Re: Interesting case of IMMUTABLE significantly hurting performance

On Wednesday, April 9, 2025, Olleg Samoylov <splarv(at)ya(dot)ru> wrote:
>
> On 10.04.2025 01:08, Tom Lane wrote:
>
>> Yeah. The assumption is that you had a reason for marking the
>> function IMMUTABLE and you want the planner to treat it that way
>> even if it isn't really. (There are valid use-cases for that, for
>> instance if you want calls to the function to be constant-folded.)
>> regards, tom lane
>>
>
> Well, to_char(bigint, text) indeed not immutable, because in some pattern
> it uses get information from locale. For instance,'SLDG' patterns. But in
> case of
>
> CREATE OR REPLACE FUNCTION formatted_num_immutable(p_summa bigint)
> RETURNS text
> LANGUAGE sql
> IMMUTABLE STRICT
> RETURN ltrim(to_char(p_summa, '999 999 999 999 999 999 999 999'));
>
> to_char do not use locale information in this pattern. So it is correct
> conclude that to_char is immutable with this pattern and
> formatted_num_immutable too. I did not lie to the planner.
>
> So this is looked "strange", immutable function marked as immutable
> function can not be inlined, but exactly the same function marked as
> volatile do.
>
>
Yeah, the inlining is an optimization, and while it seems like it could
perform more tests or maybe make slightly different/more adjustments, it
really isn’t worth the development effort or runtime cost to do so. Make
your function volatility match the most volatile function you internally
call - constant input arguments don’t change this.

There is no reason to perform number formatting immutably - function call
results involving table data are not memoized.

David J.