my $query = $self->{dbh}->prepare("select * from $database.$self->{hit_table} where time between ? and ? order by visit_id, time", {'mysql_use_result' => $self->{optimize}});
$query->execute($start_time + 0, $end_time + 0);
while (my $row = $query->fetchrow_hashref())
{
   # Do calculation
}