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Re: pgsql: Remove item, not sure what it refers to:

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Kris Jurka <books(at)ejurka(dot)com>,Bruce Momjian <momjian(at)svr1(dot)postgresql(dot)org>,pgsql-committers(at)postgresql(dot)org
Subject: Re: pgsql: Remove item, not sure what it refers to:
Date: 2005-04-25 13:52:02
Message-ID: 20050425135202.GG27470@ns.snowman.net (view raw or flat)
Thread:
Lists: pgsql-committers
* Bruce Momjian (pgman(at)candle(dot)pha(dot)pa(dot)us) wrote:
> Thanks, TODO item readded with a clearer description:
> 	
> 	* Allow ORDER BY ... LIMIT 1 to select high/low value without sort or
> 	  index using a sequential scan for highest/lowest values
> 	
> 	  Right now, if no index exists, ORDER BY ... LIMIT 1 requires we sort
> 	  all values to return the high/low value.  Instead The idea is to do a
> 	  sequential scan to find the high/low value, thus avoiding the sort.

Could we take this perhaps a step further and consider things like
'LIMIT 10' and come up with an approximate point where the trade-off
exists?  Actually, thinking about this a minute more perhaps there isn't
even a trade-off to be made...  What you're suggesting is basically a
size-of-1 temporary memory structure for the 'sort'.  Isn't there
already a memory structure used to perform the sorting though?  Could it
be adjusted such that it's of a fixed size when 'LIMIT' is given, as
above?

Just some thoughts, while I think the specific 'LIMIT 1' case is
probably pretty common I think the 'LIMIT 10' or 'LIMIT 50' (or however
many you want to display on the webpage...) is a pretty common use case
too and it sounds like we could improve those too with this mechanism.

Thoughts?

	Thanks,

		Stephen

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