Re: deadlock error - version 8.4 on CentOS 6

Steve Clark <steve(dot)clark(at)netwolves(dot)com> writes:
> No. But I examined the pg_log/log_file and saw an error indicating it was autovacuum:

> 2016-10-27 09:47:02 EDT:srm2api:12968:LOG: sending cancel to blocking autovacuum PID 12874
> 2016-10-27 09:47:02 EDT:srm2api:12968:DETAIL: Process 12968 waits for ExclusiveLock on relation 955454549 of database 955447411.
> 2016-10-27 09:47:02 EDT:srm2api:12968:STATEMENT: lock table t_unit_status_log in exclusive mode
> 2016-10-27 09:47:02 EDT::12874:ERROR: canceling autovacuum task
> 2016-10-27 09:47:02 EDT::12874:CONTEXT: automatic vacuum of table "srm2.public.t_unit_status_log"

That kicked the autovacuum off the table, but it didn't help because you
still had a deadlock condition afterwards:

> 2016-10-27 09:47:02 EDT:srm2api:9189:ERROR: deadlock detected at character 8
> 2016-10-27 09:47:02 EDT:srm2api:9189:DETAIL: Process 9189 waits for RowExclusiveLock on relation 955454549 of database 955447411; blocked by process 12968.
> Process 12968 waits for ExclusiveLock on relation 955454518 of database 955447411; blocked by process 9189.
> Process 9189: update t_unit_status_log set status_date = now ( ) , unit_active = 'y' , last_updated_date = now ( ) , last_updated_by = current_user , devices_down = $1 where unit_serial_no = $2
> Process 12968: lock table t_unit in exclusive mode

> So I feel pretty confident this is the issue. I guess I should retry the update in my application.

Retrying might be a usable band-aid, but really this is an application
logic error. The code that is trying to do "lock table t_unit in
exclusive mode" must already hold some lower-level lock on t_unit, which
is blocking whatever the "update t_unit_status_log" command wants to do
with t_unit. Looks like a classic lock-strength-upgrade mistake to me.

regards, tom lane