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Re: Speed with offset clause

From: Yves Vindevogel <yves(dot)vindevogel(at)implements(dot)be>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Speed with offset clause
Date: 2005-06-24 20:23:43
Message-ID: d1a17c01d826791cf2ae54381f902c3a@implements.be (view raw or flat)
Thread:
Lists: pgsql-performance
Hi,

Indeed, I would have to do it through a function, where I check the 
number of pages, ....
It puts my weakest point in the middle then.

I could simply rewrite my query like you state, just to check.
I think all my queries are on one table only.  (I report in a website 
on one table, that has been denormalized into other smaller tables for 
speed)
But the problem is on the big table.

I'm currently looking at another possibility, and that is generating 
XML files based upon my database.  This would increase disk space 
enormously, but limit my problems with the database.
Since I am using Cocoon for the website, this is not such a problematic 
decision, disks are cheap and I need only a few modifications to my 
code.

On 24 Jun 2005, at 21:22, John A Meinel wrote:

> Yves Vindevogel wrote:
>
>> Hi again all,
>>
>> My queries are now optimised. They all use the indexes like they 
>> should.
>> However, there's still a slight problem when I issue the "offset" 
>> clause.
>>
>> We have a table that contains 600.000 records
>> We display them by 25 in the webpage.
>> So, when I want the last page, which is: 600k / 25 = page 24000 - 1 = 
>> 23999, I issue the offset of 23999 * 25
>> This take a long time to run, about 5-10 seconds whereas offset below 
>> 100 take less than a second.
>>
>> Can I speed this up ?
>>
>>
>> Met vriendelijke groeten,
>> Bien à vous,
>> Kind regards,
>>
>> *Yves Vindevogel*
>> *Implements*
>>
> Postgres has the optimization that it will plan a query, and once it 
> reaches the limit, it can stop even though there is more data 
> available.
> The problem you are having is that it has to go through "offset" rows 
> first, before it can apply the limit.
> If you can, (as mentioned in the other post), try to refine your index 
> so that you can reverse it for the second half of the data.
>
> This is probably tricky, as you may not know how many rows you have 
> (or the amount might be changing).
>
> A potentially better thing, is if you have an index you are using, you 
> could use a subselect so that the only portion that needs to have 60k 
> rows is a single column.
>
> Maybe an example:
> Instead of saying:
>
> SELECT * FROM table1, table2 WHERE table1.id = table2.id ORDER BY 
> table1.date OFFSET x LIMIT 25;
>
> You could do:
>
> SELECT * FROM
>    (SELECT id FROM table1 OFFSET x LIMIT 25) as subselect
>    JOIN table1 ON subselect.id = table1.id
>    , table2
>    WHERE table1.id = table2.id;
>
> That means that the culling process is done on only a few rows of one 
> table, and the rest of the real merging work is done on only a few 
> rows.
>
> It really depends on you query, though, as what rows you are sorting 
> on has a big influence on how well this will work.
>
> John
> =:->
>
>
>
Met vriendelijke groeten,
Bien à vous,
Kind regards,

Yves Vindevogel
Implements


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