title: elasticsearch add field to all documents
body: <p>I'm new here. </p>
  
  <p>I have about 200 thousand documents in one index, all have same type. I want to add one more field "category" (which is a keyword string) to every single document.</p>
  
  <p>Is there a convenient way to achieve this? I know normally one query only gets 10000 documents </p>
  
  <p>Thank you very much</p>