title: how to use two datasets in flink in the same filter function
body: <p>i am working with two different [Flink 1.1.2]-datasets of different sizes, lets call dem DS_little and DS_Big. what i want to do now is to only keep the entries of DS_Big that have a corresponding entry in DS_little.</p>
  
  <p>something like:</p>
  
  <pre><code>if (first element of DS_Big = any element of DS_little)
  {keep it and add an additional entry of DS_little to DS_Big}
  else{forget it}
  </code></pre>
  
  <p>is there a clever way to do such?</p>
  
  <p>thanks in advance for any potential help,</p>
  
  <p>best t.</p>