Re: Parallel Seq Scan

On Fri, Nov 13, 2015 at 10:46 AM, Thom Brown <thom(at)linux(dot)com> wrote:
>>> And perhaps associated PIDs?
>>
>> Yeah, that can be useful, if others also feel like it is important, I can
>> look into preparing a patch for the same.
>
> Thanks.

Thom, what do you think the EXPLAIN output should look like,
specifically? Or anyone else who feels like answering.

I don't think it would be very useful to repeat the entire EXPLAIN
output n times, once per worker. That sounds like a loser. But we
could add additional lines to the output for each node, like this:

Parallel Seq Scan on foo (cost=0.00..XXX rows=YYY width=ZZZ) (actual
time=AAA..BBB rows=CCC loops=1)
Leader: actual time=AAA..BBB rows=CCC loops=1
Worker 0: actual time=AAA..BBB rows=CCC loops=1
Worker 1: actual time=AAA..BBB rows=CCC loops=1
Worker 2: actual time=AAA..BBB rows=CCC loops=1

If "buffers" is specified, we could display the summary information
after the Parallel Seq Scan as normal and then display an additional
per-worker line after the "Leader" line and each "Worker N" line. I
think displaying the worker index is more useful than displaying the
PID, especially if we think that a plan tree like this might ever get
executed multiple times with different PIDs on each pass.

Like? Dislike? Other ideas?

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