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Performance problem with correlated sub-query

From: "Howard, Steven (US - Tulsa)" <sthoward(at)DELOITTE(dot)com>
To: <pgsql-general(at)PostgreSQL(dot)org>
Subject: Performance problem with correlated sub-query
Date: 2004-04-29 13:34:06
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
I have created a web app that stores and displays all the messages from
my database maintenance jobs that run each night. The web app uses Java
servlets and has PostgreSQL 7.0 as the back end.


When the user requests the first page, he gets a list of all the servers
with maintenance records in the database, and a drop down list of all
the dates of maintenance records. If the user chooses a date first, then
the app uses a prepared statement with the date contained in a
parameter, and this executes very quickly - no problems.


However, if the web page user does not choose a date, then the app uses
a correlated sub-query to grab only the current (latest) day's
maintenance records. The query that is executed is:


select servername, databasename, message from messages o where
o.date_of_msg = 

            (select max(date_of_msg) from messages i where i.servername
= o.servername);


And this is a dog. It takes 15 - 20 minutes to execute the query (there
are about 200,000 rows in the table). I have an index on (servername,
date_of_msg), but it doesn't seem to be used in this query.


Is there a way to improve the performance on this query?




Steve Howard


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Subject: Unable to use index?
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