On Thu, Jan 29, 2015 at 4:09 PM, Jim Nasby <jim.na...@bluetreble.com> wrote:
>> The difference between the autovacuum-run vacuum and the cron-run vacuum
>> is that the one running out of cron will just keep holding the lock
>> until it's actually able to truncate the end of the relation, no?  I
>> recall discussion previously that we need a way to either support that
>> in autovacuum for (a configurable set of) regular relations or come up
>> with a solution that doesn't require that lock.
> AFAICT, in master, there is no difference in truncation between auto and
> manual vacuum. What we do is attempt to acquire the truncation lock for up
> to 5 seconds, giving up after that. Once we do have the lock, we check to
> see how many pages we can actually truncate. During that check, we test
> every ~20ms or so to see if someone else is waiting on our exclusive lock;
> if they are we stop counting and will only truncate the relation up to that
> point.

I don't think this is true, and I don't think it's been true for a
long time, if ever.  The difference between a manual vacuum and
autovacuum is that autovacuum commits suicide when it conflicts with
somebody else's lock request, and a manual vacuum doesn't.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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