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Re: BUG #4224: issue with LIMIT and ORDER BY

From: hubert depesz lubaczewski <depesz(at)depesz(dot)com>
To: Lawrence Cohan <lawrencec(at)1shoppingcart(dot)com>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #4224: issue with LIMIT and ORDER BY
Date: 2008-06-05 18:41:27
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Lists: pgsql-bugs
On Thu, Jun 05, 2008 at 06:15:29PM +0000, Lawrence Cohan wrote:
> Following queries run FOREVER in PG if an index exists on the "id" column
> which is a integer - serial and PKey on the table. 
> SELECT id FROM orders WHERE merchant_id = xxxxxx ORDER BY id DESC LIMIT 31
> -- or 30, 29, 28, 27, 26, 25
> or 
> SELECT id FROM clients WHERE merchant_id = XXXXXX ORDER BY id LIMIT 3 -- or
> 1, 2.
> With different limits we get different results but  the queris are running
> forever with DESC as well. 

my guess is that you:
1. don't have index on merchant_id
2. have a lot of rows in this table
3. very little rows have given merchant_id

you can easily fix the situation with:
create index q on clients (merchant_id, id);


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