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Message-ID: <c857b827-3a17-4600-aa98-d0710216ccfe@aklaver.com>
Date: Wed, 9 Apr 2025 14:43:11 -0700
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Subject: Re: Interesting case of IMMUTABLE significantly hurting performance
To: Nico Williams <nico@cryptonector.com>,
 Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Olleg Samoylov <splarv@ya.ru>,
 "pgsql-generallists.postgresql.org" <pgsql-general@lists.postgresql.org>
References: <662101e2-5ca5-4bf5-9307-2519efde8690@ya.ru>
 <8687df7d49bc4b28b41c7cf4e9ad1e15d383b2fa.camel@cybertec.at>
 <Z/bk5E6pqIBWTF3j@ubby>
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From: Adrian Klaver <adrian.klaver@aklaver.com>
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On 4/9/25 14:21, Nico Williams wrote:
> On Wed, Apr 09, 2025 at 10:50:00PM +0200, Laurenz Albe wrote:
>> The IMMUTABLE function cannot be inlined because to_char() is not IMMUTABLE.
> 
> Q: Why would to_char() not be IMMUTABLE?
> 
> A: Because it makes use of locales, and I guess the guc-timezone GUC,
>     which could change if the expression is ultimately used in a PlPgSQL
>     fragment, or if it's in a prepared statement.  (I think.)
> 
> That to_char is not immutable is not documented though.  Though it's
> clear when looking at the docs for the `jsonb_.*_tz()` functions.

 From here:

https://www.postgresql.org/docs/current/catalog-pg-proc.html

select proname, provolatile, prosrc  from pg_proc where proname='to_char';
  proname | provolatile |       prosrc
---------+-------------+---------------------
  to_char | s           | timestamptz_to_char
  to_char | s           | numeric_to_char
  to_char | s           | int4_to_char
  to_char | s           | int8_to_char
  to_char | s           | float4_to_char
  to_char | s           | float8_to_char
  to_char | s           | interval_to_char
  to_char | s           | timestamp_to_char

Where 's' is:

"It is s for “stable” functions, whose results (for fixed inputs) do not 
change within a scan."

> 
> Nico

-- 
Adrian Klaver
adrian.klaver@aklaver.com