Re: Performance problem with correlated sub-query

I apologize for my mistake in the version. I have 7.3, and not 7.0.

However, I'll upgrade to 7.4. and work with some of the other
suggestions made also.

-----Original Message-----
From: Paul Thomas [mailto:paul(at)tmsl(dot)demon(dot)co(dot)uk]
Sent: Thursday, April 29, 2004 9:20 AM
To: Howard, Steven (US - Tulsa)
Cc: pgsql-general @ postgresql . org
Subject: Re: [GENERAL] Performance problem with correlated sub-query

On 29/04/2004 14:34 "Howard, Steven (US - Tulsa)" wrote:
> 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.

7.0? That's positively ancient!
>
> 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.

PG doesn't use indexes for things like count(), max, min()...

You can avoid using max() by something like

select my_date from my_table order by my_date desc limit 1;

which will use the index.

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

In addition to the above, I'd strongly recommend upgrading to 7.4 to
take
advantage of the last ~4 years of continuous improvements.

--
Paul Thomas
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