Re: Selecting K random rows - efficiently!

2007/10/26, Patrick TJ McPhee <ptjm@interlog.com>:
> In article <ffnid8$1q2t$1@news.hub.org>, cluster  <skrald@amossen.dk> wrote:
> % > How important is true randomness?
> %
> % The goal is an even distribution but currently I have not seen any way
> % to produce any kind of random sampling efficiently. Notice the word
>
> How about generating the ctid randomly? You can get the number of pages
> from pg_class and estimate the number of rows either using the number
> of tuples in pg_class or just based on what you know about the data.
> Then just generate two series of random numbers, one from 0 to the number
> of pages and the other from 1 to the number of rows per page, and keep
> picking rows until you have enough numbers. Assuming there aren't too
> many dead tuples and your estimates are good, this should retrieve n rows
> with roughly n look-ups.
>
> If your estimates are low, there will be tuples which can never be selected,
> and so far as I know, there's no way to construct a random ctid in a stock
> postgres database, but apart from that it seems like a good plan. If
> efficiency is important, you could create a C function which returns a
> series of random tids and join on that.
> --
>

SELECT id, ...
   FROM data
  WHERE id = ANY(ARRAY(
                       SELECT (random()*max_id)::int
                          FROM generate_series(1,20)))
  LIMIT 1;

-- max_id is external constant

Pavel Stehule