HeapTuple->t_tableOid==0 after SPI_exec

Hi,

i notice that when HeapTuple data are populated by a trigger
then the table oid can be retrieved from HeapTuple->t_tableOid.

When HeapTuple is populated by 
SPI_exec("select * from foobar when id=667");
tuple = SPI_tuptable->tvals[0] (id is PK and row with 667 exists)
then tuple->t_tableOid is always 0.

Is it a known issue??
Am i missing something?

-Achilleus