Slow performance with left outer join

From: "Marten Verhoeven" <m(dot)verhoeven(at)van-beek(dot)nl>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Slow performance with left outer join
Date: 2008-01-21 15:00:02
Message-ID: D4E656958EEB52498883B7906D789CD54018EF@alpha2
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Hi,

Since I moved from PostgreSQL 7.3 to 8.2 I have a query which suddenly runs very slow. In 7.3 it was really fast. It seems that the query analyser makes other choices, which I don't understand.

I have the query:

SELECT * FROM fpuArticle
LEFT OUTER JOIN fpuArticleText ON a_No=at_a_No AND coalesce(at_Type,1)=1 AND coalesce(at_Language,0)=0
WHERE strpos(lower(coalesce(a_Code,'') || ' ' || coalesce(at_Text,'')), 'string')>0

when I use a normal join, this query is very fast, but with this left outer join it is slow.

This is the query analysis:

Nested Loop Left Join (cost=1796.69..3327.98 rows=5587 width=516)
Join Filter: (fpuarticle.a_no = fpuarticletext.at_a_no)
Filter: (strpos(lower((((COALESCE(fpuarticle.a_code, ''::character varying))::text || ' '::text) || (COALESCE(fpuarticletext.at_text, ''::character varying))::text)), 'string'::text) > 0)
-> Seq Scan on fpuarticle (cost=0.00..944.62 rows=16762 width=386)
-> Materialize (cost=1796.69..1796.70 rows=1 width=130)
-> Seq Scan on fpuarticletext (cost=0.00..1796.69 rows=1 width=130)
Filter: ((COALESCE((at_type)::integer, 1) = 1) AND (COALESCE(at_language, 0::numeric) = 0::numeric))

It seems that the filter on at_type and at_Language is used at the and, while it is much faster to use it at the beginning. Why is this, and how can I influence this?

With kind regards

Marten Verhoeven
Van Beek B.V.

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