From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
cc: Tender Wang <tndrwang@gmail.com>, pgsql-bugs@lists.postgresql.org,
        Alexander Lakhin <exclusion@gmail.com>,
        jian he <jian.universality@gmail.com>
Subject: Re: BUG #18314: PARALLEL UNSAFE function does not prevent parallel
 index build
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Comments: In-reply-to Michael Paquier <michael@paquier.xyz>
	message dated "Thu, 07 Mar 2024 07:40:57 +0900"
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Date: Wed, 06 Mar 2024 18:01:50 -0500
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Michael Paquier <michael@paquier.xyz> writes:
> Still the issue fixed here and the issue of the other thread are
> different?  Relying on the relcache tricks the planner to spawn
> workers for nothing because they would just fail if the internals of
> the function are unsafe to run in the workers, and the function is
> marked as such.

Sure, but the current patch just trades one set of errors for others.
As I said, if we care about this problem then the right fix will
involve serializing/deserializing already-const-folded expression
trees instead of expecting that the workers can regenerate those.
I don't personally think it's worth the trouble, especially not if
we can get rid of the need for exception-using functions to be marked
parallel unsafe.

> In some cases users like to use properties as hints
> to give to the planner about what to use or not.  I'd like to believe
> (and perhaps that's a cute thought) that functions are marked as
> parallel-unsafe for a purpose, which is that they cannot run in
> workers as they rely on a shared state.  Whether the internals of the
> functions are implemented correctly is up to the user.  I mean, it's
> perfectly possible to mark a function as immutable while its internals
> are volatile, and I've seen people rely on that to push down clauses.

It's the other direction that I'm concerned about: if we inline a SQL
function and find that its innards are less volatile than the original
marking, we are free to exploit that, and people will complain if we
don't.  Similarly, if the innards are more parallel safe than the
original marking, we should be able to exploit that.

			regards, tom lane