From: Tom Lane <tgl@sss.pgh.pa.us>
To: Joel Mukuthu <jom@upright.co>
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: Unexpected behaviour of a RAISE statement in an IMMUTABLE
 function
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Comments: In-reply-to Joel Mukuthu <jom@upright.co>
	message dated "Wed, 23 Nov 2022 15:12:43 +0100"
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Date: Wed, 23 Nov 2022 11:20:26 -0500
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Joel Mukuthu <jom@upright.co> writes:
> CREATE FUNCTION raise_exception_immutable(IN err_message text)
> RETURNS void
> LANGUAGE 'plpgsql'
> IMMUTABLE
> AS $BODY$
> BEGIN
> RAISE EXCEPTION
> USING MESSAGE = err_message;
> END;
> $BODY$;

A function with side-effects (like raising an error) isn't
really immutable [1].  We do fudge that a bit, since hardly
anything could be marked immutable if there were a strict
rule about it --- but when the primary point of the function
is to cause that side-effect, you can't fudge it.

> 4. This raises an exception that was surprising to me:

> SELECT raise_exception_immutable('foo') WHERE false;
> -- ERROR: foo
> -- CONTEXT: PL/pgSQL function raise_exception_immutable(text) line 3 at
> RAISE

The allegedly-immutable function is evaluated during constant folding.

> 5. This does not raises an exception, that was also surprising to me:

> SELECT raise_exception_immutable(format('foo')) WHERE false;

format() isn't immutable, only stable; so constant-folding can't
reach the error.

			regards, tom lane

[1] https://www.postgresql.org/docs/current/xfunc-volatility.html