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Message-ID: <20070102124518.1503.qmail@web26301.mail.ukl.yahoo.com>
Date: Tue, 2 Jan 2007 13:45:18 +0100 (CET)
From: =?iso-8859-1?q?Rolf=20=D8stvik?= <rolfostvik@yahoo.no>
Subject: Re: Worse perfomance on 8.2.0 than on 7.4.14 
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <21166.1167694227@sss.pgh.pa.us>
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--- Tom Lane <tgl@sss.pgh.pa.us> skrev:

> =?iso-8859-1?q?Rolf=20=D8stvik?=
> <rolfostvik@yahoo.no> writes:
> > First i have some queries to give you a feel of
> size
> > of datasets and plans and times. 
> 
> You said earlier that essentially all the rows of
> step_result_subset
> have step_parent = 0 ... is that really true?

Not true, but i am sorry if it could be intepreted
that way.
What i tried to say was 
 step_result_subset contain 17 179 506 rows
 uut_Result_subset  contain    176 555 rows

There is one entry in step_result_subset with the
condition step_parent = 0 for each entry in
uut_result_subset (there is 176 555 rows in
step_result_subset which have step_parent = 0).

For this (sample) query i have found that if i select
just a little bigger data set (setting start_date_time
to an earlier date) the plan selected by the server
does the best job and gives a more stable execution
time independent of size of data sets. I also have
found that my theories of the best solution has been
wrong. 


If you (Tom) still want me to do the following steps
then please tell me.

>  I can
> hardly believe
> that either 7.4 or 8.2 would use an indexscan for
> Q-A if so.
> 
> I'd be interested to see the results of
> 
> prepare foo(int) as select id from
> step_result_subset sr
>     where uut_result = $1 and step_parent = 0;
> explain analyze execute foo(42);
> 
> (use some representative uut_result value instead of
> 42).  If it doesn't
> want to use an indexscan for this, disable plan
> types until it does.
> This would perhaps shed some light on why 8.2
> doesn't want to use a scan
> like that as the inside of a nestloop.
> 
> 			regards, tom lane
> 

Best regards
Rolf Østvik


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