title: Weird performance difference when not caching uuid
body: <p>I read this <a href="https://www.elastic.co/blog/performance-considerations-elasticsearch-indexing" rel="nofollow">documentation article</a> regarding Java UUIDs which says that using Java's random uuid is the worst choice.</p>
  
  <p>I have a large amount of documents in one index where I have a Java random UUID in one of the fields, but not as the document id.</p>
  
  <p>I'm running the following query which takes about 6000ms to return:</p>
  
  <pre><code>{
      "query": {
          "filtered": {
              "filter": {
                  "and": [
                      {
                          "term": {
                              "events.d_id": "1175cbe1-5329-8463-7e7b-547aca5dbe83"
                          }
                      },
                      {
                          "term": {
                              "events.a_id": "4495665825523018"
                          }
                      }
                  ]
              }
          }
      }
  }
  </code></pre>
  
  <p>However, when setting <code>_cache</code> to <code>false</code> on that term, I'm getting results back in 4ms:</p>
  
  <pre><code>{
      "query": {
          "filtered": {
              "filter": {
                  "and": [
                      {
                          "term": {
                              "events.d_id": "1175cbe1-5329-8463-7e7b-547aca5dbe83",
                              "_cache": false
                          }
                      },
                      {
                          "term": {
                              "events.a_id": "4495665825523018"
                          }
                      }
                  ]
              }
          }
      }
  }
  </code></pre>
  
  <p>I did another test where I changed the <code>events.d_id</code> to a non-existent value, but in the same format, the query would still take 6000ms and return 0 results. However when I changed the <code>-</code> to <code>_</code> the query took 4ms. It's like ES treats this specific format differently. Please note that the field is defined as <code>string</code> and <code>not_analyzed</code></p>