Re: Interesting case of IMMUTABLE significantly hurting performance

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