Re: Odd system-column handling in postgres_fdw join pushdown patch

With this patch, all instances of tableoid, cmin, cmax etc. will get a
non-NULL value irrespective of whether they appear on nullable side of the
join or not.

e.g. select t1.c1, t1.tableoid, t2.c1, t2.tableoid from ft4 t1 left join
ft5 t2 on (t1.c1 = t2.c1); run in contrib_regression gives output
c1 | tableoid | c1 | tableoid
-----+----------+----+----------
2 | 54282 | | 54285
4 | 54282 | | 54285
6 | 54282 | 6 | 54285
8 | 54282 | | 54285
10 | 54282 | | 54285
12 | 54282 | 12 | 54285

but the same query run on local tables select t1.c1, t1.tableoid, t2.c1,
t2.tableoid from "S 1"."T 3" t1 left join "S 1"."T 4" t2 on (t1.c1 =
t2.c1); gives output
c1 | tableoid | c1 | tableoid
-----+----------+----+----------
2 | 54258 | |
4 | 54258 | |
6 | 54258 | 6 | 54266
8 | 54258 | |
10 | 54258 | |
12 | 54258 | 12 | 54266

BTW, why do we want to set the column values with invalid values, and not
null? Wouldn't setting them NULL will be a better way?

On Tue, Apr 5, 2016 at 12:11 PM, Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
wrote:

> On 2016/03/29 15:37, Etsuro Fujita wrote:
>
>> I added two helper functions: GetFdwScanTupleExtraData and
>> FillFdwScanTupleSysAttrs. The FDW author could use the former to get
>> info about system attributes other than ctids and oids in fdw_scan_tlist
>> during BeginForeignScan, and the latter to set values for these system
>> attributes during IterateForeignScan (InvalidTransactionId for
>> xmins/xmaxs, InvalidCommandId for cmins/cmaxs, and valid values for
>> tableoids). Attached is a proposed patch for that. I also slightly
>> simplified the changes to make_tuple_from_result_row and
>> conversion_error_callback made by the postgres_fdw join pushdown patch.
>> What do you think about that?
>>
>
> I revised comments a little bit. Attached is an updated version of the
> patch. I think this issue should be fixed in advance of the PostgreSQL
> 9.6beta1 release.
>
> Best regards,
> Etsuro Fujita
>

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