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Re: Question about indexes

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Question about indexes
Date: 2004-01-30 15:22:34
Message-ID: 200401301522.i0UFMYr16926@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Greg Stark wrote:
> 
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> 
> > In any case, this discussion is predicated on the assumption that the
> > operations involving the bitmap are a significant fraction of the total
> > time, which I think is quite uncertain.  Until we build it and profile
> > it, we won't know that.
> 
> The other thought I had was that it would be difficult to tell when to follow
> this path. Since the main case where it wins is when the individual indexes
> aren't very selective but the combination is very selective, and we don't have
> inter-column correlation statistics ...

We actually have heap access cost and index access cost.  You could
compare costs of looking at all of index A's heap vs. looking at index
B and then hopefully fewer heap rows.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
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