"k...@rice.edu" <k...@rice.edu> wrote: > On Wed, Sep 19, 2012 at 02:39:12PM -0500, Kevin Grittner wrote: >> In another thread, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> 2. Apply the WHERE condition to each row from 1, and drop rows >>> that don't pass it. >> >> People expect that the results will be consistent with this >> model, even if the implementation is optimized "under the >> covers". I think correct semantics should trump performance >> here. > It seems like this is what happens here except that the function > is evaluated once for the WHERE and not once per ROW. Both of > these meet the criterion for 2 above and Tom's earlier comments > both hold. There really needs to be some way to specify that when an expression is evaluated for each row in a set, a function used within that expression is not optimized away for some rows. Fortunately we have a way: http://www.postgresql.org/docs/9.2/interactive/sql-createfunction.html | VOLATILE indicates that the function value can change even within | a single table scan, so no optimizations can be made. Relatively | few database functions are volatile in this sense; some examples | are random(), [...] The behavior in the OP's query would certainly be sane if the function were not VOLATILE; as it is, I have a hard time seeing this as anything but a bug.
There is a workaround, if you don't mind ugly: CREATE FUNCTION random_really_i_mean_it(dummy int) RETURNS double precision LANGUAGE plpgsql VOLATILE AS $$ BEGIN -- no need to reference dummy parameter RETURN random(); END; $$; 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_really_i_mean_it(i) < 5::DOUBLE PRECISION/_n; -Kevin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers