title: efficiancy of indexing frequently update collection in mongodb
body: <p>I am newbie with MongoDB so my questions might be trivial ... I want to allow my users to upload their address book. the document have the following structure </p>
  
  <pre><code>{ 
  "_id" : "56f29ecc2a00001800dbdf54", 
  "contacts" : [
      {
          "name" : "John", 
          "phoneNumber" : [
              "+18144040000"
          ]
      }, 
      {
          "name" : "Andrew ", 
          "phoneNumber" : [
              "+14129123456"
          ]
      }
  ]
  }
  </code></pre>
  
  <p>I would like to run search by phone number in order to find useres with mutual contacts
  i.e </p>
  
  <pre><code>{"contacts.phoneNumber":"+14129123456"}
  </code></pre>
  
  <p>my question is - <strong>will it be efficient to add this index</strong> </p>
  
  <pre><code>db.addresses.createIndex( { "contacts.phoneNumber": 1 }, { unique: false }, {background: true} )
  </code></pre>
  
  <p>considering the fact that the user will frequently update his address book from his phone which overrides the current data or insert new one. this upload is a single document with an array of contacts each hold an array of phone numbers .
  each upload/update will contain hundreds/thousands of records ?</p>