On Wed, Oct 28, 2015 at 3:55 PM, Kouhei Kaigai <kai...@ak.jp.nec.com> wrote: > At PGconf.EU, I could have a talk with Robert about this topic, > then it became clear we have same idea. > >> +--------+ >> |sub-plan | * Sub-Plan 1 ... Index Scan on p1 >> |index on *-----> * Sub-Plan 2 ... PartialSeqScan on p2 >> |shared | * Sub-Plan 2 ... PartialSeqScan on p2 >> |memory | * Sub-Plan 2 ... PartialSeqScan on p2 >> +---------+ * Sub-Plan 3 ... Index Scan on p3 >> > In the above example, I put non-parallel sub-plan to use only > 1 slot of the array, even though a PartialSeqScan takes 3 slots. > It is a strict rule; non-parallel aware sub-plan can be picked > up once. > The index of sub-plan array is initialized to 0, then increased > to 5 by each workers when it processes the parallel-aware Append. > So, once a worker takes non-parallel sub-plan, other worker can > never take the same slot again, thus, no duplicated rows will be > produced by non-parallel sub-plan in the parallel aware Append. > Also, this array structure will prevent too large number of > workers pick up a particular parallel aware sub-plan, because > PartialSeqScan occupies 3 slots; that means at most three workers > can pick up this sub-plan. If 1st worker took the IndexScan on > p1, and 2nd-4th worker took the PartialSeqScan on p2, then the > 5th worker (if any) will pick up the IndexScan on p3 even if > PartialSeqScan on p2 was not completed.
Actually, this is not exactly what I had in mind. I was thinking that we'd have a single array whose length is equal to the number of Append subplans, and each element of the array would be a count of the number of workers executing that subplan. So there wouldn't be multiple entries for the same subplan, as you propose here. To distinguish between parallel-aware and non-parallel-aware plans, I plan to put a Boolean flag in the plan itself. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
