Re: [HACKERS] Re: [QUESTIONS] MySQL benchmark page

On Tue, 3 Feb 1998, Bruce Momjian wrote:

> > Vacuum right now locks pg_class because of the statistics? If
> > that is the case, if we made vacuum *just* garbage collecting,it wouldn't
> > have to lock pg_class, only "vacuum analyze" wouldhave to do that?
> >
> > So, I was misunderstanding in that I was thinking that 'vacuum
> > analyze' only needed the read-lock :(
>
> Maybe I am wrong. I have not looked at it.

Okay, just sitting here thinking about it, and that doesn't really
make sense (if its true)...

Vacuum should be locking the table itself for a garbage cleanup,
since it has to move around records, and I wouldn't imagine you'd want to
have someone doing a SELECT at the same time. So, that locks the *table*
itself, but shouldn't affect pg_class (statistically)

Once the vacuum is finished its garbage cleanup phase (which,
granted, could take several minutes), then the statistics phase would come
into play...but again, a lock on pg_class shouldn't have to be imposed
until the 'update' of the table takes place, should it?

So, why is pg_class locked for the duration of a vacuum when the
vacuum is being performed for the whole database when it should (I think)
only need to be locked when updates are happening to it?