Re: Ok, why isn't it using *this* index?
От | ADBAAMD |
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Тема | Re: Ok, why isn't it using *this* index? |
Дата | |
Msg-id | 3AC79BCB.4040507@bell.ca обсуждение исходный текст |
Ответ на | Ok, why isn't it using *this* index? (Paul Tomblin <ptomblin@xcski.com>) |
Ответы |
Re: Ok, why isn't it using *this* index?
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Список | pgsql-general |
Paul Tomblin wrote: > Quoting ADBAAMD (adba.amdocs@bell.ca): > > Ok, so if I understand you correctly, the fact that about 90% of the > records have country='USA' and about 9% of the records have > country='CANADA' means that it's never going to use the index because it > on average, a query is going to be for USA, and a sequential scan is going > to be better. > > I think I understand now. If this is correct, then doesn't it make sense > just to drop that index? At least until I get a lot more data from other > countries? Probably you are right. You could also try other index access methods besides the standard b-tree. I don't know about the situation in pgsql, but in Oracle we have even an index access method create specifically to serve low selectivity data: bitmaps. While I could find a succint description of access methods at http://www.postgresql.org/users-lounge/docs/7.0/postgres/indices.html, I don't know how each of them work exactly, nor if any would be any good for low selectivity situations. Is there any docs on that? -- _ / \ Leandro Guimarães Faria Corsetti Dutra +55 (11) 3040 8913 \ / Amdocs at Bell Canada +1 (514) 786 87 47 X Support Center, São Paulo, Brazil mailto:adbaamd@bell.ca / \ http://terravista.pt./Enseada/1989/ mailto:leandrod@amdocs.com
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