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, 28 insertions, 0 deletions

diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index f71899463b8..7c539de5cf2 100644
--- a/src/backend/executor/execUtils.c
+++ b/src/backend/executor/execUtils.c
@@ -1243,6 +1243,34 @@ ExecGetReturningSlot(EState *estate, ResultRelInfo *relInfo)
 }
 
 /*
+ * Return a relInfo's all-NULL tuple slot for processing returning tuples.
+ *
+ * Note: this slot is intentionally filled with NULLs in every column, and
+ * should be considered read-only --- the caller must not update it.
+ */
+TupleTableSlot *
+ExecGetAllNullSlot(EState *estate, ResultRelInfo *relInfo)
+{
+	if (relInfo->ri_AllNullSlot == NULL)
+	{
+		Relation	rel = relInfo->ri_RelationDesc;
+		MemoryContext oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
+		TupleTableSlot *slot;
+
+		slot = ExecInitExtraTupleSlot(estate,
+									  RelationGetDescr(rel),
+									  table_slot_callbacks(rel));
+		ExecStoreAllNullTuple(slot);
+
+		relInfo->ri_AllNullSlot = slot;
+
+		MemoryContextSwitchTo(oldcontext);
+	}
+
+	return relInfo->ri_AllNullSlot;
+}
+
+/*
  * Return the map needed to convert given child result relation's tuples to
  * the rowtype of the query's main target ("root") relation.  Note that a
  * NULL result is valid and means that no conversion is needed.