Re: Parallel query only when EXPLAIN ANALYZEd

On 30 September 2016 at 10:47, Jay Knight <jay@jayknight.com> wrote:
>>What's max_worker_processes set to?
>
> 8
>
>>One theory would be that, the worker might not have been available
>>when you performed the query execution, but it just happened to be
>>when you did the EXPLAIN ANALYZE
>
> This happens consistently this way.  Every time I run it with explain
> analyze it uses parallel workers, and every time I run it without it
> doesn't.  I just enabled auto_explain, and see that it is "planning" to use
> a worker, but doesn't launch it.  Is there a way to know why a planned
> worker doesn't get launched?

It seems not, at least not unless you're willing to attach a debugger.

> 2016-09-29 16:45:44 CDT LOG:  duration: 50703.595 ms  plan:
>     Query Text: select count(*) from t1
>     join t2 on t1.item = t2.item and t1.low <= t2.high and t1.high >= t2.low
>     Finalize Aggregate  (cost=14609058.99..14609059.00 rows=1 width=8)
> (actual time=50703.584..50703.584 rows=1 loops=1)
>       ->  Gather  (cost=14609058.88..14609058.99 rows=1 width=8) (actual
> time=50703.581..50703.581 rows=1 loops=1)
>             Workers Planned: 1
>             Workers Launched: 0
>             ->  Partial Aggregate  (cost=14608958.88..14608958.89 rows=1
> width=8) (actual time=50703.579..50703.579 rows=1 loops=1)
>                   ->  Nested Loop  (cost=0.42..13608937.28 rows=400008641
> width=0) (actual time=0.534..50577.673 rows=3669891 loops=1)
>                         ->  Parallel Seq Scan on t1  (cost=0.00..3386.71
> rows=176471 width=12) (actual time=0.041..18.351 rows=300000 loops=1)
>                         ->  Index Only Scan using t2_item_low_high_idx on t2
> (cost=0.42..63.77 rows=1333 width=12) (actual time=0.167..0.168 rows=12
> loops=300000)
>                               Index Cond: ((item = t1.item) AND (low <=
> t1.high) AND (high >= t1.low))
>                               Heap Fetches: 0

That's interesting.

How about trying to increase max_worker_processes to say, 16. I know
you've said you've tried multiple times and it seems consistent, but
increasing this seems to be worth a try, if anything, to rule that
out.

--
 David Rowley                   http://www.2ndQuadrant.com/
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