Re: Question about indexes - Mailing list pgsql-hackers

From Bruce Momjian
Subject Re: Question about indexes
Date
Msg-id 200401301522.i0UFMYr16926@candle.pha.pa.us
Whole thread Raw
In response to Re: Question about indexes  (Greg Stark <gsstark@mit.edu>)
List pgsql-hackers
Greg Stark wrote:
> 
> Tom Lane <tgl@sss.pgh.pa.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@candle.pha.pa.us               |  (610)
359-1001+  If your life is a hard drive,     |  13 Roberts Road +  Christ can be your backup.        |  Newtown Square,
Pennsylvania19073
 


pgsql-hackers by date:

Previous
From: Bruce Momjian
Date:
Subject: Re: Question about indexes
Next
From: Alvaro Herrera
Date:
Subject: Re: Question about indexes