On 02/11/2011, at 2:16 PM, Tom Lane wrote: > Royce Ausburn <royce.ml@xxxxxxxxxxx> writes: >> [ random() is evaluated only once in ] >> test=# select (select random()) from generate_series(1,10); -- rows are the same > >> I understand that it's likely an optimisation thing -- postgres knows >> that the subselect doesn't depend on the FROM rows so it evaluates it >> only once, but is this really correct behaviour? > > I can't find anything in the SQL standard that explicitly addresses this > point, but a bit of googling turns up documentation for both Oracle and > DB/2 that say they evaluate uncorrelated subqueries just once: > http://www.smart-soft.co.uk/Oracle/advanced-sql-tutorial.htm > http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=/sqlp/rbafycorrs.htm > whereas correlated subqueries are evaluated once per row. > > So we're in good company here. Even if we were not, I'd be loath to > change it, because people have historically relied on this behavior to > force single evaluation of what otherwise would be considered volatile > functions. That's good enough for me, Tom. Thanks =) -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
