Re: EvalPlanQual behaves oddly for FDW queries involving system columns

On 2015/05/11 8:50, Tom Lane wrote:
> Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> writes:
>> [ EvalPlanQual-v6.patch ]
>
> I've started to study this in a little more detail, and I'm not terribly
> happy with some of the API decisions in it.

Thanks for taking the time to review the patch!

> In particular, I find the addition of "void *fdw_state" to ExecRowMark
> to be pretty questionable.  That does not seem like a good place to keep
> random state.  (I realize that WHERE CURRENT OF keeps some state in
> ExecRowMark, but that's a crock not something to emulate.)  ISTM that in
> most scenarios, the state that LockForeignRow/FetchForeignRow would like
> to get at is probably the FDW state associated with the ForeignScanState
> that the tuple came from.  Which this API doesn't help with particularly.
> I wonder if we should instead add a "ScanState*" field and expect the
> core code to set that up (ExecOpenScanRelation could do it with minor
> API changes...).

Sorry, I don't understand clearly what you mean, but that (the idea of
expecting the core to set it up) sounds inconsistent with your comment
on the earlier version of the API "BeginForeignFetch" [1].

> I'm also a bit tempted to pass the TIDs to LockForeignRow and
> FetchForeignRow as Datums not ItemPointers.  We have the Datum format
> available already at the call sites, so this is free as far as the core
> code is concerned, and would only cost another line or so for the FDWs.
> This is by no means sufficient to allow FDWs to use some other type than
> "tid" for row identifiers; but it would be a down payment on that problem,
> and at least would avoid nailing the rowids-are-tids assumption into yet
> another global API.

That is a good idea.

> Also, as I mentioned, I'd be a whole lot happier if we had a way to test
> this...

Attached is a postgres_fdw patch that I used for the testing.  If you
try it, edit postgresGetForeignRowMarkType as necessary.  I have to
confess that I did the testing only in the normal conditions by the patch.

Sorry for the delay.  I took a vacation until yesterday.

Best regards,
Etsuro Fujita

[1] http://www.postgresql.org/message-id/14504.1428446647@sss.pgh.pa.us