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Re: Is it possible to speed this query up?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: arnaulist(at)andromeiberica(dot)com
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Is it possible to speed this query up?
Date: 2006-07-26 20:24:47
Message-ID: 17741.1153945487@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Arnau <arnaulist(at)andromeiberica(dot)com> writes:
> the explain analyze shouts the following:

The expensive part appears to be this indexscan:

>                       ->  Index Scan using pk_agndusrgrp_usergroup on 
> agenda_users_groups ug  (cost=0.00..123740.26 rows=2936058 width=30) 
> (actual time=0.101..61921.260 rows=2836638 loops=1)

Since there's no index condition, the planner is evidently using this
scan just to obtain sort order.  I think ordinarily it would use a
seqscan and then sort the final result, which'd be a lot faster if the
whole result were being selected.  But you have a LIMIT and it's
mistakenly guessing that only a small part of the table will need to be
scanned before the LIMIT is satisfied.

Bottom line: try dropping the LIMIT.  If you really need the limit to be
enforced on the SQL side, you could try declaring the query as a cursor
and only fetching 150 rows from it.

			regards, tom lane

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