Tweak nodeBitmapAnd to stop evaluating sub-plan scans if it finds it's

got an empty bitmap after any step; the remaining subplans can no longer
affect the result.  Per a suggestion from Ilia Kantor.

src/backend/executor/nodeBitmapAnd.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapAnd.c,v 1.2 2005/04/20 15:48:36 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeBitmapAnd.c,v 1.3 2005/08/28 22:47:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -143,6 +143,16 @@ MultiExecBitmapAnd(BitmapAndState *node)
 			tbm_intersect(result, subresult);
 			tbm_free(subresult);
 		}
+
+		/*
+		 * If at any stage we have a completely empty bitmap, we can fall
+		 * out without evaluating the remaining subplans, since ANDing them
+		 * can no longer change the result.  (Note: the fact that indxpath.c
+		 * orders the subplans by selectivity should make this case more
+		 * likely to occur.)
+		 */
+		if (tbm_is_empty(result))
+			break;
 	}
 
 	if (result == NULL)

src/backend/nodes/tidbitmap.c

@@ -23,7 +23,7 @@
  * Copyright (c) 2003-2005, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/tidbitmap.c,v 1.5 2005/07/24 02:25:26 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/tidbitmap.c,v 1.6 2005/08/28 22:47:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -510,6 +510,15 @@ tbm_intersect_page(TIDBitmap *a, PagetableEntry *apage, const TIDBitmap *b)
 	}
 }
 
+/*
+ * tbm_is_empty - is a TIDBitmap completely empty?
+ */
+bool
+tbm_is_empty(const TIDBitmap *tbm)
+{
+	return (tbm->nentries == 0);
+}
+
 /*
  * tbm_begin_iterate - prepare to iterate through a TIDBitmap
  *

src/backend/optimizer/path/indxpath.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.187 2005/07/28 20:26:20 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.188 2005/08/28 22:47:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -538,6 +538,11 @@ choose_bitmap_and(PlannerInfo *root, RelOptInfo *rel, List *paths)
 	 * cases, the partial index will sort before competing non-partial
 	 * indexes and so it makes the right choice, but perhaps we need to
 	 * work harder.
+	 *
+	 * Note: outputting the selected sub-paths in selectivity order is a good
+	 * thing even if we weren't using that as part of the selection method,
+	 * because it makes the short-circuit case in MultiExecBitmapAnd() more
+	 * likely to apply.
 	 */
 
 	/* Convert list to array so we can apply qsort */

src/include/nodes/tidbitmap.h

@@ -15,7 +15,7 @@
  *
  * Copyright (c) 2003-2005, PostgreSQL Global Development Group
  *
- * $PostgreSQL: pgsql/src/include/nodes/tidbitmap.h,v 1.1 2005/04/17 22:24:02 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/tidbitmap.h,v 1.2 2005/08/28 22:47:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -49,6 +49,8 @@ extern void tbm_add_tuples(TIDBitmap *tbm, const ItemPointer tids, int ntids);
 extern void tbm_union(TIDBitmap *a, const TIDBitmap *b);
 extern void tbm_intersect(TIDBitmap *a, const TIDBitmap *b);
 
+extern bool tbm_is_empty(const TIDBitmap *tbm);
+
 extern void tbm_begin_iterate(TIDBitmap *tbm);
 extern TBMIterateResult *tbm_iterate(TIDBitmap *tbm);