Re: Invalid optimization of VOLATILE function in WHERE clause?

Florian(dot)Schoppmann(at)emc(dot)com (Florian Schoppmann) writes:
> In PostgreSQL 9.1 and 9.2 (possibly also in earlier versions), the query

> --8<--
> WITH source AS (
> SELECT i FROM generate_series(1,10) AS i
> )
> SELECT
> i
> FROM
> source, (
> SELECT
> count(*) AS _n
> FROM source
> ) AS _stats
> WHERE
> random() < 5::DOUBLE PRECISION/_n;
> -->8--

[ doesn't do what you think it should ]

I can't get excited about this. Any time you put a volatile function
into WHERE, you're playing with fire. The docs warn against it:
http://www.postgresql.org/docs/9.2/static/sql-expressions.html#SYNTAX-EXPRESS-EVAL

To do what you want, I'd suggest wrapping the join into a sub-select
with an "OFFSET 0" clause, which will serve as an optimization fence
that prevents the random() call from being pushed down.

regards, tom lane