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Re: formatting of SQL sent by PHP to postgres

From: "David Green" <david(at)sagerobot(dot)com>
To: <pgsql-general(at)postgresql(dot)org>
Subject: Re: formatting of SQL sent by PHP to postgres
Date: 2003-10-30 20:19:31
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On Thu, 30 Oct 2003, Scott Marlowe wrote:

> "It's far more likely that optimizing your SQL queries will yield the
> greatest increase in performance.  Things like replacing "select max(id)
> from table" with "select id from table order by id desc limit 1" etc..."

When I first read this I was surprised that this kind of change could even
a difference. I tested it and it makes a lot of difference.

On a table with 21,000 records I ran 2 queries. One using "Max(Num)" and one
using the "order by num desc limit 1". The "Max(Num)" query took 51 msec and
the other took 0.09 msec. I tried the same thing on SQL Server and the 2
queries run in exactly the same amount of time. Why does it make so much of
difference in PostgreSQL? I did notice in the query plan, the second query
able to use the index on the Num field - this may be the speed difference..

I'm running pgsql v7.3.2 on redhat 9.

Also, are there any other "tricks" for optimizing this way? I have a vb app
I'm porting to PostgreSQL from SQL Server and it seems a lot of the queries,
etc take a lot longer... I'm starting to think it may be ODBC or something
slowing me up but that I can ask about on the other mailing list...

David Green
Sage Automation, Inc.

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