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CREATE TABLE message (
  message_id int(11) DEFAULT '0' NOT NULL auto_increment,
...
  PRIMARY KEY (message_id),
  KEY for_user (for_user)
);

  my $expiration_seconds = 500;
  $dbh->prepare("SELECT * FROM message 
     WHERE for_user=0 AND unix_timestamp(now())-unix_timestamp(tstamp)
+ < $expiration_seconds ORDER BY tstamp");

  my $UID = getUserIdSomehow();
  $dbh->prepare("SELECT * FROM message 
     WHERE for_user=$UID ORDER BY tstamp");