On Sat, Mar 5, 2016 at 10:11 PM, Tom Lane <
tgl@sss.pgh.pa.us> wrote:
>
> Amit Kapila <
amit.kapila16@gmail.com> writes:
> > On Fri, Mar 4, 2016 at 11:31 PM, Tom Lane <
tgl@sss.pgh.pa.us> wrote:
> >> (BTW, I found what seemed to be a couple of pre-existing bugs of
> >> the same kind, eg create_mergejoin_path was different from the
> >> other two kinds of join as to setting parallel_degree.)
>
> > I think the reason for keeping parallel_degree as zero for mergejoin path
> > is that currently it can't participate in parallelism.
>
> Is there some reason why hash and nestloop are safe but merge isn't?
>
I think it is because we consider to pushdown hash and nestloop to workers, but not merge join and the reason for not pushing mergejoin is that currently we don't have executor support for same, more work is needed there. I think even if we set parallel_degree as you are doing in patch for merge join is harmless, but ideally there is no need to set it as far as what we support today in terms of parallelism.
