title: How to iterate through indexed field to add field from another index
body: <p>I'm rather new to elasticsearch, so i'm coming here in hope to find advices. 
  I have two indices in elastic from two different csv files. </p>
  
  <p>The index_1 has this mapping:</p>
  
  <pre><code>{'settings': {
              'number_of_shards' : 3
      },
      'mappings': {
          'properties': {
              'place': {'type': 'keyword' },
              'address': {'type': 'keyword' },
          }
      }
  }
  </code></pre>
  
  <p>The file is about 400 000 documents long.
  The index_2 with a much smaller file(about 50 documents) has this mapping:</p>
  
  <pre><code>    {'settings': {
              "number_of_shards" : 1
      },
      'mappings': {
          'properties': {
              'place': {'type': 'text' },
              'address': {'type': 'keyword' },
          }
      }
  }
  </code></pre>
  
  <p>The field "place" in index_2 is all of the unique values from the field "place" in index_1. 
  In both indices the "address" fields are postcodes of datatype keyword with a structure: 0000AZ.</p>
  
  <p>Based on the "place" field keyword in index_1 I want to assign the term of field "address" from index_2.  </p>
  
  <p>I have tried using the pandas library but the index_1 file is too large. I have also to tried creating modules based off pandas and elasticsearch, quite unsuccessfully. Although I believe this is a promising direction. A good solution would be to stay into the elasticsearch library as much as possible as these indices will be later be used for further analysis.</p>