Re: inconsistent behaviour of set-returning functions in sub-query with random() - Mailing list pgsql-general

Tom van Tilburg <tom.van.tilburg@gmail.com> writes:
> I'm often using the WHERE clause random() > 0.5 to pick a random subset of
> my data. Now I noticed that when using a set-returning function in a
> sub-query, I either get the whole set or none (meaning that the WHERE
> random() > 0.5 clause is interpreted *before* the set is being generated).
> e.g.:
>
> SELECT num FROM (
>     SELECT unnest(Array[1,2,3,4,5,6,7,8,9,10]) num) AS foo WHERE random() > 0.5;

Hmm, I think this is an optimizer bug.  There are two legitimate behaviors
here:

    SELECT * FROM unnest(ARRAY[1,2,3,4,5,6,7,8,9,10]) WHERE random() > 0.5;

should (and does) re-evaluate the WHERE for every row output by unnest().

    SELECT unnest(ARRAY[1,2,3,4,5,6,7,8,9,10]) WHERE random() > 0.5;

should evaluate WHERE only once, since that happens before expansion of the
set-returning function in the targetlist.  (If you're an Oracle user and
you imagine this query as having an implicit "FROM dual", the WHERE should
be evaluated for the single row coming out of the FROM clause.)

In the case you've got here, given the placement of the WHERE in the outer
query, you'd certainly expect it to be evaluated for each row coming out
of the inner query.  But the optimizer is deciding it can push the WHERE
clause down to become a WHERE of the sub-select.  That is legitimate in a
lot of cases, but not when there are SRF(s) in the sub-select's
targetlist, because that pushes the WHERE to occur before the SRF(s),
analogously to the change between the two queries I wrote.

I'm a bit hesitant to change this in existing releases.  Given the lack
of previous complaints, it seems more likely to break queries that were
behaving as-expected than to make people happy.  But we could change it
in v10 and up, especially since some other corner-case changes in
SRF-in-tlist behavior are afoot.

In the meantime, you could force it to work as you wish by inserting the
all-purpose optimization fence "OFFSET 0" in the sub-select:

=# SELECT num FROM (
    SELECT unnest(Array[1,2,3,4,5,6,7,8,9,10]) num OFFSET 0) AS foo WHERE random() > 0.5;
 num
-----
   1
   4
   7
   9
(4 rows)


                        regards, tom lane