Re: [HACKERS] New design for FK-based join selectivity estimation
От | ronan.dunklau@dalibo.com |
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Тема | Re: [HACKERS] New design for FK-based join selectivity estimation |
Дата | |
Msg-id | 3012483.BymkNIPMEm@ronan_laptop обсуждение исходный текст |
Ответ на | Re: [HACKERS] New design for FK-based join selectivity estimation (Adrien Nayrat <adrien.nayrat@dalibo.com>) |
Ответы |
Re: [HACKERS] New design for FK-based join selectivity estimation
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Список | pgsql-hackers |
On mardi 13 décembre 2016 09:10:47 CET Adrien Nayrat wrote: > Hi hackers, > > The commit 100340e2dcd05d6505082a8fe343fb2ef2fa5b2a introduce an > estimation error : [....] > > Estimated row is 10x larger since 100340e2d > > Regards, Hello, I think I understand what the problem is. In get_foreign_key_join_selectiviy, we remove the restrict info clauses which match a foreign key. This is done so that the selectivy is not applied twice (once in the function itself, once when processing the restrictinfos). The problem is, for semi and anti joins, we assume that we have nohing to do (costsize.c:4253): else if (jointype == JOIN_SEMI || jointype == JOIN_ANTI) { /* * For JOIN_SEMI and JOIN_ANTI, the selectivityis defined as the * fraction of LHS rows that have matches. If the referenced * table is on theinner side, that means the selectivity is 1.0 * (modulo nulls, which we're ignoring for now). We already * covered the other case, so no work here. */ } This results in assuming that the whole outerrel will match, no matter the selectivity of the innerrel. If I understand it correctly and the above is right, I think we should ignore SEMI or ANTI joins altogether when considering FKs, and keep the corresponding restrictinfos for later processing since they are already special-cased later on. Regards, -- Ronan Dunklau
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