Tweak hash join code to use an additional heuristic for deciding whether

it's worth probing the outer relation for emptiness before building the
hash table.  To wit, if we're rescanning a join previously performed,
remember whether we found it nonempty the previous time, and don't bother
with the probe if it was nonempty.  This buys back the performance lost
in examples like Mario Weilguni's.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index e9fb41f653..7371f95007 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.144 2005/11/26 22:14:57 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.145 2005/11/28 23:46:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1118,6 +1118,7 @@ typedef struct MergeJoinState
  *		hj_FirstOuterTupleSlot	first tuple retrieved from outer plan
  *		hj_NeedNewOuter			true if need new outer tuple on next call
  *		hj_MatchedOuter			true if found a join match for current outer
+ *		hj_OuterNotEmpty		true if outer relation known not empty
  * ----------------
  */
 
@@ -1142,6 +1143,7 @@ typedef struct HashJoinState
 	TupleTableSlot *hj_FirstOuterTupleSlot;
 	bool		hj_NeedNewOuter;
 	bool		hj_MatchedOuter;
+	bool		hj_OuterNotEmpty;
 } HashJoinState;