Re: Selecting K random rows - efficiently!

In article <ffnid8$1q2t$1(at)news(dot)hub(dot)org>, cluster <skrald(at)amossen(dot)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.
--

Patrick TJ McPhee
North York Canada
ptjm(at)interlog(dot)com