On Wed, Sep 19, 2012 at 9:30 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Florian.Schoppmann@emc.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.
I like the more standard CTE approach to optimization fencing where it works:
postgres=# WITH source AS ( SELECT i, random() r FROM generate_series(1,10) AS i
)
SELECT i
FROM source, ( SELECT count(*) AS _n FROM source ) AS _stats
WHERE r < 5::DOUBLE PRECISION/_n;
merlin
