$ST=$DBH->prepare(<<EOSQL);
if not exists (select * from tbl where hostname=?)
   insert tbl (hostname, timestamp) values (?, ?)
else
   if not exists (select * from tbl where hostname=? and timestamp>?)
      update tbl set timestamp=? where hostname=?
EOSQL

$ST->execute($hostname, $hostname, $timestamp, $hostname, $timestamp,
             $timestamp, $hostname);