On Thu, Mar 15, 2018 at 4:48 PM, Stephen Frost <sfrost@xxxxxxxxxxx> wrote:
Greetings,
* Enrico Thierbach (eno@xxxxxxxxxxxx) wrote:
> I am using `SELECT * FROM queue ... FOR UPDATE SKIP LOCKED` to implement a
> queueing system.
>
> Now I wonder if it is possible, given the id of one of the locked rows in
> the queue table, to find out which connection/which transaction owns the
> lock.
Sure, you can attempt to lock the record and then run pg_blocking_pids()
(in another session) against the pid which is trying to acquire the
lock.
Session #1:
Connect
SELECT * FROM queue ... FOR UPDATE SKIP LOCKED;
... gets back some id X
... waits
Session #2:
Connect
SELECT pg_backend_pid(); -- save this for the 3rd session
SELECT * FROM queue WHERE id = X FOR UPDATE;
... get blocked waiting for #1
... waits
Session #3:
SELECT pg_blocking_pids(SESSION_2_PID);
-- returns PID of Session #1
Obviously there's race conditions and whatnot (what happens if session
#1 releases the lock?), but that should work to figure out who is
blocking who.
If you're on a version of PG without pg_blocking_pids then you can look
in the pg_locks view, though that's a bit more annoying to decipher.
Thanks!
Stephen
> Now I wonder if it is possible, given the id of one of the locked rows in
> the queue table, to find out which connection/which transaction owns
You have not specified which version of PostgreSQL, but try this query.
SELECT c.datname,
c.pid as pid,
c.client_addr,
c.usename as user,
c.query,
c.wait_event,
c.wait_event_type,
/* CASE WHEN c.waiting = TRUE
THEN 'BLOCKED'
ELSE 'no'
END as waiting,
*/
l.pid as blocked_by,
c.query_start,
current_timestamp - c.query_start as duration
FROM pg_stat_activity c
LEFT JOIN pg_locks l1 ON (c.pid = l1.pid and not l1.granted)
LEFT JOIN pg_locks l2 on (l1.relation = l2.relation and l2.granted)
LEFT JOIN pg_stat_activity l ON (l2.pid = l.pid)
LEFT JOIN pg_stat_user_tables t ON (l1.relation = t.relid)
WHERE pg_backend_pid() <> c.pid
ORDER BY datname,
query_start;
--
Melvin Davidson
Maj. Database & Exploration Specialist
Universe Exploration Command – UXC
Employment by invitation only!
Maj. Database & Exploration Specialist
Universe Exploration Command – UXC
Employment by invitation only!
