Stephen Frost wrote:
-- Start of PGP signed section.
> * Bruce Momjian (pgman@candle.pha.pa.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.
Yes, I think the final optimization will allow >1 values for LIMIT.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
