Subject: Re: Interesting case of IMMUTABLE significantly hurting performance

From: Olleg Samoylov <[email-masked]>
Date: Thu, 10 Apr 2025 08:33:02 +0300

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.
-- 
Olleg

---

From: "David G. Johnston" <[email-masked]>
Date: Thu, 10 Apr 2025 07:01:18 -0700

On Wednesday, April 9, 2025, Olleg Samoylov <[email-masked]> wrote:
> 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
>
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.