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Re: query performance question

From: "Dave Dutcher" <dave(at)tridecap(dot)com>
To: "'gulsah'" <gulsah_129(at)yahoo(dot)com>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: query performance question
Date: 2006-05-01 00:36:11
Message-ID: 00a601c66cb7$3f2c9860$ (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
You are pulling a fair amount of data from the database and doing a lot
of computation in the SQL.  I'm not sure how fast this query could be
expected to run, but I had one idea.  If you've inserted and deleted a
lot into this table, you will need to run vacuum ocasionally.  If you
haven't been doing that, I would try a VACUUM FULL ANALYZE on the table.
(That will take a lock on the table and prevent clients from reading
data while it is running.)

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of gulsah
Sent: Friday, April 28, 2006 6:31 AM
To: pgsql-performance(at)postgresql(dot)org
Subject: [PERFORM] query performance question


I have a performance problem with Postgresql version 8.1 installed on a
Fedora Core release 4 (Stentz) with kernel version 2.6.11.

The machine I am working on has 512MB of RAM and Pentium III 800 MHz

I have only one table in the database which consists of 256 columns and
10000 rows. Each column is of float type and each row corresponds to a
vector in my application. What I want to do is to compute the distance
between a predefined vector in hand and the ones in the database.

The computation proceeds according to the following pseudocode:

        for(i=1; i<=256 ; i++){
                distance += abs(x1_i - x2_i);

where x1_i denotes the vector in hand's i coordinate and x2_i denotes
the i
coordinate of the vector in the database.

The distance computation have to be done for all the vectors in the
by means of a query and the result set should be sorted in terms of the
computed distances.

When I implement the query and measure the time spent for it in an
I see that the query is handled in more than 8 seconds which is
undesirable in
my application.

Here what I want to ask you all is that, is it a normal performance for
computer with the properties that I have mentioned above? Is there any
in your mind to increase the performance of my query?

To make it more undestandable, I should give the query for vectors with
3, but in my case their size is 256.

id as vectorid,
abs(40.9546-x2_1)+abs(-72.9964-x2_2)+abs(53.5348-x2_3) as distance
from vectordb
order by distance

Thank you all for your help.



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