Re: Schema variables - new implementation for Postgres 15

út 25. 1. 2022 v 6:18 odesílatel Julien Rouhaud <rjuju123(at)gmail(dot)com> napsal:

> Hi,
>
> On Mon, Jan 24, 2022 at 12:33:11PM +0100, Pavel Stehule wrote:
> >
> > here is updated patch with locking support
>
> Thanks for updating the patch!
>
> While the locking is globally working as intended, I found a few problems
> with
> it.
>
> First, I don't think that acquiring the lock in
> get_session_variable_type_typmod_collid() and
> prepare_variable_for_reading() is
> the correct approach. In transformColumnRef() and transformLetStmt() you
> first
> call IdentifyVariable() to check if the given name is a variable without
> locking it and later try to lock the variable if you get a valid Oid.
> This is
> bug prone as any other backend could drop the variable between the two
> calls
> and you would end up with a cache lookup failure. I think the lock should
> be
> acquired during IdentifyVariable. It should probably be optional as one
> codepath only needs the information to raise a warning when a variable is
> shadowed, so a concurrent drop isn't a problem there.
>

There is a problem, because before the IdentifyVariable call I don't know
if the variable will be shadowed or not.

If I lock a variable inside IdentifyVariable, then I need to remember if I
did lock there, or if the variable was locked already, and If the variable
is shadowed and if lock is fresh, then I can unlock the variable.

Regards

Pavel