MCE::Loop::init {
   max_workers => 3, chunk_size => 1000,
   input_data => db_iter(),

   user_begin => sub {
      my ($mce) = @_;
      my $dsn = "DBI:Oracle:host=db_server;port=db_port;sid=db_name";
      $mce->{dbh} = DBI->connect($dsn, 'db_user', 'db_passwd') ||
          die "Could not connect to database: $DBI::errstr";
   },

   user_end => sub {
      my ($mce) = @_;
      $mce->{dbh}->disconnect;
   },
};

mce_loop {
   my ($mce, $chunk_ref, $chunk_id) = @_;
   my $dbh = $mce->{dbh}; my $ret = '';

   foreach my $row (@{ $chunk_ref }) {
      $ret .= $row->[$i_color] .": ". $row->[$i_desc] ."\n";
   }

   MCE->print($ret);
};