Re: Oddity in EXPLAIN for foreign/custom join pushdown plans

On Tue, Jul 26, 2016 at 11:20 PM, Etsuro Fujita
<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> I noticed that currently the core doesn't show any information on the target
> relations involved in a foreign/custom join in EXPLAIN, by itself.

I think that's a feature, not a bug.

> postgres_fdw shows the target relations in the Relations line, as shown
> above, but I think that the core should show such information independently
> of FDWs; in the above example replace "Foreign Scan" with "Foreign Join on
> public.ft1 t1, public.ft2 t2".

I disagree with that. Currently, when we say that something is a join
(Merge Join, Hash Join, Nested Loop) we mean that the executor is
performing a join, but that's not the case here. The executor is
performing a scan. The remote side, we suppose, is performing a join
for us, but we are not performing a join: we are performing a scan.
So I think the fact that it shows up in the plan as "Foreign Scan" is
exactly right. We are scanning some foreign thing, and that thing may
internally be doing other stuff, like joins and aggregates, but all
we're doing is scanning it.

Also, I don't really see the point of moving this from postgres_fdw to
core. If, at some point in time, there are many FDWs that implement
sophisticated pushdown operations and we figure out that they are all
duplicating the code to do the EXPLAIN printout, and they're all
printing basically the same thing, perhaps not in an entirely
consistent way, then we could try to unify all of them into one
implementation in core. But that's certainly not where we are right
now. I don't see any harm at all in leaving this under the control of
the FDW, and in fact, I think it's better. Neither the postgres_fdw
format nor what you want to replace it with are so unambiguously
awesome that some other FDW author might not come up with something
better.

I think we should leave this the way it is.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company