TABLES      => ['column1 TableA, 'column1 TableB'],
         JOIN_ON     => 'TableA.column2 = TableB.column2',
         CONSTRAINTS => {
             'TableA.username'            => '__CREDENTIAL_1__',
             'md5_base64:salt:password' => '__CREDENTIAL_2__', 
             'TableB.column3'            => __CREDENTIAL_3__',
         }

FILTERS => { salt => \&salt_filter },
                ],

 sub salt_filter {
     my $param = shift;
     my $value = shift;
     $value = $dbh->prepare ("SELECT salt FROM TableB JOIN TableA
ON TableB.column2 = TableA.column2 ");
     return $value;
 }