[{"_id":1,"array":["a","b","c"]},{"_id":2,"array":["a","b","c"]},{"_id":3,"array":["x","a","b","c"]},{"_id":4,"array":["x","b","c","a"]}]

db.collection.aggregate([{"$match":{$expr:{"$eq":[{"$slice":["$array",2]},["a","b"]]}}}])