2007/10/26, Patrick TJ McPhee <ptjm@xxxxxxxxxxxx>:
> In article <ffnid8$1q2t$1@xxxxxxxxxxxx>, cluster <skrald@xxxxxxxxxx> 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
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
