Clone magic method.
Select queries have dependent objects that must be deep-cloned. The connection object itself, however, should not be cloned as that would duplicate the connection itself.
Overrides SelectQueryInterface::__clone
public function __clone() {
$this->uniqueIdentifier = uniqid('', TRUE);
// We need to deep-clone the query we're wrapping, which in turn may
// deep-clone other objects. Exciting!
$this->query = clone $this->query;
}