Right now a leaf node will have the "local" path, but not the full context. For example, in both of the following queries there will be an equality node with the path "foo":

{foo: 4}
{a: {$elemMatch: {foo: 4}}

In the second query, the full path for that equality is really "a.foo". As a result, a lot of planning logic has to manually keep track of the full path to accomodate this. For example this logic in QueryPlannerIXSelect::getFields()