Re: Problems with ALTER DOMAIN patch

On Tue, 2002-12-10 at 22:56, Tom Lane wrote:
> Rod Taylor <rbt(at)rbt(dot)ca> writes:
> >> 2. Insufficient locking, guise 2: there's no protection against someone
> >> else adding a column or table while you're processing an ALTER DOMAIN,
> >> either. This means that constraint checks will be missed. Example:
>
> > Locking the entry in pg_type doesn't prevent that?
>
> If there were such a thing as "locking the entry in pg_type", it might
> prevent that, but (a) there isn't, and (b) your code wouldn't invoke it
> if there were. Reading a row should surely not be tantamount to
> invoking an exclusive lock on it.

Hrm... Yes.. I came to that conclusion while walking home. My concepts
of locking, and what actually happens in PostgreSQL are two completely
different things.

> In any case, other backends might have the pg_type entry in their
> syscaches, in which case their references to the type would be quite
> free of any actual read of the pg_type row that might fall foul of
> your hypothetical lock.

So... Basically I'm cooked.

> relation's pg_class row. We have no such locks on types at present,
> but I think it may be time to invent 'em.

I'd be happy to use them once created.

Thanks again for the help.

--
Rod Taylor <rbt(at)rbt(dot)ca>

PGP Key: http://www.rbt.ca/rbtpub.asc