From: Peter Eisentraut <peter_e@gmx.net>
To: pgsql-performance@postgresql.org
Subject: Index scan startup time
Date: Thu, 30 Mar 2006 13:59:10 +0200
Message-ID: <200603301359.11038.peter_e@gmx.net> (raw)
[Apologies if this already went through. I don't see it in the archives.]
Normally one expects that an index scan would have a startup time of nearly
zero. Can anyone explain this:
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 plan choice and the statistics look right, but why does it take 3 minutes
before doing anything? Or is the measurement of the actual start time
inaccurate? This is quite reproducible, so it's not just a case of a
temporary I/O bottleneck, say.
(PostgreSQL 8.0.3)
--
Peter Eisentraut
http://developer.postgresql.org/~petere/
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To: pgsql-performance@postgresql.org
Cc: peter_e@gmx.net
Subject: Re: Index scan startup time
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