Re: Problem while updating a foreign table pointing to a partitioned table on foreign server

From: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: ashutosh(dot)bapat(at)enterprisedb(dot)com, tgl(at)sss(dot)pgh(dot)pa(dot)us, alvherre(at)2ndquadrant(dot)com, robertmhaas(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Problem while updating a foreign table pointing to a partitioned table on foreign server
Date: 2018-08-14 11:49:02
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(2018/08/09 22:04), Etsuro Fujita wrote:
> (2018/08/08 17:30), Kyotaro HORIGUCHI wrote:
>> I choosed to expand tuple descriptor for junk column added to
>> foreign relaions. We might be better to have new member in
>> ForeignScan but I didn't so that we can backpatch it.
> I've not looked at the patch closely yet, but I'm not sure that it's a
> good idea to expand the tuple descriptor of the target relation on the
> fly so that it contains the remotetableoid as a non-system attribute of
> the target table. My concern is: is there not any risk in affecting some
> other part of the planner and/or the executor? (I was a bit surprised
> that the patch passes the regression tests successfully.)

I spent more time looking at the patch. ISTM that the patch well
suppresses the effect of the tuple-descriptor expansion by making
changes to code in the planner and executor (and ruleutils.c), but I'm
still not sure that the patch is the right direction to go in, because
ISTM that expanding the tuple descriptor on the fly might be a wart.

>> What the patch does are:
>> - This abuses ForeignScan->fdw_scan_tlist to store the additional
>> junk columns when foreign simple relation scan (that is, not a
>> join).
> I think that this issue was introduced in 9.3, which added postgres_fdw
> in combination with support for writable foreign tables, but
> fdw_scan_tlist was added to 9.5 as part of join-pushdown infrastructure,
> so my concern is that we might not be able to backpatch your patch to
> 9.3 and 9.4.

Another concern about this:

You wrote:
> Several places seems to be assuming that fdw_scan_tlist may be
> used foreign scan on simple relation but I didn't find that
> actually happens.

Yeah, currently, postgres_fdw and file_fdw don't use that list for
simple foreign table scans, but it could be used to improve the
efficiency for those scans, as explained in fdwhandler.sgml:

Another <structname>ForeignScan</structname> field that can be
filled by FDWs
is <structfield>fdw_scan_tlist</structfield>, which describes the
tuples returned by
the FDW for this plan node. For simple foreign table scans this
can be
set to <literal>NIL</literal>, implying that the returned tuples
have the
row type declared for the foreign table. A
non-<symbol>NIL</symbol> value must be a
target list (list of <structname>TargetEntry</structname>s)
containing Vars and/or
expressions representing the returned columns. This might be
used, for
example, to show that the FDW has omitted some columns that it noticed
won't be needed for the query. Also, if the FDW can compute
used by the query more cheaply than can be done locally, it could add
those expressions to <structfield>fdw_scan_tlist</structfield>.
Note that join
plans (created from paths made by
<function>GetForeignJoinPaths</function>) must
always supply <structfield>fdw_scan_tlist</structfield> to
describe the set of
columns they will return.

You wrote:
> I'm not sure whether the following ponits are valid.
> - If fdw_scan_tlist is used for simple relation scans, this would
> break the case. (ExecInitForeignScan, set_foreignscan_references)

Some FDWs might already use that list for the improved efficiency for
simple foreign table scans as explained above, so we should avoid
breaking that.

If we take the Param-based approach suggested by Tom, I suspect there
would be no need to worry about at least those things, so I'll try to
update your patch as such, if there are no objections from you (or
anyone else).

Best regards,
Etsuro Fujita

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