Re: Query only slow on first run

[Craig James <craig_james(at)emolecules(dot)com>]

tmp wrote:
> > what exactly is that
> > "random_number" column
>
> A random float that is initialized when the row is created and never 
> modified afterwards. The physical row ordering will clearly not match 
> the random_number ordering. However, other queries uses a row ordering 
> by the primary key so I don't think it would make much sense to make the 
> index on random_number a clustering index just in order to speed up this 
> single query.
>
> >  and why are you desirous of ordering by it?
>
> In order to simulate a random pick of K rows. See [1].

A trick that I used is to sample the random column once, and create a much smaller table of the first N rows, where N is the sample size you want, and use that.

If you need a different N samples each time, you can create a temporary table, put your random N rows into that, do an ANALYZE, and then join to this smaller table.  The overall performance can be MUCH faster even though you're creating and populating a whole table, than the plan that Postgres comes up with. This seems wrong-headed (why shouldn't Postgres be able to be as efficient on its own?), but it works.

Craig