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