Re: What Causes Access Exclusive Lock?

On Fri, 24 Jun 2016, 6:23 a.m. Jeff Janes, <jeff(dot)janes(at)gmail(dot)com> wrote:

> On Thu, Jun 23, 2016 at 10:54 AM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com>
> wrote:
> >
> >
> > On Fri, 24 Jun 2016, 1:47 a.m. Jeff Janes, <jeff(dot)janes(at)gmail(dot)com> wrote:
> >>
> >> On Thu, Jun 23, 2016 at 8:14 AM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com>
> >> wrote:
> >> >
> >> > Hi,
> >> >
> >> > I just wanted to understand what are the commands which will acquire
> >> > Access
> >> > Exclusive Lock on a table? In my knowledge below operations will
> acquire
> >> > access exclusive lock:-
> >> >
> >> > 1. VACUUM FULL
> >> > 2. ALTER TABLE
> >> > 3. DROP TABLE
> >> > 4. TRUNCATE
> >> > 5. REINDEX
> >> > 6. LOCK command with Access Exclusive Mode (or no mode specified)
> >> >
> >> > I am using PostgreSQL v9.4.
> >>
> >> A regular VACUUM (not a FULL one), including autovac, will take an
> >> ACCESS EXCLUSIVE lock if it believes there are enough empty
> >> (truncatable) pages at the end of the table to be worth truncating and
> >> returning that storage to the OS. On master it will quickly abandon
> >> the lock if it detects someone else wants it, but that does not work
> >> on a standby.
> >
> >
> > Thanks! This is helpful. I believe going by this explaination I can try
> to
> > reproduce this issue manually.
> >
> > Is this part about regular vacuum acquiring an AccessExclusive Lock
> > documented? I did not see a reference to it on page for Explicit Locking.
>
> Not that I know of. I don't think any part of the user documentation
> attempts to make an exhaustive list of all actions which take which
> level of locks. It only provides some illustrative examples.
>

Thanks!
But is it something which is worth mentioning on the page about VACUUM?

> Cheers,
>
> Jeff
>
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