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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2006-06-07 17:08:07 +0000 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2006-06-07 17:08:07 +0000 |
| commit | ae0c8d09fb05d39c4f2c548bfc4720dadbd83532 (patch) | |
| tree | 41c52c469cf75c5207158592862ac6fb95f8957a /src | |
| parent | ca9d50304ff664ec2fbf6ddf4cafe3d031c319cb (diff) | |
| download | postgresql-ae0c8d09fb05d39c4f2c548bfc4720dadbd83532.tar.gz postgresql-ae0c8d09fb05d39c4f2c548bfc4720dadbd83532.zip | |
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.
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/optimizer/path/indxpath.c | 14 |
1 files changed, 6 insertions, 8 deletions
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c index eaf4e1cb337..75f2487757e 100644 --- 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) |