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.
Few basic checks..
- What does explain analyze says for the slow query?
- Have you vacuumed and analyzed recently?
- Have you done basic optimisations from default state? Check http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html and http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html
And 7.0 is way too old. If you can afford to upgrade, upgrade to 7.4.2.
HTH
Shridhar
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