Re: On-disk bitmap index patch
От | Hannu Krosing |
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Тема | Re: On-disk bitmap index patch |
Дата | |
Msg-id | 1153749575.2909.3.camel@localhost.localdomain обсуждение исходный текст |
Ответ на | Re: On-disk bitmap index patch (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: On-disk bitmap index patch
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Список | pgsql-hackers |
Ühel kenal päeval, P, 2006-07-23 kell 20:25, kirjutas Tom Lane: > Gavin Sherry <swm@linuxworld.com.au> writes: > > On Sun, 23 Jul 2006, Tom Lane wrote: > >> However, the main problem I've got with this is that a new index AM is a > >> pretty large burden, and no one's made the slightest effort to sell > >> pghackers on taking this on. > > > For low cardinality sets, bitmaps greatly out perform btree. > > If the column is sufficiently low cardinality, you might as well just do > a seqscan --- you'll be hitting most of the heap's pages anyway. I'm > still waiting to be convinced that there's a sweet spot wide enough to > justify supporting another index AM. (I'm also wondering whether this > doesn't overlap the use-case for GIN.) IIRC they quoted the cardinality of 10000 as something that is still faster than btree for several usecases. And also for AND-s of several indexes, where indexes are BIG, your btree indexes may be almost as big as tables but the resulting set of pages is small. -- ---------------- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com
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