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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.
This commit is contained in:
Tom Lane 2006-06-07 17:08:07 +00:00
parent ca9d50304f
commit ae0c8d09fb
1 changed files with 6 additions and 8 deletions
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14
src/backend/optimizer/path/indxpath.c
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@ -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)