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0ee5a39862
the number of rows likely to be produced by a query such as SELECT * FROM t1 LEFT JOIN t2 USING (key) WHERE t2.key IS NULL; What this is doing is selecting for t1 rows with no match in t2, and thus it may produce a significant number of rows even if the t2.key table column contains no nulls at all. 8.2 thinks the table column's null fraction is relevant and thus may estimate no rows out, which results in terrible plans if there are more joins above this one. A proper fix for this will involve passing much more information about the context of a clause to the selectivity estimator functions than we ever have. There's no time left to write such a patch for 8.3, and it wouldn't be back-patchable into 8.2 anyway. Instead, put in an ad-hoc test to defeat the normal table-stats-based estimation when an IS NULL test is evaluated at an outer join, and just use a constant estimate instead --- I went with 0.5 for lack of a better idea. This won't catch every case but it will catch the typical ways of writing such queries, and it seems unlikely to make things worse for other queries. |
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