/*
 * call-seq:
 *    res.ftablecol( column_number ) -> Fixnum
 *
 * Returns the column number (within its table) of the table from 
 * which the column _column_number_ is made up.
 *
 * Raises ArgumentError if _column_number_ is out of range or if
 * the column number from its table is undefined for that column.
 */
static VALUE
pgresult_ftablecol(VALUE self, VALUE column_number)
{
    int col_number = NUM2INT(column_number);
    PGresult *pgresult = get_pgresult(self);

    int n;

    if( col_number < 0 || col_number >= PQnfields(pgresult)) 
        rb_raise(rb_eArgError,"Invalid column index: %d", col_number);

    n = PQftablecol(pgresult, col_number);
    return INT2FIX(n);
}