I rethinked on this from the first.
> Sorry.
>
> > Hi, this looks to be a bug of cost_index(). The attached patch
> > would fix that.
>
> No, that's wrong. please forget the patch. The qual in qpquals
> should be indexquals which is excluded because it is not
> necessary to be applied. The right way would be remove the cost
> for qpqual in cost_index().
Your patch allows index only scan even if a qual contains
non-index column when the qual can be removed by implication from
index predicates.
So the 'implied' clauses is not needed ever after. It should be
excluded from cost estimation and it is not needed on execution
even if index only scan is found not to be doable finally.
So the implicit quals may be removed on building index paths but
I think check_index_only is not the place.
Removing implied quals from index quals is not only for index
*only* scan so the place for removing such quals is in
build_index_paths, in the loop of step 1. After removing the
quals there, check_index_only will naturally give disired result.
# I remember that I have tried the same or similar thing. I don't
# recall what made me give up then.
regards,
--
Kyotaro Horiguchi
NTT Open Source Software Center