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

On Wed, Apr 13, 2016 at 2:36 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Apr 13, 2016 at 2:11 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> So, clearly that's not good.  It should at least be consistent.  But
>> more than that, the fact that postgres_fdw sets the xmax to 0xffffffff
>> is also pretty wacky.  We might use such a value as a sentinel for
>> some data type, but for transaction IDs that's just some random normal
>> transaction ID, and it's NOT coming from t1.  I haven't tracked down
>> where it *is* coming from yet, but can't imagine it's any place very
>> principled.
>
> And, yeah, it's not very principled.
>
> rhaas=# select ft1.xmin, ft1.xmax, ft1.cmin from ft1;
>  xmin |    xmax    | cmin
> ------+------------+-------
>    96 | 4294967295 | 16392
>    96 | 4294967295 | 16392
>    96 | 4294967295 | 16392
>    96 | 4294967295 | 16392
> (4 rows)
>
> What's happening here is that heap_getattr() is being applied to a
> HeapTupleHeaderData which contains DatumTupleFields.  So 96 is
> datum_len_, 4294967295 is the -1 recorded in datum_typmod, and 16392
> is the compose type OID recorded in datum_typeid, which happens in
> this case to be the OID of ft1.  Isn't that special?
>
> It's hard for me to view this as anything other than a bug in
> postgres_fdw - which of course means that this open item boils down to
> the complaint that the way system columns are handled by join pushdown
> isn't bug-compatible with the existing behavior....

OK, here's a patch.  What I did is:

1. For a regular FDW scan, zero the xmin, xmax, and cid of the tuple
before returning it from postgres_fdw, so that we don't expose the
datum-tuple fields.   I can't see any reason this isn't safe, but I
might be missing something.

2. When a join is pushed down, deparse system columns using something
like "CASE WHEN r1.* IS NOT NULL THEN 0 END", except for the table OID
column, which gets deparsed with the table OID in place of 0.  This
delivers the correct behavior in the presence of outer joins.

Review appreciated.

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