Feedback-ID: i76984098:Fastmail
Message-ID: <2efb8ad0-7f1f-43b3-80e1-fdd0a7f182ca@aklaver.com>
Date: Thu, 10 Apr 2025 08:38:18 -0700
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Interesting case of IMMUTABLE significantly hurting performance
To: Olleg Samoylov <splarv@ya.ru>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: "pgsql-generallists.postgresql.org" <pgsql-general@lists.postgresql.org>
References: <662101e2-5ca5-4bf5-9307-2519efde8690@ya.ru>
 <8687df7d49bc4b28b41c7cf4e9ad1e15d383b2fa.camel@cybertec.at>
 <CAKFQuwY51Us6LeBxtdL7MbHpJkxYpWtMOHJAhUoXXN2p5f1JfQ@mail.gmail.com>
 <373227.1744236488@sss.pgh.pa.us>
 <379a1e13-c775-4fb5-a444-b41308996cc4@ya.ru>
Content-Language: en-US
From: Adrian Klaver <adrian.klaver@aklaver.com>
In-Reply-To: <379a1e13-c775-4fb5-a444-b41308996cc4@ya.ru>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Archived-At: <https://www.postgresql.org/message-id/2efb8ad0-7f1f-43b3-80e1-fdd0a7f182ca%40aklaver.com>
Precedence: bulk

On 4/9/25 22:33, Olleg Samoylov 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.

I doubt the code actually looks at the template, it just does the lookup 
on pg_proc and sees:

proname | provolatile |       prosrc
---------+-------------+---------------------

to_char | s           | int8_to_char

> 
> So this is looked "strange", immutable function marked as immutable 
> function can not be inlined, but exactly the same function marked as 
> volatile do.

-- 
Adrian Klaver
adrian.klaver@aklaver.com