Message-ID: <4F7D843E.4060304@ulb.ac.be>
Date: Thu, 05 Apr 2012 13:38:38 +0200
From: Julien Cigar <jcigar@ulb.ac.be>
Organization: Belgian Biodiversity Platform
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To: pgsql-sql@postgresql.org
Subject: bad plan
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Hello,

I have an extremely bad plan for one of my colleague's query. Basically 
PostgreSQL chooses to seq scan instead of index scan. This is on:

antabif=# select version();
                                                  version
----------------------------------------------------------------------------------------------------------
  PostgreSQL 9.0.7 on amd64-portbld-freebsd8.2, compiled by GCC cc (GCC) 
4.2.1 20070719  [FreeBSD], 64-bit

The machines has 4GB of RAM with the following config:
- shared_buffers: 512MB
- effective_cache_size: 2GB
- work_mem: 32MB
- maintenance_work_mem: 128MB
- default_statistics_target: 300
- temp_buffers: 64MB
- wal_buffers: 8MB
- checkpoint_segments = 15

The tables have been ANALYZE'd. I've put the EXPLAIN ANALYZE on:

- http://www.pastie.org/3731956 : with default config
- http://www.pastie.org/3731960 : this is with enable_seq_scan = off
- http://www.pastie.org/3731962 : I tried to play on the various cost 
settings but it's doesn't change anything, except setting 
random_page_cost to 1 (which will lead to bad plans for other queries, 
so not a solution)
- http://www.pastie.org/3732035 : with enable_hashagg and 
enable_hashjoin to false

I'm currently out of idea why PostgreSQL still chooses a bad plan for 
this query ... any hint :) ?

Thank you,
Julien

-- 
No trees were killed in the creation of this message.
However, many electrons were terribly inconvenienced.


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