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$ST=$DBH->prepare(<<EOSQL);
if not exists (select * from tbl where timestamp > ?)
...
EOSQL

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

$ST=$DBH->prepare(<<EOSQL);
if not exists (select * from tbl where hostname=?)
...

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