Robert Treat <xzilla@users.sourceforge.net> writes:
> Don't know if I would agree for sure, but i the second vacuum could see
> that it is being blocked by the current vacuum, exiting out would be a
> bonus, since in most scenarios you don't need to run that second vacuum
> so it just ends up wasting resources (or clogging other things up with
> it lock)
This would be reasonable if we could do it, but the present lock manager
doesn't provide any way to tell what sort of lock is blocking you.
There are some cases in which it isn't obvious anyway. For instance,
suppose an ALTER TABLE (which wants an exclusive lock) is queued up
waiting for the currently-running VACUUM. An incoming new VACUUM
request will queue behind the ALTER. Which lock would you say is
blocking it ... and does an honest answer to that question jibe with
your preference about whether the second VACUUM should give up?
A chintzy way out would be for VACUUM to just exit if it can't
immediately acquire lock, regardless of the cause. This wouldn't be
too useful for VACUUM FULL, but plain VACUUM is not blocked by very many
common operations other than another VACUUM, so most of the time it
would do what you want. I could possibly be talked into supporting an
option to do that.
regards, tom lane