title: Select distinct secondary key values in BerkeleyDB JE
body: <p>I have a Berkeley DB JE setup using the DPL.</p>
  
  <p>I have a secondary key field which is a string, and I would like to retrieve all distinct values for this key. No additional fitlering is required, I simply want all the distinct values.</p>
  
  <p>I could iterate over all entries and add values to sets, but this seems very inefficient as I have on the order of 10s of values from hundreds of thousands of rows.</p>