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query performance question

From: gulsah <gulsah_129(at)yahoo(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: query performance question
Date: 2006-04-28 11:30:59
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
 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 CPU.
 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 database
 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 application
 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 a
 computer with the properties that I have mentioned above? Is there any solution
 in your mind to increase the performance of my query?
 To make it more undestandable, I should give the query for vectors with size
 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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Subject: Re: Why so slow?
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Subject: Re: CPU usage goes to 100%, query seems to ran forever

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