From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: RFC: replace pg_stat_activity.waiting with something more descriptive |
Date: | 2015-07-27 18:46:11 |
Message-ID: | CA+TgmoYx2zwK476MR2HuDJD4aZyTtked7LBP9tV--XyANVxK7Q@mail.gmail.com |
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On Mon, Jul 27, 2015 at 2:43 PM, Alvaro Herrera
<alvherre(at)2ndquadrant(dot)com> wrote:
> Robert Haas wrote:
>> On Mon, Jul 27, 2015 at 2:32 PM, Alvaro Herrera
>> <alvherre(at)2ndquadrant(dot)com> wrote:
>
>> > I think this is already possible, is it not? You just have to look for
>> > an identically-identified pg_locks entry with granted=true. That gives
>> > you a PID and vxid/xid. You can self-join pg_locks with that, and join
>> > to pg_stat_activity.
>> >
>> > I remember we discussed having a layer of system views on top of
>> > pg_stat_activity and pg_locks, probably defined recursively, that would
>> > show the full graph of waiters/lockers.
>>
>> It isn't necessarily the case that A is waiting for a unique process
>> B. It could well be the case that A wants AccessExclusiveLock and
>> many processes hold a variety of other lock types.
>
> Sure, but I don't think this makes it impossible to figure out who's
> locking who. I think the only thing you need other than the data in
> pg_locks is the conflicts table, which is well documented.
>
> Oh, hmm, one thing missing is the ordering of the wait queue for each
> locked object. If process A holds RowExclusive on some object, process
> B wants ShareLock (stalled waiting) and process C wants AccessExclusive
> (also stalled waiting), who of B and C is woken up first after A
> releases the lock depends on order of arrival.
Agreed - it would be nice to expose that somehow.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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