Compound index selection in query planner is broken

[apkar]

Document Layer allows users to create compound indexes and updates them properly. But, the query planner is not yet using them. Query planner uses them just like Simple indexes. For example, if we have a compound index on fields a and b, query planner treats the index as a Simple Index on a. So, if there is a query with predicate a == "foo" and b == "bar" - query planner would scan the index with bounds foo, foo0, and runs FilterPlan on it to look for bar. Instead, it should scan with bounds foo:bar, foo:bar0.

[apkar]

In [2]: coll = db['test_comp_ind']

In [3]: coll.drop_indexes()

In [4]: coll.create_index([('a', pymongo.ASCENDING), ('b', pymongo.ASCENDING), ('c', pymongo.ASCENDING)])
Out[4]: u'a_1_b_1_c_1'

In [5]: coll.find({'a': 'aaa', 'b': 'bbb'}).explain()
Out[5]:
{u'explanation': {u'source_plan': {u'projection': u'{}',
   u'source_plan': {u'filter': u'ANY(ExtPath((b\\x00) matching EQUALS(\'"bbb"\'))',
    u'source_plan': {u'bounds': {u'begin': u'(aaa\\x00', u'end': u'(aaa\\x00'},
     u'index name': u'a_1_b_1_c_1',
     u'type': u'index scan'},
    u'type': u'filter'},
   u'type': u'projection'},
  u'type': u'non-isolated'}}

In [6]: coll.find({'a': 'aaa', 'b': 'bbb', 'c': 'ccc'}).explain()
Out[6]:
{u'explanation': {u'source_plan': {u'projection': u'{}',
   u'source_plan': {u'filter': u'AND(ANY(ExtPath((c\\x00) matching EQUALS(\'"ccc"\')), ANY(ExtPath((b\\x00) matching EQUALS(\'"bbb"\')))',
    u'source_plan': {u'bounds': {u'begin': u'(aaa\\x00', u'end': u'(aaa\\x00'},
     u'index name': u'a_1_b_1_c_1',
     u'type': u'index scan'},
    u'type': u'filter'},
   u'type': u'projection'},
  u'type': u'non-isolated'}}

In [7]:

Both queries should use the index a_1_b_1_c_1. Bug is in prefix matching, its always matching only the first key.