		 * to the result relation, which should never be a flattened UNION ALL
+		 * subquery.
 		 */
 
 		for (cnt = 0; cnt < nappinfos; cnt++)
@@ -283,9 +290,17 @@ adjust_appendrel_attrs_mutator(Node *node,
 					elog(ERROR, "attribute %d of relation \"%s\" does not exist",
 						 var->varattno, get_rel_name(appinfo->parent_reloid));
 				if (IsA(newnode, Var))
+				{
+					((Var *) newnode)->varreturningtype = var->varreturningtype;
 					((Var *) newnode)->varnullingrels = var->varnullingrels;
-				else if (var->varnullingrels != NULL)
-					elog(ERROR, "failed to apply nullingrels to a non-Var");
+				}
+				else
+				{
+					if (var->varreturningtype != VAR_RETURNING_DEFAULT)
+						elog(ERROR, "failed to apply returningtype to a non-Var");
+					if (var->varnullingrels != NULL)
+						elog(ERROR, "failed to apply nullingrels to a non-Var");
+				}
 				return newnode;
 			}
 			else if (var->varattno == 0)
@@ -339,6 +354,8 @@ adjust_appendrel_attrs_mutator(Node *node,
 					rowexpr->colnames = copyObject(rte->eref->colnames);
 					rowexpr->location = -1;
 
+					if (var->varreturningtype != VAR_RETURNING_DEFAULT)
+						elog(ERROR, "failed to apply returningtype to a non-Var");
 					if (var->varnullingrels != NULL)
 						elog(ERROR, "failed to apply nullingrels to a non-Var");