Re: Parallel Seq Scan

Okay, with my pgbench_accounts partitioned into 300, I ran:

SELECT DISTINCT bid FROM pgbench_accounts;

The query never returns, and I also get this:

grep -r 'starting background worker process "parallel worker for PID 12165"' postgresql-2015-03-25_112522.log  | wc -l
2496

2,496 workers?  This is with parallel_seqscan_degree set to 8.  If I set it to 2, this number goes down to 626, and with 16, goes up to 4320.

Here's the query plan:

                                               QUERY PLAN                                               
---------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=38856527.50..38856529.50 rows=200 width=4)
   Group Key: pgbench_accounts.bid
   ->  Append  (cost=0.00..38806370.00 rows=20063001 width=4)
         ->  Seq Scan on pgbench_accounts  (cost=0.00..0.00 rows=1 width=4)
         ->  Funnel on pgbench_accounts_1  (cost=0.00..192333.33 rows=100000 width=4)
               Number of Workers: 8
               ->  Partial Seq Scan on pgbench_accounts_1  (cost=0.00..1641000.00 rows=100000 width=4)
         ->  Funnel on pgbench_accounts_2  (cost=0.00..192333.33 rows=100000 width=4)
               Number of Workers: 8
               ->  Partial Seq Scan on pgbench_accounts_2  (cost=0.00..1641000.00 rows=100000 width=4)
         ->  Funnel on pgbench_accounts_3  (cost=0.00..192333.33 rows=100000 width=4)
               Number of Workers: 8
...
               ->  Partial Seq Scan on pgbench_accounts_498  (cost=0.00..10002.10 rows=210 width=4)
         ->  Funnel on pgbench_accounts_499  (cost=0.00..1132.34 rows=210 width=4)
               Number of Workers: 8
               ->  Partial Seq Scan on pgbench_accounts_499  (cost=0.00..10002.10 rows=210 width=4)
         ->  Funnel on pgbench_accounts_500  (cost=0.00..1132.34 rows=210 width=4)
               Number of Workers: 8
               ->  Partial Seq Scan on pgbench_accounts_500  (cost=0.00..10002.10 rows=210 width=4)

Still not sure why 8 workers are needed for each partial scan.  I would expect 8 workers to be used for 8 separate scans.  Perhaps this is just my misunderstanding of how this feature works.

--