Re: function(contants) evaluated for every row

On Wed, Nov 24, 2010 at 21:52, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Bruce Momjian <bruce(at)momjian(dot)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?

Regards,
Marti