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Re: Question on Index usage

From: Michael Monnerie <michael(dot)monnerie(at)is(dot)it-management(dot)at>
To: pgsql-admin(at)postgresql(dot)org
Subject: Re: Question on Index usage
Date: 2009-01-22 01:24:05
Message-ID: 200901220224.05787@zmi.at (view raw or flat)
Thread:
Lists: pgsql-admin
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.  Maybe something to do with the selectivity of
> that clause?

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

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

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

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

mfg zmi
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