With Look Ahead enabled, the user gets feedback on the number of objects that are returned from a relationship or edge expansion. This information is easily determined for relationships, but with edges, the server must expand each one to do the following things:
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Account for interim objects appearing at configurations that are not in scope.
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Apply property or interface criteria.
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Count the number of objects at the end of the edge.
A relationship is an object that is instantiated in the database, rather than a view of data with criteria. The code can count the number of relationship objects that are in scope without expanding them. Using Look Ahead on relationships rather than edges makes the process more efficient.