How to do it...

1. Assuming you have already created an index on the city field, create one by executing the command db.mockdata.createIndex({'city': 1}) again.
2. Run a find() query:

 > plan = db.mockdata.find({city:'Boston', first_name: 'Sara'}).explain("executionStats")

1. Examine the executionStats:

 > plan['executionStats']

You should see the following result:

{   
  "executionSuccess" : true,
  "nReturned" : 1,
  "executionTimeMillis" : 0,
  "totalKeysExamined" : 9,
  "totalDocsExamined" : 9,
  "executionStages" : {
    "stage" : "FETCH",
    "filter" : {
      "first_name" : {
        "$eq" : "Sara"
      }
    },
    "nReturned" : 1,
    "executionTimeMillisEstimate" : 0,
    "works" : 10,
    "advanced" : 1,
    "needTime" : 8,
    "needYield" : 0,
    "saveState" : 0,
    "restoreState" : 0,
    "isEOF" : 1,
    "invalidates" : 0,
    "docsExamined" : 9,
    "alreadyHasObj" : 0,
    "inputStage" : {
      "stage" : "IXSCAN",
      "nReturned" : 9,
      "executionTimeMillisEstimate" : 0,
      "works" : 10,
      "advanced" : 9,
      "needTime" : 0,
      "needYield" : 0,
      "saveState" : 0,
      "restoreState" : 0,
      "isEOF" : 1,
      "invalidates" : 0,
      "keyPattern" : {
        "city" : 1
      },
      "indexName" : "city_1",
      "isMultiKey" : false,
      "multiKeyPaths" : {
        "city" : [ ]
      },
      "isUnique" : false,
      "isSparse" : false,
      "isPartial" : false,
      "indexVersion" : 2,
      "direction" : "forward",
      "indexBounds" : {
        "city" : [
          "[\"Boston\", \"Boston\"]"
        ]
      },
      "keysExamined" : 9,
      "seeks" : 1,
      "dupsTested" : 0,
      "dupsDropped" : 0,
      "seenInvalidated" : 0
    }
  }
}

1. Now drop this index:

 > db.mockdata.dropIndex('city_1')

You should see an output similar to this:

 { "nIndexesWas" : 2, "ok" : 1 }

1. Create a compound index on city and name:

 > db.mockdata.createIndex({'city': 1, 'first_name': 1})

You should see an output similar to this:

 {
 "createdCollectionAutomatically" : false,
 "numIndexesBefore" : 1,
 "numIndexesAfter" : 2,
 "ok" : 1
 }

1. Let's run the same fetch query again and examine the plan:

> plan = db.mockdata.find({city:'Boston', first_name: 'Sara'}).explain("executionStats")

> plan['executionStats']

You should see an output similar to this:

{
  "executionSuccess": true,
  "nReturned": 1,
  "executionTimeMillis": 0,
  "totalKeysExamined": 1,
  "totalDocsExamined": 1,
  "executionStages": {
    "stage": "FETCH",
    "nReturned": 1,
    "executionTimeMillisEstimate": 0,
    "works": 2,
    "advanced": 1,
    "needTime": 0,
    "needYield": 0,
    "saveState": 0,
    "restoreState": 0,
    "isEOF": 1,
    "invalidates": 0,
    "docsExamined": 1,
    "alreadyHasObj": 0,
    "inputStage": {
      "stage": "IXSCAN",
      "nReturned": 1,
      "executionTimeMillisEstimate": 0,
      "works": 2,
      "advanced": 1,
      "needTime": 0,
      "needYield": 0,
      "saveState": 0,
      "restoreState": 0,
      "isEOF": 1,
      "invalidates": 0,
      "keyPattern": {
        "city": 1,
        "first_name": 1
      },
      "indexName": "city_1_first_name_1",
      "isMultiKey": false,
      "multiKeyPaths": {
        "city": [],
        "first_name": []
      },
      "isUnique": false,
      "isSparse": false,
      "isPartial": false,
      "indexVersion": 2,
      "direction": "forward",
      "indexBounds": {
        "city": [
          "[\"Boston\", \"Boston\"]"
        ],
        "first_name": [
          "[\"Sara\", \"Sara\"]"
        ]
      },
      "keysExamined": 1,
      "seeks": 1,
      "dupsTested": 0,
      "dupsDropped": 0,
      "seenInvalidated": 0
    }
  }
}