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Re: Abnormal performance difference between Postgres and MySQL

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Farhan Husain <russoue(at)gmail(dot)com>, 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 02:00:58
Message-ID: 138.1235440858@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> writes:
> 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.

Uh, no, it's sorting 3192000 rows --- look at the input scan.  Evidently
only the first 31 of those rows are getting fetched out of the sort,
though.

			regards, tom lane

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