From: Peter Eisentraut <peter_e@gmx.net>
To: pgsql-performance@postgresql.org
Subject: Index scan startup time
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Date: Thu, 30 Mar 2006 13:59:10 +0200
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[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/