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Re: plans for bitmap indexes?

From: Greg Stark <gsstark(at)mit(dot)edu>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: plans for bitmap indexes?
Date: 2004-10-26 15:42:08
Message-ID: 87is8xtrjj.fsf@stark.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hannu Krosing <hannu(at)tm(dot)ee> writes:

> I repeat here my earlier proposal of making the bitmap indexes
> page-level and clustering data automatically on AND of all defined
> bitmap indexes. 

The problem with page-level bitmaps is that they could be much less effective.
Consider a query like 'WHERE foo = ? AND bar = ? AND baz = ?" where each of
those matches about 1% of your tuples. If you have 100 tuples per page then
each of those bitmaps will find a tuple in about half the pages. So the
resulting AND will find about 1/8th of the pages as candidates. In reality the
number of pages it should have to fetch should be more like 1 in a million.

The other problem is that for persist on-disk indexes they require more work
to update. You would have to recheck every other tuple in the page to
recalculate the bit value instead of just being able to flip one bit.

-- 
greg


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