title: mongodb runs slow on IXSCAN
body: <p>i cannot figure out with some requests in mongodb, i created an index which match all fields from request but it didn't help</p>
  
  <p>is the problem in index ? which fields i should include in index?
  here is a part of slowlog</p>
  
  <pre><code>query: { 
  $query: { 
      moderation.blocked: false, 
      project_id: "9fd6db37-049f-4af3-bea4-8301345dc109", 
      dates.posted: { $gt: 1458720426.177019 }, 
      parent_ids: "31cd1be3-adbd-4108-a05e-58c283176738", 
      moderation.visible: true }, 
      $orderby: { dates.posted: 1 } 
  } 
  planSummary: IXSCAN { 
      parent_ids: 1, 
      dates.posted: 1, 
      owner_id: 1, 
      project_id: 1, 
      moderation.blocked: 1, 
      moderation.visible: -1 
  } 
  ntoreturn:0 
  ntoskip:0 
  nscanned:0 
  nscannedObjects:0 
  keyUpdates:0 
  writeConflicts:0 
  numYields:1 
  nreturned:0 
  reslen:20 
  locks:{ Global: { acquireCount: { r: 4 } }, Database: { acquireCount: { r: 2 } }, Collection: { acquireCount: { r: 2 } } } 235ms
  </code></pre>
  
  <p>explain() for same query</p>
  
  <pre><code>{
  'queryPlanner': {
      'plannerVersion': 1,
      'parsedQuery': {
          '$and': [{
              'moderation.blocked': {
                  '$eq': False
              }
          }, {
              'moderation.visible': {
                  '$eq': True
              }
          }, {
              'parent_ids': {
                  '$eq': '31cd1be3-adbd-4108-a05e-58c283176738'
              }
          }, {
              'project_id': {
                  '$eq': '9fd6db37-049f-4af3-bea4-8301345dc109'
              }
          }, {
              'dates.posted': {
                  '$gt': 1458720426.177019
              }
          }]
      },
      'namespace': 'comments.comments',
      'rejectedPlans': [],
      'winningPlan': {
          'filter': {
              'moderation.visible': {
                  '$eq': True
              }
          },
          'stage': 'FETCH',
          'inputStage': {
              'indexName': 'parent_ids_1_dates.posted_1_owner_id_1_project_id_1_moderation.blocked_1_moderation.visible_-1',
              'keyPattern': {
                  'parent_ids': 1,
                  'moderation.blocked': 1,
                  'owner_id': 1,
                  'moderation.visible': -1,
                  'dates.posted': 1,
                  'project_id': 1
              },
              'isMultiKey': True,
              'indexBounds': {
                  'parent_ids': ['["31cd1be3-adbd-4108-a05e-58c283176738", "31cd1be3-adbd-4108-a05e-58c283176738"]'],
                  'moderation.blocked': ['[false, false]'],
                  'owner_id': ['[MinKey, MaxKey]'],
                  'moderation.visible': ['[MaxKey, MinKey]'],
                  'dates.posted': ['(1458720426.177019, inf.0]'],
                  'project_id': ['["9fd6db37-049f-4af3-bea4-8301345dc109", "9fd6db37-049f-4af3-bea4-8301345dc109"]']
              },
              'stage': 'IXSCAN',
              'direction': 'forward'
          }
      },
      'indexFilterSet': False
  },
  'executionStats': {
      'totalDocsExamined': 0,
      'allPlansExecution': [],
      'totalKeysExamined': 0,
      'executionSuccess': True,
      'nReturned': 0,
      'executionStages': {
          'invalidates': 0,
          'filter': {
              'moderation.visible': {
                  '$eq': True
              }
          },
          'isEOF': 1,
          'needTime': 0,
          'alreadyHasObj': 0,
          'advanced': 0,
          'inputStage': {
              'invalidates': 0,
              'isEOF': 1,
              'needTime': 0,
              'dupsDropped': 0,
              'dupsTested': 0,
              'keyPattern': {
                  'parent_ids': 1,
                  'moderation.blocked': 1,
                  'owner_id': 1,
                  'moderation.visible': -1,
                  'dates.posted': 1,
                  'project_id': 1
              },
              'advanced': 0,
              'seenInvalidated': 0,
              'nReturned': 0,
              'indexBounds': {
                  'parent_ids': ['["31cd1be3-adbd-4108-a05e-58c283176738", "31cd1be3-adbd-4108-a05e-58c283176738"]'],
                  'moderation.blocked': ['[false, false]'],
                  'owner_id': ['[MinKey, MaxKey]'],
                  'moderation.visible': ['[MaxKey, MinKey]'],
                  'dates.posted': ['(1458720426.177019, inf.0]'],
                  'project_id': ['["9fd6db37-049f-4af3-bea4-8301345dc109", "9fd6db37-049f-4af3-bea4-8301345dc109"]']
              },
              'needFetch': 0,
              'saveState': 0,
              'keysExamined': 0,
              'matchTested': 0,
              'executionTimeMillisEstimate': 0,
              'indexName': 'parent_ids_1_dates.posted_1_owner_id_1_project_id_1_moderation.blocked_1_moderation.visible_-1',
              'restoreState': 0,
              'works': 1,
              'direction': 'forward',
              'stage': 'IXSCAN',
              'isMultiKey': True
          },
          'stage': 'FETCH',
          'saveState': 0,
          'docsExamined': 0,
          'nReturned': 0,
          'restoreState': 0,
          'works': 1,
          'executionTimeMillisEstimate': 0,
          'needFetch': 0
      },
      'executionTimeMillis': 0
  }
  </code></pre>