Message-ID: <41EFE954.7080205@archonet.com>
Date: Thu, 20 Jan 2005 17:24:36 +0000
From: Richard Huxton <dev@archonet.com>
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To: Greg Stark <gsstark@mit.edu>
Cc: Andrei Bintintan <klodoma@ar-sd.net>, pgsql-sql@postgresql.org,
	pgsql-performance@postgresql.org
Subject: Re: [SQL] OFFSET impact on Performance???
References: <019f01c4fee1$1b58d600$0b00a8c0@forge>
	<41EF9FD3.5020400@archonet.com>
	<020d01c4fef6$595bb9d0$0b00a8c0@forge>
	<87pt00c9zd.fsf@stark.xeocode.com>
In-Reply-To: <87pt00c9zd.fsf@stark.xeocode.com>
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Greg Stark wrote:
> "Andrei Bintintan" <klodoma@ar-sd.net> writes:
> 
> 
>>>If you're using this to provide "pages" of results, could you use a cursor?
>>
>>What do you mean by that? Cursor?
>>
>>Yes I'm using this to provide "pages", but If I jump to the last pages it goes
>>very slow.
> 
> 
> The best way to do pages for is not to use offset or cursors but to use an
> index. This only works if you can enumerate all the sort orders the
> application might be using and can have an index on each of them.
> 
> To do this the query would look something like:
> 
> SELECT * FROM tab WHERE col > ? ORDER BY col LIMIT 50
> 
> Then you take note of the last value used on a given page and if the user
> selects "next" you pass that as the starting point for the next page.

Greg's is the most efficient, but you need to make sure you have a 
suitable key available in the output of your select.

Also, since you are repeating the query you could get different results 
as people insert/delete rows. This might or might not be what you want.

A similar solution is to partition by date/alphabet or similar, then 
page those results. That can reduce your resultset to a manageable size.
--
   Richard Huxton
   Archonet Ltd