The attached patch (against HEAD) implements

  COPY x (a,d,c,b) from stdin;
  COPY x (a,c) to stdout;

as well as the corresponding changes to pg_dump to use the new
functionality.  This functionality is not available when using
the BINARY option.  If a column is not specified in the COPY FROM
statement, its default values will be used.

In addition to this functionality, I tweaked a couple of the
error messages emitted by the new COPY <options> checks.

Brent Verner