Arrange for hash join to skip scanning the outer relation if it detects

that the inner one is completely empty.  Per recent discussion.  Also some
cosmetic cleanups in nearby code.

src/backend/executor/nodeHash.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.86 2004/08/29 04:12:31 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.87 2004/09/22 19:13:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -32,8 +32,8 @@
 /* ----------------------------------------------------------------
  *		ExecHash
  *
- *		build hash table for hashjoin, all do partitioning if more
- *		than one batches are required.
+ *		build hash table for hashjoin, doing partitioning if more
+ *		than one batch is required.
  * ----------------------------------------------------------------
  */
 TupleTableSlot *
@@ -81,6 +81,7 @@ ExecHash(HashState *node)
 		slot = ExecProcNode(outerNode);
 		if (TupIsNull(slot))
 			break;
+		hashtable->hashNonEmpty = true;
 		econtext->ecxt_innertuple = slot;
 		ExecHashTableInsert(hashtable, econtext, hashkeys);
 		ExecClearTuple(slot);
@@ -189,7 +190,7 @@ ExecEndHash(HashState *node)
 /* ----------------------------------------------------------------
  *		ExecHashTableCreate
  *
- *		create a hashtable in shared memory for hashjoin.
+ *		create an empty hashtable data structure for hashjoin.
  * ----------------------------------------------------------------
  */
 HashJoinTable
@@ -226,12 +227,13 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
 	 * The hashtable control block is just palloc'd from the executor's
 	 * per-query memory context.
 	 */
-	hashtable = (HashJoinTable) palloc(sizeof(HashTableData));
+	hashtable = (HashJoinTable) palloc(sizeof(HashJoinTableData));
 	hashtable->nbuckets = nbuckets;
 	hashtable->totalbuckets = totalbuckets;
 	hashtable->buckets = NULL;
 	hashtable->nbatch = nbatch;
 	hashtable->curbatch = 0;
+	hashtable->hashNonEmpty = false;
 	hashtable->innerBatchFile = NULL;
 	hashtable->outerBatchFile = NULL;
 	hashtable->innerBatchSize = NULL;

src/backend/executor/nodeHashjoin.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeHashjoin.c,v 1.65 2004/09/17 18:28:53 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeHashjoin.c,v 1.66 2004/09/22 19:13:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -127,6 +127,17 @@ ExecHashJoin(HashJoinState *node)
 		hashNode->hashtable = hashtable;
 		(void) ExecProcNode((PlanState *) hashNode);
 
+		/*
+		 * If the inner relation is completely empty, and we're not doing
+		 * an outer join, we can quit without scanning the outer relation.
+		 */
+		if (!hashtable->hashNonEmpty && node->js.jointype != JOIN_LEFT)
+		{
+			ExecHashTableDestroy(hashtable);
+			node->hj_HashTable = NULL;
+			return NULL;
+		}
+
 		/*
 		 * Open temp files for outer batches, if needed. Note that file
 		 * buffers are palloc'd in regular executor context.
@@ -138,10 +149,8 @@ ExecHashJoin(HashJoinState *node)
 	}
 
 	/*
-	 * Now get an outer tuple and probe into the hash table for matches
+	 * run the hash join process
 	 */
-	outerTupleSlot = node->js.ps.ps_OuterTupleSlot;
-
 	for (;;)
 	{
 		/*
@@ -226,7 +235,7 @@ ExecHashJoin(HashJoinState *node)
 			 * Only the joinquals determine MatchedOuter status, but all
 			 * quals must pass to actually return the tuple.
 			 */
-			if (ExecQual(joinqual, econtext, false))
+			if (joinqual == NIL || ExecQual(joinqual, econtext, false))
 			{
 				node->hj_MatchedOuter = true;
 

src/include/executor/hashjoin.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/hashjoin.h,v 1.32 2004/08/29 04:13:06 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/executor/hashjoin.h,v 1.33 2004/09/22 19:13:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -47,7 +47,7 @@ typedef struct HashJoinTupleData
 
 typedef HashJoinTupleData *HashJoinTuple;
 
-typedef struct HashTableData
+typedef struct HashJoinTableData
 {
 	int			nbuckets;		/* buckets in use during this batch */
 	int			totalbuckets;	/* total number of (virtual) buckets */
@@ -57,6 +57,8 @@ typedef struct HashTableData
 	int			nbatch;			/* number of batches; 0 means 1-pass join */
 	int			curbatch;		/* current batch #, or 0 during 1st pass */
 
+	bool		hashNonEmpty;	/* did inner plan produce any rows? */
+
 	/*
 	 * all these arrays are allocated for the life of the hash join, but
 	 * only if nbatch > 0:
@@ -90,8 +92,8 @@ typedef struct HashTableData
 
 	MemoryContext hashCxt;		/* context for whole-hash-join storage */
 	MemoryContext batchCxt;		/* context for this-batch-only storage */
-} HashTableData;
+} HashJoinTableData;
 
-typedef HashTableData *HashJoinTable;
+typedef HashJoinTableData *HashJoinTable;
 
 #endif   /* HASHJOIN_H */