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.

4 files changed, 26 insertions, 0 deletions

diff --git a/src/backend/executor/nodeBitmapAnd.c b/src/backend/executor/nodeBitmapAnd.c
index 2f23a1ae04..f6044c4a85 100644
--- a/src/backend/executor/nodeBitmapAnd.c
+++ b/src/backend/executor/nodeBitmapAnd.c
@@ -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)
diff --git a/src/backend/nodes/tidbitmap.c b/src/backend/nodes/tidbitmap.c
index 095138c8ab..1064a38c34 100644
--- a/src/backend/nodes/tidbitmap.c
+++ b/src/backend/nodes/tidbitmap.c
@@ -511,6 +511,15 @@ 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
  *
  * NB: after this is called, it is no longer allowed to modify the contents
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 2fdfe97896..bb6b73faf3 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -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 */
diff --git a/src/include/nodes/tidbitmap.h b/src/include/nodes/tidbitmap.h
index 656fd8e467..6962c8d070 100644
--- a/src/include/nodes/tidbitmap.h
+++ b/src/include/nodes/tidbitmap.h
@@ -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);