Remove "fuzzy comparison" logic in qsort comparison function for

choose_bitmap_and().  It was way too fuzzy --- per comment, it was meant to be
1% relative difference, but was actually coded as 0.01 absolute difference,
thus causing selectivities of say 0.001 and 0.000000000001 to be treated as
equal.  I believe this thinko explains Maxim Boguk's recent complaint.  While
we could change it to a relative test coded like compare_fuzzy_path_costs(),
there's a bigger problem here, which is that any fuzziness at all renders the
comparison function non-transitive, which could confuse qsort() to the point
of delivering completely wrong results.  So forget the whole thing and just
do an exact comparison.

diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index eaf4e1cb33751d7f3ed8c52824a1db8742cde2b5..75f2487757e852f6db6f1c88ff3d451412e1c6b6 100644 (file)
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.207 2006/06/06 17:59:57 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.208 2006/06/07 17:08:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -674,20 +674,18 @@ bitmap_path_comparator(const void *a, const void *b)
        Cost            bcost;
        Selectivity aselec;
        Selectivity bselec;
-       Selectivity diff;
 
        cost_bitmap_tree_node(pa, &acost, &aselec);
        cost_bitmap_tree_node(pb, &bcost, &bselec);
 
        /*
-        * Since selectivities are often pretty crude, don't put blind faith
-        * in them; if the selectivities are within 1% of being the same, treat
-        * them as equal and sort by cost instead.
+        * If selectivities are the same, sort by cost.  (Note: there used to be
+        * logic here to do "fuzzy comparison", but that's a bad idea because it
+        * fails to be transitive, which will confuse qsort terribly.)
         */
-       diff = aselec - bselec;
-       if (diff < -0.01)
+       if (aselec < bselec)
                return -1;
-       if (diff > 0.01)
+       if (aselec > bselec)
                return 1;
 
        if (acost < bcost)