The answer sheet explained the following. But why can’t one use regular expression ‘' to match the direct reports. like '/Stuart\ Spencer/’ ?
"The Materialized Paths approach falls short on:
Collect all direct reports of an employee with one single and efficient query We can use a regular expression to match all reports of a given employee. However, this regular expression query, to be efficient and use a supporting index, requires the prepending of left end side of the reports_to values. In this particular case, if we do not have the information of the root of the organization chart, the CEO name, we would not be able to create an efficient query"