Re: postgres_fdw join pushdown (was Re: Custom/Foreign-Join-APIs)

On Thu, Dec 10, 2015 at 11:20 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Dec 8, 2015 at 6:40 AM, Etsuro Fujita
> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> > IMO I want to see the EvalPlanQual fix in the first version for 9.6.
>
> +1.
>
> I think there is still a lot functionality that is offered without
EvalPlanQual fix. As long as we do not push joins when there are RowMarks
involved, implementation of that hook is not required. We won't be able to
push down joins for DMLs and when there are FOR SHARE/UPDATE clauses in the
query. And there are huge number of queries, which will be benefitted by
the push down even without that support. There's nothing in this patch,
which comes in way of implementing the EvalPlanQual fix. It can be easily
added after committing the first version. On the other hand, getting
minimal (it's not really minimal, it's much more than that) support for
postgres_fdw support committed opens up possibility to work on multiple
items (as listed in my mail) in parallel.

I am not saying that we do not need EvalPlanQual fix in 9.6. But it's not
needed in the first cut. If we get the first cut in first couple of months
of 2016, there's plenty of room for the fix to go in 9.6. It would be
really bad situation if we could not get postgres_fdw join pushdown
supported in 9.6 because EvalPlanQual hook could not be committed while the
rest of the code is ready. EvalPlanQual fix in core was being discussed
since April 2015. It took 8 months to get that fixed. Hopefully we won't
need that long to implement the hook in postgres_fdw, but that number says
something about the complexity of the feature.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company