On Wed, Aug 8, 2018 at 11:33 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Oh ... never mind that last. The parent Append will run its children > sequentially, so that the Gathers execute one at a time, and at no > point will more than 2 workers be active.
Yep. > Nonetheless, it's a damfool query plan, because we'll be going through > parallel worker startup/shutdown overhead 4 times for no benefit. > We really should put in some kind of logic to force those sibling > Gathers to be aggregated into one, or else disallow them altogether. Disallowing them could be a big performance regression. Combining them is reasonable, but it's not clear to me how you'd fit that into the planner structure. There's no convenient RelOptInfo to associate with the aggregated-Gather. It can't use the parent RelOptInfo unless all of the children have a partial path, and in that case this plan never would have been generated in the first place. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
