Re: Performance of ORDER BY RANDOM to select random rows?

On Sun, Aug 11, 2013 at 9:59 PM, Victor Hooi <victorhooi@yahoo.com> wrote:
> Hmm, aha, so the ORDER BY RANDOM behaviour hasn't changed - just to confirm
> - this means that Postgres will duplicate the table, add a new column,
> generate random numbers for every record, then sort by that new column,
> right?

It doesn't duplicate the table, it sec scans it and uses top-N sort if
we use limit, and memory or disc sort depending on the data size if we
don't use limit.

> I've just read the above anecdotally on the internet, but I'm curious if the
> actual implementation is documented somewhere officially apart from the
> source? Running the query through EXPLAIN didn't seem to tell me much
> additional information.

I can not say about official docs, but you will find a good sorting
explanation here
http://www.depesz.com/2013/05/09/explaining-the-unexplainable-part-3/

> @Sergey - Thanks for the tip about using WITH RECURSIVE. I'm actually doing
> something similar in my application code in Django - basically take the max
> id, then generate a random integer between 0 and max id. However, it is
> dependent on how evenly distributed the record IDs are - in our case, if we
> delete a large number of records, it might affect things.

You can try to look at pg_stats.histogram_bounds to work the issue
around, however it is just my assumption, I have newer tried it.

--
Kind regards,
Sergey Konoplev
PostgreSQL Consultant and DBA

http://www.linkedin.com/in/grayhemp
+1 (415) 867-9984, +7 (901) 903-0499, +7 (988) 888-1979
gray.ru@gmail.com