On Mon, Dec 7, 2020 at 11:32 AM Hou, Zhijie <houzj.f...@cn.fujitsu.com> wrote: > > Hi > > + /* > + * Flag to let the planner know that the SELECT query is for CTAS. > This is > + * used to calculate the tuple transfer cost from workers to gather > node(in > + * case parallelism kicks in for the SELECT part of the CTAS), to > zero as > + * each worker will insert its share of tuples in parallel. > + */ > + if (IsParallelInsertInCTASAllowed(into, NULL)) > + query->isForCTAS = true; > > > + /* > + * We do not compute the parallel_tuple_cost for CTAS because the > number of > + * tuples that are transferred from workers to the gather node is > zero as > + * each worker, in parallel, inserts the tuples that are resulted > from its > + * chunk of plan execution. This change may make the parallel plan > cheap > + * among all other plans, and influence the planner to consider this > + * parallel plan. > + */ > + if (!(root->parse->isForCTAS && > + root->query_level == 1)) > + run_cost += parallel_tuple_cost * path->path.rows; > > I noticed that the parallel_tuple_cost will still be ignored, > When Gather is not the top node. > > Example: > Create table test(i int); > insert into test values(generate_series(1,10000000,1)); > explain create table ntest3 as select * from test where i < 200 limit > 10000; > > QUERY PLAN > ------------------------------------------------------------------------------- > Limit (cost=1000.00..97331.33 rows=1000 width=4) > -> Gather (cost=1000.00..97331.33 rows=1000 width=4) > Workers Planned: 2 > -> Parallel Seq Scan on test (cost=0.00..96331.33 rows=417 width=4) > Filter: (i < 200) > > > The isForCTAS will be true because [create table as], the > query_level is always 1 because there is no subquery. > So even if gather is not the top node, parallel cost will still be ignored. > > Is that works as expected ? >
I don't think that is expected and is not the case without this patch. The cost shouldn't be changed for existing cases where the write is not pushed to workers. -- With Regards, Amit Kapila.
