Verena Ruff <lists@triosolutions.at> writes:
> But I define a new primary key constraint in the child table, using the
> column frm the parent. Shouldn't create this an index implicitly?
Yes, and I'm sure it did. The more relevant question is how much data
do you have in the child table? The planner generally won't bother with
an indexscan if the table is only one or two disk pages; trying to use
the index in such cases would end up fetching *more* disk pages overall.
regards, tom lane