Re: CPU hogged by concurrent SELECT..FOR UPDATE SKIP LOCKED

On 20-Aug.-2020 17:42, Michael Lewis wrote:
> Can you share an explain analyze for the query that does the select for
> update? I wouldn't assume that partition pruning is possible at all with
> hash, and it would be interesting to see how it is finding those rows.

Well this got interesting  - the already moved error showed up: Note,
the actual process partitions are regular table partitions, these are
not hashed.  Only the incoming and completed are hashed due to row
counts at either end of the processing; in flight (where the issue shows
up) is quite small:

[queuedb] # explain analyze select queueid,txobject,objectid,state from
mq.queue where (state = 'tx_active' or state='tx_fail_retryable') and
txobject = 'ticket' limit 250 for update skip locked;
ERROR:  40001: tuple to be locked was already moved to another partition
due to concurrent update
LOCATION:  heapam_tuple_lock, heapam_handler.c:405
Time: 579.131 ms
[queuedb] # explain analyze select queueid,txobject,objectid,state from
mq.queue where (state = 'tx_active' or state='tx_fail_retryable') and
txobject = 'ticket' limit 250 for update skip locked;
ERROR:  40001: tuple to be locked was already moved to another partition
due to concurrent update
LOCATION:  heapam_tuple_lock, heapam_handler.c:405
Time: 568.008 ms
[queuedb] # explain analyze select queueid,txobject,objectid,state from
mq.queue where (state = 'tx_active' or state='tx_fail_retryable') and
txobject = 'ticket' limit 250 for update skip locked;
      QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..25.71 rows=250 width=34) (actual
time=1306.041..1306.338 rows=250 loops=1)
   ->  LockRows  (cost=0.00..7934.38 rows=77150 width=34) (actual
time=1306.040..1306.315 rows=250 loops=1)
         ->  Append  (cost=0.00..7162.88 rows=77150 width=34) (actual
time=520.685..1148.347 rows=31500 loops=1)
               ->  Seq Scan on queue_tx_active  (cost=0.00..6764.50
rows=77000 width=34) (actual time=520.683..1145.258 rows=31500 loops=1)
                     Filter: ((txobject = 'ticket'::mq.queue_object)
AND ((state = 'tx_active'::mq.tx_state) OR (state =
'tx_fail_retryable'::mq.tx_state)))
               ->  Seq Scan on queue_tx_fail_retryable
 (cost=0.00..12.62 rows=150 width=34) (never executed)
                     Filter: ((txobject = 'ticket'::mq.queue_object)
AND ((state = 'tx_active'::mq.tx_state) OR (state =
'tx_fail_retryable'::mq.tx_state)))
 Planning Time: 0.274 ms
 Execution Time: 1306.380 ms
(9 rows)

Time: 1317.150 ms (00:01.317)
[queuedb] #