Re: ORDER BY and LIMIT not propagated on inherited

From: Matteo Beccati <php(at)beccati(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: ORDER BY and LIMIT not propagated on inherited
Date: 2005-09-02 15:00:50
Message-ID: 43186922.4060703@beccati.com
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Hi,

>>The correct strategy IMHO would
>>be applying the order by and limit for each child table (which results
>>in an index scan, if possible), appending, then finally sorting a bunch
>>of rows, and limiting again.
>
> This would be a win in some cases, and in many others a loss (ie, wasted
> sort steps). The hard part is determining when to apply it.

I don't actually know how many smaller separate sorts compare to a
single big sort, but I guess the difference wouldn't be so big if the
LIMIT is low. Add to this that you don't need to append the whole
rowsets, but just smaller ones.

Best regards
--
Matteo Beccati
http://phpadsnew.com
http://phppgads.com

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