From: Tom Lane Date: Wed, 7 Jun 2006 17:08:15 +0000 (+0000) Subject: Remove "fuzzy comparison" logic in qsort comparison function for X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=a2b476df7c292c72348c4dcacf5152f960942585;p=users%2Fbernd%2Fpostgres.git 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 d7874f43a8..e337773535 100644 --- a/src/backend/optimizer/path/indxpath.c +++ b/src/backend/optimizer/path/indxpath.c @@ -610,20 +610,18 @@ 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)