Re: Revised Patch to allow multiple table locks in "Unison"

Fernando Nasser <fnasser(at)cygnus(dot)com> writes:
> I guess the principle (for Oracle folks) was that, for the user, there should
> be no distinction between a real table and a view. Thus, it should not matter
> for the user if the thing that is being locked is a real table or if it
> is actually being implemented as a view. Consider that it may have been
> a table one day, but the DBA changed it into a view. So that SQL will
> not work anymore and give the "ERROR: LOCK TABLE: v is not a table" message.
> This violates the Data Independence notion.

I don't really buy this, because it makes life difficult for DBAs who
want to do creative things with views. Update rules don't necessarily
touch exactly the same set of tables that are mentioned in the select
rule. But that's the only set that a LOCK implementation might possibly
know about.

Consider: for the view as view (ie, select) there's no real need to do
locking at all. The implicit read locks that will be grabbed as the
view is expanded will do fine. For updates, the behavior can and should
be defined by the rewrite rules that the DBA supplies. (Hmm, I'm not
sure that LOCK is one of the allowed query types in a rule --- if not,
it probably should be, so that the rule author can ensure the right
kinds of locks are grabbed in the right sequence.)

Another serious issue, which gets back to your original point, is that
we have no good idea what order to lock the base tables in. If we had
a concurrent-lock implementation it wouldn't matter, but in the absence
of one I am not sure it's a good idea to put in a LOCK that is going to
lock base tables in some arbitrary order.

regards, tom lane