On Oct 31, 2013, at 11:04 AM, Joe Love <j...@primoweb.com> wrote: > In postgres 9.2 I have a function that is relatively expensive. When I write > a query such as: > > select expensive_function(o.id),o.* from offeirng o where valid='Y' order by > name limit 1; > > the query runs slow and appears to be running the function on each ID, which > in this case should be totally unnecessary as it really only needs to run on > 1 row. > > When I rewrite the query like so: > > select expensive_function(o.id), o.* > from (select *offering where valid='Y' order by name limit 1) o; > > the expensive function only runs once and thus, much faster. I would think > that the optimizer could handle this situation, especially when limit or > offset is used and the expensive function is not used in a group by, order by > or where.
Does anyone know what the SQL standard says about this, if anything? I can't see any way that this would change the result set, but of course if the function has external effects this would make a difference...
