To: "Peter Hardman" <peter@ssbg.zetnet.co.uk>
cc: pgsql-performance@postgresql.org
Subject: Re: PostgreSQL runs a query much slower than BDE and MySQL 
In-reply-to: <44E4430D.15373.6A0C60@peter.ssbg.zetnet.co.uk> 
References: <44E35A5D.19933.35F376@peter.ssbg.zetnet.co.uk>
	<44E4430D.15373.6A0C60@peter.ssbg.zetnet.co.uk>
Comments: In-reply-to "Peter Hardman" <peter@ssbg.zetnet.co.uk>
	message dated "Thu, 17 Aug 2006 10:21:01 +0100"
Date: Thu, 17 Aug 2006 09:11:50 -0400
Message-ID: <5321.1155820310@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Peter Hardman" <peter@ssbg.zetnet.co.uk> writes:
> I wonder whether Paradox and MySQL are just not doing the sort (this
> seems to be what eats up the time), since the output of the subquery
> is in fact already in the proper order.

MSSQL (from the other thread).  I feel fairly safe in assuming that
MySQL's query optimizer is not nearly in the league to do this query
effectively.  (I like the theory Arjen mentioned that what you are
measuring there is the effects of their query cache rather than a
smart fundamental implementation.)  I wonder whether MSSQL has an
EXPLAIN equivalent ...

Anywy, your point about the sort being redundant is a good one, and
offhand I'd have expected PG to catch that; I'll have to look into
why it didn't.  But that's not going to explain a 10x speed
difference, because the sort isn't 90% of the runtime.

			regards, tom lane