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

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'm not sure what to
do about that.  Elsewhere, trying to show the leader's information
separately when VERBOSE is used has been discussed, and I think that's
a good idea, but it may not be enough.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company