my %want;
$want{$_} = 1 for map { keys %{$self->_column_groups->{$_}} } @groups;
if (my @want = grep { !exists $self->{'_column_data'}{$_} } keys %want) {
+ #warn "@want";
my $sth = $self->_get_sth('select', \@want, $self->_table_name,
$self->_ident_cond);
$sth->execute($self->_ident_values);