Re: Overhead of union versus union all
От | Simon Riggs |
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Тема | Re: Overhead of union versus union all |
Дата | |
Msg-id | 1247232164.11347.568.camel@ebony.2ndQuadrant обсуждение исходный текст |
Ответ на | Re: Overhead of union versus union all (Bruce Momjian <bruce@momjian.us>) |
Ответы |
Re: Overhead of union versus union all
Re: Overhead of union versus union all |
Список | pgsql-general |
On Fri, 2009-07-10 at 08:59 -0400, Bruce Momjian wrote: > > I think it should be possible to use predtest theorem proving to > discard > > the sort/hash step in cases where we can prove the sets are > disjoint. > > Often there are top-level quals that can be compared in the WHERE > > clauses of the sub-queries, so a shallow search could be quite > > profitable in allowing us to rewrite a UNION into a UNION ALL. > > I assume we would still need the distinct removal step; we just avoid > the sort/hash. I mean it seems possible to prove that the distinct removal step is not necessary, by proving that the various sub-queries are already disjoint. It's a common manual optimization, so automating it seems a reasonable future goal. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
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