# Store
my %storage;
foreach my $row (@$result_set) {
    my $hash_key = encode_hash_key(row);
    my $new_row = $row;
    if (exists $storage{$hash_key}) {
        $new_row = merge_rows($row, $storage{$hash_key});
    }
    $storage{$hash_key} = $new_row;
}