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, 7 insertions, 5 deletions

diff --git a/src/backend/nodes/makefuncs.c b/src/backend/nodes/makefuncs.c
index b14d4d6adf4..007612563ca 100644
--- a/src/backend/nodes/makefuncs.c
+++ b/src/backend/nodes/makefuncs.c
@@ -80,12 +80,14 @@ makeVar(int varno,
 	var->varlevelsup = varlevelsup;
 
 	/*
-	 * Only a few callers need to make Var nodes with non-null varnullingrels,
-	 * or with varnosyn/varattnosyn different from varno/varattno.  We don't
-	 * provide separate arguments for them, but just initialize them to NULL
-	 * and the given varno/varattno.  This reduces code clutter and chance of
-	 * error for most callers.
+	 * Only a few callers need to make Var nodes with varreturningtype
+	 * different from VAR_RETURNING_DEFAULT, non-null varnullingrels, or with
+	 * varnosyn/varattnosyn different from varno/varattno.  We don't provide
+	 * separate arguments for them, but just initialize them to sensible
+	 * default values.  This reduces code clutter and chance of error for most
+	 * callers.
 	 */
+	var->varreturningtype = VAR_RETURNING_DEFAULT;
 	var->varnullingrels = NULL;
 	var->varnosyn = (Index) varno;
 	var->varattnosyn = varattno;