Re: Parallel Seq Scan - Mailing list pgsql-hackers

On 18 December 2014 at 16:03, Amit Kapila <amit.kapila16@gmail.com> wrote:

On Thu, Dec 18, 2014 at 9:22 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Mon, Dec 8, 2014 at 10:40 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Sat, Dec 6, 2014 at 5:37 PM, Stephen Frost <sfrost@snowman.net> wrote:
> > >
> >
> > So to summarize my understanding, below are the set of things
> > which I should work on and in the order they are listed.
> >
> > 1. Push down qualification
> > 2. Performance Data
> > 3. Improve the way to push down the information related to worker.
> > 4. Dynamic allocation of work for workers.
> >
> >
>
> I have worked on the patch to accomplish above mentioned points
> 1, 2 and partly 3 and would like to share the progress with community.

Sorry forgot to attach updated patch in last mail, attaching it now.

When attempting to recreate the plan in your example, I get an error:

 ➤ psql://thom@[local]:5488/pgbench

# create table t1(c1 int, c2 char(500)) with (fillfactor=10);
CREATE TABLE
Time: 13.653 ms

 ➤ psql://thom@[local]:5488/pgbench

# insert into t1 values(generate_series(1,100),'amit');
INSERT 0 100
Time: 4.796 ms

 ➤ psql://thom@[local]:5488/pgbench

# explain select c1 from t1;
ERROR:  could not register background process
HINT:  You may need to increase max_worker_processes.
Time: 1.659 ms

 ➤ psql://thom@[local]:5488/pgbench

# show max_worker_processes ;
 max_worker_processes
----------------------
 8
(1 row)

Time: 0.199 ms

# show parallel_seqscan_degree ;
 parallel_seqscan_degree
-------------------------
 10
(1 row)

Should I really need to increase max_worker_processes to >= parallel_seqscan_degree?  If so, shouldn't there be a hint here along with the error message pointing this out?  And should the error be produced when only a *plan* is being requested?

Also, I noticed that where a table is partitioned, the plan isn't parallelised:

# explain select distinct bid from pgbench_accounts;


                                       QUERY PLAN                                      
----------------------------------------------------------------------------------------
 HashAggregate  (cost=1446639.00..1446643.99 rows=499 width=4)
   Group Key: pgbench_accounts.bid
   ->  Append  (cost=0.00..1321639.00 rows=50000001 width=4)
         ->  Seq Scan on pgbench_accounts  (cost=0.00..0.00 rows=1 width=4)
         ->  Seq Scan on pgbench_accounts_1  (cost=0.00..4279.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_2  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_3  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_4  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_5  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_6  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_7  (cost=0.00..2640.00 rows=100000 width=4)
...
         ->  Seq Scan on pgbench_accounts_498  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_499  (cost=0.00..2640.00 rows=100000 width=4)
         ->  Seq Scan on pgbench_accounts_500  (cost=0.00..2640.00 rows=100000 width=4)
(504 rows)

Is this expected?