Re: FDW: possible resjunk columns in AddForeignUpdateTargets

2013/11/8 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at> writes:
>> What I would like to do is add a custom resjunk column
>> (e.g. a bytea) in AddForeignUpdateTargets that carries a row identifier
>> from the scan state to the modify state.
>> Would that be possible? Can I have anything else than a Var
>> in a resjunk column?
>
> [ thinks for awhile... ] Hm. In principle you can put any expression
> you want into the tlist during AddForeignUpdateTargets. However, if it's
> not a Var then the planner won't understand that it's something that needs
> to be supplied by the table scan, so things won't work right in any but
> the most trivial cases (maybe not even then :-().
>
> What I'd try is creating a Var that has the attno of ctid
> (ie, SelfItemPointerAttributeNumber) but the datatype you want, ie bytea.
> This won't match what the catalogs say your table's ctid is, but I think
> that nothing will care much about that.

Apologies for reinvigorating this thread, but I'm running into a similar wall
myself and would like to clarify if this approach will work at all.

My foreign data source is returning a fixed-length string as a unique row
identifier; in AddForeignUpdateTargets() I can create a Var like this:

var = makeVar(parsetree->resultRelation,
SelfItemPointerAttributeNumber,
BPCHAROID,
32,
InvalidOid,
0);

but is it possible to store something other than a TIDOID here, and if so how?

Regards

Ian Barwick