> 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. > I don't have strong preference here. Both of design can implement the requirement; none-parallel sub-plans are never picked up twice, and parallel-aware sub-plans can be picked up multiple times. So, I'll start with the above your suggestion.
Thanks, -- NEC Business Creation Division / PG-Strom Project KaiGai Kohei <kai...@ak.jp.nec.com> -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
