Re: explain analyze output with parallel workers - question about meaning of information for explain.depesz.com

On Tue, Nov 28, 2017 at 9:37 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Nov 27, 2017 at 6:54 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>>> Anything "below" "Gather"?
>>>
>> I think it is "actual_time * 1" for anything below Gather.
>
> The actual time amounts below Gather show total elapsed time divided
> by loop count, just as they do anywhere else in the plan.  The loop
> count tracks the number of times the plan was executed, just as it
> does anywhere else in the plan.  So for example if there are 5
> participants which each execute the node once, and the times spent are
> 5 s, 60 s, 60 s, 60 s, and 60 s, you'll get actual time = 49 s, loops
> = 5.
>
> If you want to know the total amount of time spent under the node,
> then you have to multiply the actual time (49 s in this example) by
> the loop count (5 in this example) just as you would for any other
> plan node.  However, you have to keep in mind that, for a parallel
> query, the total time spent under the node is not the same as the
> elapsed time.  In this example, if all 5 workers started at the same
> time and ran the node continuously without a break, the *elapsed* time
> until they all finished would be 60 s, not 49 s, a value that EXPLAIN
> will report nowhere.  But they can also start and stop executing under
> that node repeatedly and they need not all start at the same time,
> making the concept of elapsed time a bit unclear -- earliest start to
> latest finish would be one way, but that will be misleading (perhaps)
> if they spend 5 seconds a piece but start staggered at 4 second
> intervals.
>
> It's really hard to understand what's actually going on with a
> parallel query unless you look at each worker individually, and even
> then sometimes it's not as clear as it could be.
>

I think stats at Gather node itself gives a somewhat right picture.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com