On Thu, Jan 31, 2019 at 6:00 PM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> > - 0003 doesn't have any handling for parallel query at this point, so
> > even though within a single backend a single query execution will
> > always get the same PartitionDesc for the same relation, the answers
> > might not be consistent across the parallel group.
>
> That doesn't sound good. I think the easiest would be to just serialize
> the PartitionDesc and send it to the workers instead of them recomputing
> it, but then I worry that this might have bad performance when the
> partition desc is large. (Or maybe sending bytes over pqmq is faster
> than reading all those catalog entries and so this isn't a concern
> anyway.)
I don't think we'd be using pqmq here, or shm_mq either, but I think
the bigger issues is that starting a parallel query is already a
pretty heavy operation, and so the added overhead of this is probably
not very noticeable. I agree that it seems a bit expensive, but since
we're already waiting for the postmaster to fork() a new process which
then has to initialize itself, this probably won't break the bank.
What bothers me more is that it's adding a substantial amount of code
that could very well contain bugs to fix something that isn't clearly
a problem in the first place.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company