Suppose we have indices on 'a', 'b', and we receive the following query:

db.coll.find({$or: [{a: 1, b: 1}, {a: 2, b: 2}]});

We could end up enumerating 9 possible solutions, as there are three possibilities for each branch (use 'a', use 'b', and use index intersection). If all combinations are considered than 3 * 3 = 9. This is the search space explosion problem for $or that is currently being tracked in SERVER-13184.

Regardless of the approach taken for SERVER-13184, we can mitigate the problem by not enumerating index intersection solutions if the AND is not top-level. For the example above, this would reduce the number of possibilities to 2 * 2 = 4.