Re: EvalPlanQual behaves oddly for FDW queries involving system columns

On 2015/03/11 17:37, Ashutosh Bapat wrote:
> Now I can reproduce the problem.
>
> Sanity
> --------
> Patch compiles cleanly and make check passes. The tests in file_fdw and
> postgres_fdw contrib modules pass.
>
> The patch works as expected in the test case reported.

Thanks for the testing!

> I have only one doubt.
> In EvalPlanQualFetchRowMarks(). tuple->t_self is assigned from
> td->t_ctid. CTID or even t_self may be valid for foreign tables based on
> postgres_fdw but may not be valid for other FDWs. So, this assignment
> might put some garbage in t_self, rather we should set it to invalid as
> done prior to the patch. I might have missed some previous thread, we
> decided to go this route, so ignore the comment, in that case.

Good point. As the following code and comment I added to ForeignNext, I
think that FDW authors should initialize the tup->t_data->t_ctid of each
scan tuple with its own TID. If the authors do that, the t_self is
guaranteed to be assigned the right TID from the whole-row Var (ie,
td->t_ctid) in EvalPlanQualFetchRowMarks.

/* We assume that t_ctid is initialized with its own TID */
tup->t_self = tup->t_data->t_ctid;

IMHO, I'm not sure it's worth complicating the code as you mentioned.
(I don't know whether there are any discussions about this before.)

Note that file_fdw needs no treatment. In that case, in ForeignNext,
the tup->t_data->t_ctid of each scan tuple is initialized with (0,0) (if
necessary), and then the t_self will be correctly assigned (0,0) throguh
the whole-row Var in EvalPlanQualFetchRowMarks. So, no inconsistency!

> Apart from this, I do not have any comments here.

Thanks again.

Best regards,
Etsuro Fujita