Re: Extracting only the columns needed for a query

> Approach B: after parsing and/or after planning

If we wanted to do something about this, making the planner record
the set of used columns seems like the thing to do.  We could avoid
the expense of doing it when it's not needed by setting up an AM/FDW/
etc property or callback to request it.

Another reason for having the planner do this is that presumably, in
an AM that's excited about this, the set of fetched columns should
play into the cost estimates for the scan.  I've not been paying
enough attention to the tableam work to know if we've got hooks for
the AM to affect scan costing ... but if we don't, that seems like
a hole that needs plugged.

>     Approach B, however, does not work for utility statements which do
>     not go through planning.

I'm not sure why you're excited about that case?  Utility statements
tend to be pretty much all-or-nothing as far as data access goes.