Marginal improvement in sublink planning: allow unknownEqFalse optimization

to be used for SubLinks that are underneath a top-level OR clause.  Just as at
the very top level of WHERE, it's not necessary to be accurate about whether
the sublink returns FALSE or NULL, because either result has the same impact
on whether the WHERE will succeed.

src/backend/optimizer/plan/subselect.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.136 2008/08/20 15:49:30 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.137 2008/08/20 19:58:24 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1148,10 +1148,14 @@ process_sublinks_mutator(Node *node, process_sublinks_context *context)
 	 * take steps to preserve AND/OR flatness of a qual.  We assume the input
 	 * has been AND/OR flattened and so we need no recursion here.
 	 *
-	 * If we recurse down through anything other than an AND node, we are
-	 * definitely not at top qual level anymore.  (Due to the coding here, we
-	 * will not get called on the List subnodes of an AND, so no check is
-	 * needed for List.)
+	 * (Due to the coding here, we will not get called on the List subnodes of
+	 * an AND; and the input is *not* yet in implicit-AND format.  So no check
+	 * is needed for a bare List.)
+	 *
+	 * Anywhere within the top-level AND/OR clause structure, we can tell
+	 * make_subplan() that NULL and FALSE are interchangeable.  So isTopQual
+	 * propagates down in both cases.  (Note that this is unlike the meaning
+	 * of "top level qual" used in most other places in Postgres.)
 	 */
 	if (and_clause(node))
 	{
@@ -1174,14 +1178,14 @@ process_sublinks_mutator(Node *node, process_sublinks_context *context)
 		return (Node *) make_andclause(newargs);
 	}
 
-	/* otherwise not at qual top-level */
-	locContext.isTopQual = false;
-
 	if (or_clause(node))
 	{
 		List	   *newargs = NIL;
 		ListCell   *l;
 
+		/* Still at qual top-level */
+		locContext.isTopQual = context->isTopQual;
+
 		foreach(l, ((BoolExpr *) node)->args)
 		{
 			Node	   *newarg;
@@ -1195,6 +1199,12 @@ process_sublinks_mutator(Node *node, process_sublinks_context *context)
 		return (Node *) make_orclause(newargs);
 	}
 
+	/*
+	 * If we recurse down through anything other than an AND or OR node,
+	 * we are definitely not at top qual level anymore.
+	 */
+	locContext.isTopQual = false;
+
 	return expression_tree_mutator(node,
 								   process_sublinks_mutator,
 								   (void *) &locContext);