Re: parallel explain analyze support not exercised

On 2017-04-03 10:26:27 +0530, Rafia Sabih wrote:
> On Sat, Apr 1, 2017 at 12:25 AM, Andres Freund <andres@anarazel.de> wrote:
> > Hi,
> >
> > As visible in [1], the explain analyze codepaths of parallel query isn't
> > exercised in the tests.  That used to be not entirely trivial if the
> > output was to be displayed (due to timing), but we should be able to do
> > that now that we have the SUMMARY option.
> >
> > E.g.
> > SET max_parallel_workers = 0;
> > EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM blarg2 WHERE generate_series < 0;
> > ┌───────────────────────────────────────────────────────────┐
> > │                        QUERY PLAN                         │
> > ├───────────────────────────────────────────────────────────┤
> > │ Gather (actual rows=0 loops=1)                            │
> > │   Workers Planned: 10                                     │
> > │   Workers Launched: 0                                     │
> > │   ->  Parallel Seq Scan on blarg2 (actual rows=0 loops=1) │
> > │         Filter: (generate_series < 0)                     │
> > │         Rows Removed by Filter: 10000000                  │
> > └───────────────────────────────────────────────────────────┘
> >
> > should be reproducible.  I'd suggest additionally adding one tests that
> > throws the EXPLAIN output away, but actually enables paralellism.
> >
> > Greetings,
> >
> > Andres Freund
> >
> > [1] https://coverage.postgresql.org/src/backend/executor/execParallel.c.gcov.html
> 
> Please find the attached for the same.

> +-- to increase the parallel query test coverage
> +EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
> +                         QUERY PLAN                          
> +-------------------------------------------------------------
> + Gather (actual rows=10000 loops=1)
> +   Workers Planned: 4
> +   Workers Launched: 4
> +   ->  Parallel Seq Scan on tenk1 (actual rows=2000 loops=5)
> +(4 rows)

Is there an issue that we might not actually be able to start all four
workers?  Serious question, not rhetorical.

Regards,

Andres