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From: David G. Johnston <[email protected]>
To: Olleg Samoylov <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: pgsql-generallists.postgresql.org <[email protected]>
Subject: Re: Interesting case of IMMUTABLE significantly hurting performance
Date: Thu, 10 Apr 2025 07:01:18 -0700
Message-ID: <CAKFQuwZ_9nEHR7bfnhK8XqCiN3WkX5O0i7Rbqn1w68kFC+f67Q@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Wednesday, April 9, 2025, Olleg Samoylov <[email protected]> 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.

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