When you use the IN HIERARCHY query language clause, always remember that if you pass a null reference to the parameter, query execution will be slowed because the system will check each item to find out whether it belongs to the hierarchy root. To prevent this, the IN HIERARCHY condition should only be used with a specified item (group), so it is the best choice to make such fields the required ones. If the parameter is not specified (is a null reference), it is better to remove this condition from the query altogether.
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