OO enhancements

Tom, and others...

I've made modifications to libpq to accept multiple return type groups,
and now the thing to do is to make the back-end generate them from OO
hierarchies. Do you have any thoughts on the general approach to do
this? Should I be doing something like expandAll in parse_relation.c to
cater for different types? Do I play with ExecProject? Can anybody
remember anything about how postgres originally handled differing return
types?

Chris.