Allow SQL-language functions to return the output of an INSERT/UPDATE/DELETE

RETURNING clause, not just a SELECT as formerly.

A side effect of this patch is that when a set-returning SQL function is used
in a FROM clause, performance is improved because the output is collected into
a tuplestore within the function, rather than using the less efficient
value-per-call mechanism.

src/include/executor/functions.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/functions.h,v 1.31 2008/03/18 22:04:14 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/executor/functions.h,v 1.32 2008/10/31 19:37:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -15,6 +15,7 @@
 #define FUNCTIONS_H
 
 #include "nodes/execnodes.h"
+#include "tcop/dest.h"
 
 
 extern Datum fmgr_sql(PG_FUNCTION_ARGS);
@@ -24,4 +25,6 @@ extern bool check_sql_fn_retval(Oid func_id, Oid rettype,
 					bool insertRelabels,
 					JunkFilter **junkFilter);
 
+extern DestReceiver *CreateSQLFunctionDestReceiver(void);
+
 #endif   /* FUNCTIONS_H */