Date: Thu, 30 Mar 2006 14:31:34 +0200
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
To: Peter Eisentraut <peter_e@gmx.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: Index scan startup time
Message-ID: <20060330123134.GA13408@uio.no>
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	pgsql-performance@postgresql.org
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	<20060330120206.GA13075@uio.no>
	<200603301423.54510.peter_e@gmx.net>
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On Thu, Mar 30, 2006 at 02:23:53PM +0200, Peter Eisentraut wrote:
>>> EXPLAIN ANALYZE select activity_id from activity where state in (10000,
>>> 10001) order by activity_id limit 100;
>>>
>>> QUERY PLAN
>>>
>>> Limit  (cost=0.00..622.72 rows=100 width=8) (actual
>>> time=207356.054..207356.876 rows=100 loops=1)
>>>   ->  Index Scan using activity_pk on activity  (cost=0.00..40717259.91
>>> rows=6538650 width=8) (actual time=207356.050..207356.722 rows=100
>>> loops=1) Filter: ((state = 10000) OR (state = 10001))
>>> Total runtime: 207357.000 ms
>>>
>>> The table has seen VACUUM FULL and REINDEX before this.
>> The index scan is by activity_id, not by state. Do you have an index on
>> state at all?
> There is an index on state as well but the column is not selective enough.

Well, it's logical enough; it scans along activity_id until it finds one with
state=10000 or state=10001. You obviously have a _lot_ of records with low
activity_id and state none of these two, so Postgres needs to scan all those
records before it founds 100 it can output. This is the “startup cost” you're
seeing.

/* Steinar */
-- 
Homepage: http://www.sesse.net/