Ühel kenal päeval, P, 2006-07-23 kell 20:25, kirjutas Tom Lane: > Gavin Sherry <[EMAIL PROTECTED]> 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 ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
