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

On Mon, 25 Apr 2005, Stephen Frost wrote:

> * 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.

It's a question of when to stop sorting, so, yes, it should be doable.

>
> Thoughts?
>
> Thanks,
>
> Stephen
>

Regards,
Oleg
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Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
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