Re: ALTER TABLE lock strength reduction patch is unsafe

Robert Haas <robertmhaas@gmail.com> writes:
> I have been thinking for a while now that it would be sensible to make
> vacuum use a different lock type, much as we do for relation
> extension.

Hmm.  I had just been toying with the idea of introducing a new
user-visible locking level to allow separation of anti-vacuum locks from
anti-schema-alteration locks.  But I think you're probably right that it
could be done as a specialized LockTag.  That would make it not easily
user-accessible, but it's hard to think of reasons for users to lock out
vacuum anyway, unless they want to lock out everything via
AccessExclusiveLock.

> ... In particular, it's currently not
> possible to lock a table against SELECT without also locking it
> against VACUUM

Well, it still wouldn't be, since AccessExclusiveLock certainly had
better lock out vacuum.  As said above, I think the important thing
is to distinguish vacuum from schema changes.

> But that's not something I want to do in 9.1,

Definitely.
        regards, tom lane