Re: Selecting a random row

Kari,

Why not select count(*) from the table and multiply it by a true 0.0 - 1.0
pseudo random number generator?  Then adjust the outcome for the range of
uids.  If the uids (or some other column) are contiguous starting at 0,
this would be a snap.

Rick



                     
                      Tom Lane
                     
                      <tgl@sss.pgh.pa.us>            To:       Kari Lavikka <tuner@bdb.fi>
                     
                      Sent by:                       cc:       pgsql-general@postgresql.org
                     
                      pgsql-general-owner@pos        Subject:  Re: [GENERAL] Selecting a random row
                     
                      tgresql.org
                     

                     

                     
                      11/04/2004 10:25 AM
                     

                     

                     




Kari Lavikka <tuner@bdb.fi> writes:
> --
> -- Choose a random point between 0 and max_uid and select the first
> -- value from the bigger part
> --
> CREATE OR REPLACE FUNCTION random_uid() RETURNS int4 AS
>   'SELECT uid FROM users u WHERE u.status = ''a'' AND uid >=
>    cast(cast(max_uid() - 1 AS FLOAT) * random() AS INTEGER) ORDER BY uid
>    ASC LIMIT 1'
>   LANGUAGE 'sql';

This isn't going to do what you think because the random() function is
re-evaluated at every row of the table.  (For that matter, so is
max_uid(), which means performance would suck even if it worked ...)

I'd suggest rewriting in plpgsql so you can assign the (max_uid-1)*random()
expression to a variable and then just use the variable in the SELECT.

                                     regards, tom lane

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