On Thu, Jul 7, 2016 at 1:23 PM, Fujii Masao <masao.fu...@gmail.com> wrote: > Hi, > > I found $SUBJECT while trying to test parallel queries. Is this a bug? > > > In not parallel mode, EXPLAIN ANALYZE reports the information about > Sort Method as follows. > > =# EXPLAIN ANALYZE SELECT * FROM pgbench_accounts ORDER BY bid; > QUERY PLAN > ----------------------------------------------------------------------------------------------------------------------------------- > Sort (cost=180739.34..183239.34 rows=1000000 width=97) (actual > time=1501.342..1836.057 rows=1000000 loops=1) > Sort Key: bid > Sort Method: external sort Disk: 104600kB > -> Seq Scan on pgbench_accounts (cost=0.00..26394.00 rows=1000000 > width=97) (actual time=0.013..179.315 rows=1000000 loops=1) > > > However, in parallel mode, it's not reported, as follows. > > =# SET force_parallel_mode TO on; > =# EXPLAIN ANALYZE SELECT * FROM pgbench_accounts ORDER BY bid; > QUERY > PLAN > ----------------------------------------------------------------------------------------------------------------------------------------- > Gather (cost=181739.34..284239.34 rows=1000000 width=97) (actual > time=1507.138..2394.028 rows=1000000 loops=1) > Workers Planned: 1 > Workers Launched: 1 > Single Copy: true > -> Sort (cost=180739.34..183239.34 rows=1000000 width=97) (actual > time=1503.112..1901.117 rows=1000000 loops=1) > Sort Key: bid > -> Seq Scan on pgbench_accounts (cost=0.00..26394.00 > rows=1000000 width=97) (actual time=0.021..181.079 rows=1000000 > loops=1) >
I think this can never happen for force_parallel_mode TO off, because we don't generate a gather on top of sort node. The reason why we are able to push Sort below gather, because it is marked as parallel_safe (create_sort_path). I think we should not mark it as parallel_safe. Will investigate some more and send a patch. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
