From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Neill <rn214@richardneill.org>
cc: PostgreSQL Performance <pgsql-performance@postgresql.org>
Subject: Re: Why does the query planner use two full indexes,
	when a dedicated partial index exists?
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Comments: In-reply-to Richard Neill <rn214@richardneill.org>
	message dated "Thu, 27 Dec 2012 16:43:38 +0000"
Date: Thu, 27 Dec 2012 12:05:18 -0500
Message-ID: <14671.1356627918@sss.pgh.pa.us>
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Richard Neill <rn214@richardneill.org> writes:
> So, at the moment, I have 3 indexes:
>    full:     parcel_id_code
>    full:     exit_state
>    full:     parcel_id_code where exit state is null

> Am I right that when you suggest just a single, joint index
>      (parcel_id_code,exit_state)
> instead of all 3 of the others,

I think he was just recommending replacing the first and third indexes.

> it will allow me to optimally run all of the following? :
> 1.  SELECT * from tbl_tracker WHERE parcel_id_code=22345 AND exit_state 
> IS NULL
> 2.  SELECT * from tbl_tracker where parcel_id_code=44533
> 3.  SELECT * from tbl_tracker where exit_code = 2

You will need an index with exit_state as the first column to make #3
perform well --- at least, assuming that an index is going to help at
all anyway.  The rule of thumb is that if a query is going to fetch
more than a few percent of a table, an index is not useful because
it's going to be touching most table pages anyway, so a seqscan will
win.  I've forgotten now what you said the stats for exit_code values
other than null were, but it's quite possible that an index is useless
for #3.

These considerations are mostly covered in the manual:
http://www.postgresql.org/docs/9.2/static/indexes.html

			regards, tom lane


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