Re: Optimizing DISTINCT with LIMIT

On Thursday 04 December 2008 15:09, Gregory Stark wrote:
> tmp <skrald@amossen.dk> writes:

> > Also, it is my impression that many people use LIMIT to minimize the
> > evaluation time of sub queries from which the outer query only needs a
> > small subset of the sub query output.
>
> I've seen lots of queries which only pull a subset of the results too --
> but it's always a specific subset. So that means using ORDER BY or a WHERE
> clause to control it.

I use "ORDER BY random() LIMIT :some_small_number" frequently to get a "feel" 
for data.  That always builds the unrandomized relation and then sorts it.  I 
guess an alternate path for single-table queries would be to randomly choose 
a block number and then a tuple number;  but that would be biased toward long 
rows (of which fewer can appear in a block).

-- 
David Lee Lambert ... Software Developer
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