Re: Schema variables - new implementation for Postgres 15

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.