Re: Extracting only the columns needed for a query

In general implemented functionality looks good. I've checked how it
works on the existing tests, almost everywhere required columns were not
missing in scanCols (which is probably the most important part).
Sometimes exressions were checked multiple times, which could
potentially introduce some overhead, but I believe this effect is
negligible. Just to mention some counterintuitive examples, for this
kind of query

    SELECT min(q1) FROM INT8_TBL;

the same column was checked 5 times in my tests, since it's present also
in baserestrictinfo, and build_minmax_path does one extra round of
planning and invoking make_one_rel.

Another interesting
example is Values Scan (e.g. in an insert statements with multiple
records), can an abstract table AM user leverage information about
columns in it?

One case, where I believe columns were missing, is statements with
returning:

    INSERT INTO foo (col1)
      VALUES ('col1'), ('col2'), ('col3')
      RETURNING *;

Looks like in this situation there is only expression in reltarget is
for col1, but returning result contains all columns.