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Re: Abnormal performance difference between Postgres and MySQL

From: Farhan Husain <russoue(at)gmail(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Abnormal performance difference between Postgres and MySQL
Date: 2009-02-24 19:51:38
Message-ID: 3df32b6d0902241151t2f80c24ar343ad37066c89b49@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
The result set should have 31 rows, that is correct.

On Mon, Feb 23, 2009 at 7:53 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>wrote:

> On Mon, Feb 23, 2009 at 6:24 PM, Farhan Husain <russoue(at)gmail(dot)com> wrote:
> This sort here:
>
> >    ->  Sort  (cost=565372.46..568084.16 rows=1084680 width=74) (actual
> > time=5410606.604..5410606.628 rows=31 loops=1)
> >          Sort Key: a1.subj
> >          Sort Method:  quicksort  Memory: 489474kB
> >          ->  Seq Scan on jena_g1t1_stmt a1  (cost=0.00..456639.59
> > rows=1084680 width=74) (actual time=0.043..44005.780 rows=3192000
> loops=1)
>
> Seems to be the problem.  There are a few things that seem odd, the
> first is that it estimates it will return 1M ros, but returns only 31.
>  The other is that sorting 31 rows is taking 5410606 milliseconds.
>
> My first guess would be to crank up the statistics on a1.subj to a few
> hundred (going up to a thousand if necessary) re-analyzing and seeing
> if the query plan changes.
>
> I'm not expert enough on explain analyze to offer any more.
>



-- 
Mohammad Farhan Husain
Research Assistant
Department of Computer Science
Erik Jonsson School of Engineering and Computer Science
University of Texas at Dallas

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