Robert Haas <robertmh...@gmail.com> writes: > On Tue, Feb 23, 2016 at 2:06 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> Robert Haas <robertmh...@gmail.com> writes: >>> Foreign tables are supposed to be categorically excluded from >>> parallelism. Not sure why that's not working in this instance.
>> BTW, I wonder where you think that's supposed to be enforced, because >> I sure can't find any such logic. > RTEs are checked in set_rel_consider_parallel(), and I thought there > was a check there related to foreign tables, but there isn't. Oops. Even if there were, it would not fix this bug, because AFAICS the only thing that set_rel_consider_parallel is chartered to do is set the per-relation consider_parallel flag. The failure that is happening in that regression test with force_parallel_mode turned on happens because standard_planner plasters a Gather node at the top of the plan, causing the whole plan including the FDW access to happen inside a parallel worker. The only way to prevent that is to clear the wholePlanParallelSafe flag, which as far as I can tell (not that any of this is documented worth a damn) isn't something that set_rel_consider_parallel is supposed to do. It looks to me like there is a good deal of fuzzy thinking here about the difference between locally parallelizable and globally parallelizable plans, ie Gather at the top vs Gather somewhere else. I also note with dismay that turning force_parallel_mode on seems to pretty much disable any testing of local parallelism. > In view of 69d34408e5e7adcef8ef2f4e9c4f2919637e9a06, we shouldn't > blindly assume that foreign scans are not parallel-safe, but we can't > blindly assume the opposite either. Maybe we should assume that the > foreign scan is parallel-safe only if one or more of the new methods > introduced by the aforementioned commit are set, but actually that > doesn't seem quite right. That would tell us whether the scan itself > can be parallelized, not whether it's safe to run serially but within > a parallel worker. I think maybe we need a new FDW API that gets > called from set_rel_consider_parallel() with the root, rel, and rte as > arguments and which can return a Boolean. If the callback is not set, > assume false. Meh. As things stand, postgres_fdw would have to aver that it can't ever be safely parallelized, which doesn't seem like a very satisfactory answer even if there are other FDWs that work differently (and which would those be? None that use a socket-style connection to an external server.) The commit you mention above seems to me to highlight the dangers of accepting hook patches with no working use-case to back them up. AFAICT it's basically useless for typical FDWs because of this multiple-connection problem. regards, tom lane -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers