To: pgsql-admin@postgresql.org
Subject: Re: Question on Index usage
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From: Michael Monnerie <michael.monnerie@is.it-management.at>
Organization: it-management http://it-management.at
Date: Thu, 22 Jan 2009 02:24:05 +0100
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On Mittwoch 21 Januar 2009 Alvaro Herrera wrote:
> Seems like the problem is that it is not pushing the "status IN"
> condition as part of the index condition for some reason, and instead
> using it as a filter. =A0Maybe something to do with the selectivity of
> that clause?

I was reading your answer several times, and I think I understand now=20
what you mean. You mean "it doesn't use index _7 because the status IN=20
(0,1,2) isn't usable". I didn't think about that, but that's another=20
good point.

Instead, I meant: Why can't I just delete index _1, if anyway I have=20
index _7 with the same field, just plus additional fields. I thought=20
that would be redundant:

Index _1 =3D mailbox_idnr
Index _7 =3D mailbox_idnr,status,seen_flag

So I would guess that Index _1 is redundant, and I can delete it because=20
the planner will use _7 instead. After all, for searching any=20
mailbox_idnr in that table (~234k entries) it doesn't really matter if=20
you use index _1 or _7, the sort is the same (if you don't care about=20
the other fields).

mfg zmi
=2D-=20
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