Re: Parallel INSERT (INTO ... SELECT ...)

On Thu, Oct 8, 2020 at 1:42 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
>
> On Wed, Oct 7, 2020 at 7:25 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> >
> > On Wed, Oct 7, 2020 at 12:40 AM Bharath Rupireddy
> > <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> > > In parallel, we are not doing anything(due to the same reason
> > > explained in above comment) to find whether there is a foreign
> > > partition or not while deciding to go with parallel/non-parallel copy,
> > > we are just throwing an error during the first tuple insertion into
> > > the partition.
> > >
> > > errmsg("cannot perform PARALLEL COPY if partition has BEFORE/INSTEAD
> > > OF triggers, or if the partition is foreign partition"),
> > > errhint("Try COPY without PARALLEL option")));
> > >
> >
> > I'm wondering whether code similar to the following can safely be used
> > to detect a foreign partition:
> >
> > if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
> > {
> > int i;
> > PartitionDesc pd = RelationGetPartitionDesc(rel);
> > for (i = 0; i < pd->nparts; i++)
> > {
> > if (get_rel_relkind(pd->oids[i]) == RELKIND_FOREIGN_TABLE)
> > {
> > table_close(rel, NoLock);
> > return false;
> > }
> > }
> > }
> >
>
> Actually, the addition of this kind of check is still not good enough.
> Partitions can have their own constraints, triggers, column default
> expressions etc. and a partition itself can be partitioned.
> I've written code to recursively walk the partitions and do all the
> various checks for parallel-insert-safety as before, but it's doing a
> fair bit of work.
> Any other idea of dealing with this? Seems it can't be avoided if you
> want to support partitioned tables and partitions.
>

IMHO, it's good not to do all of this recursive checking right now,
which may complicate the code or may restrict the performance gain.
Having said that, in future we may have to do something about it.

Others may have better opinions on this point.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com