On Nov 24, 2010, at 15:28 , Marti Raudsepp wrote: > On Wed, Nov 24, 2010 at 21:52, Tom Lane <t...@sss.pgh.pa.us> wrote: >> Bruce Momjian <br...@momjian.us> writes: >>> Notice the to_date()'s were not converted to constants in EXPLAIN so >>> they are evaluated for every row. to_date() is marked STABLE. > >> No. This is per expectation. Only IMMUTABLE functions can be folded to >> constants in advance of the query. > > This is something that has bit me in the past. > > I realize that STABLE functions cannot be constant-folded at > planning-time. But are there good reasons why it cannot called only > once at execution-time? > > As long as *only* STABLE or IMMUTABLE functions are used in a query, > we can assume that settings like timezone won't change in the middle > of the execution of a function, thus STABLE function calls can be > collapsed -- right?
I've seen this as well be a performance issue, in particular with partitioned tables. Out of habit I now write functions that always cache the value of the function in a variable and use the variable in the actual query to avoid this particular "gotcha". Michael Glaesemann grzm seespotcode net -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
