Re: Schema variables - new implementation for Postgres 15

ne 23. 1. 2022 v 9:10 odesílatel Julien Rouhaud <rjuju123(at)gmail(dot)com> napsal:

> Hi,
>
> On Fri, Jan 21, 2022 at 09:23:34PM +0100, Pavel Stehule wrote:
> >
> > I am sending updated patches
>
> I've been looking a bit deeper at the feature and I noticed that there's no
> locking involved around the session variable usage, and I don't think
> that's
> ok. AFAICS any variable used in a session will be cached in the local hash
> table and will never try to access some catalog or cache, so I don't have
> any
> naive scenario that would immediately crash, but this has some other
> implications that seems debatable.
>
> For instance, right now nothing prevents a variable from being dropped
> while
> another session is using it.
>
> Obviously we can't lock a session variable forever just because a session
> assigned a value once ages ago, especially outside of the current
> transaction.
> But if a session set a variable in the local transaction, I don't think
> that
> it's ok to have a subsequent query failing because someone else
> concurrently
> dropped the variable.
>
> I only backlogged this current thread but I didn't see that being
> discussed.
>

Isn't there enough stability of the system cache? sinval is sent at the
moment when changes in the system catalog are visible. So inside query
execution I don't see that the variable was dropped in another session.