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Re: joining two tables slow due to sequential scan

From: "Dave Dutcher" <dave(at)tridecap(dot)com>
To: "'Tim Jones'" <TJones(at)optio(dot)com>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: joining two tables slow due to sequential scan
Date: 2006-02-10 22:14:30
Message-ID: 001f01c62e8f$63551840$8300a8c0@tridecap.com (view raw or flat)
Thread:
Lists: pgsql-performance
What version of postgres are you using?  Can you post the output from
EXPLAIN ANALYZE?
 
 
-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Tim Jones
Sent: Friday, February 10, 2006 4:07 PM
To: pgsql-performance(at)postgresql(dot)org
Subject: [PERFORM] joining two tables slow due to sequential scan
 
 
I am trying to join two tables and keep getting a sequential scan in the
plan even though there is an index on the columns I am joining on.
Basically this the deal  ... I have two tables with docid in them which
is what I am using for the join.  
 
ClinicalDocs ... (no primary key) though it does not help if I make
docid primary key
docid integer (index)
patientid integer (index)
visitid integer (index)
 ...
 
Documentversions
docid integer (index)
docversionnumber (index)
docversionidentifier (primary key)
 
It seems to do an index scan if I put the primary key as docid.  This is
what occurs when I link on the patid from ClinicalDocs to patient table.
However I can not make the docid primary key because it gets repeated
depending on how may versions of a document I have.  I have tried using
a foreign key on documentversions with no sucess. 
 
In addition this query
 
select * from documentversions join clinicaldocuments on
documentversions.documentidentifier
= clinicaldocuments.dssdocumentidentifier where
documentversions.documentstatus = 'AC'; 
 
does index scan 
but if I change the order e.g
 
select * from clinicaldocuments join documentversions on
clinicaldocuments.dssdocumentidentifier
= documentversions .documentidentifier where
clinicaldocuments.patientidentifier= 123;
 
does sequential scan what I need is bottom query
it is extremely slow ... Any ideas ?
 
Tim Jones
Healthcare Project Manager
Optio Software, Inc.
(770) 576-3555
 

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