title: mongodb keep_mutation stage explain
body: <p>I use mongo's explain() to check the performance of some queries, sometimes a keep_mutation stage will show up like the following:</p>
  
  <pre><code>  "executionStats" : {
              ...
              "executionStages" : {
                      "stage" : "KEEP_MUTATIONS",
                      "nReturned" : 1,
                      "executionTimeMillisEstimate" : 5460,
                      "works" : 79622,
                      ...
              }
     }
  </code></pre>
  
  <p>I want to know more about this stage so I search through the internet, and to my surprise, I couldn't find useful info related to it even in the official document, let alone other websites. Could someone help to explain that? </p>