From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> |
Cc: | Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Odd system-column handling in postgres_fdw join pushdown patch |
Date: | 2016-04-13 19:57:31 |
Message-ID: | CA+TgmoYPcSekUytMQScXHCD6BG9g1ofxrAWqxR1danSvR7J76w@mail.gmail.com |
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Lists: | pgsql-hackers |
On Wed, Apr 13, 2016 at 2:36 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Apr 13, 2016 at 2:11 PM, Robert Haas <robertmhaas(at)gmail(dot)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
Attachment | Content-Type | Size |
---|---|---|
postgres-fdw-syscol-zap.patch | application/x-patch | 2.3 KB |
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