On Tue, Jul 26, 2016 at 11:20 PM, Etsuro Fujita <fujita.ets...@lab.ntt.co.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 -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers