title: How to specify ordering in rethinkdb between query using index
body: <p>I want to do something like the following:</p>
  
  <p><code>
  r.db('mydb').table('tab').between(
    ['s', 0],
    ['s', 99999], {index: r.desc('s-t')})
  </code></p>
  
  <p>but this is not a correct Rql query:</p>
  
  <pre><code>RqlCompileError: DESC may only be used as an argument to ORDERBY. in:
  </code></pre>
  
  <p>Can I safely use</p>
  
  <p><code>
  r.db('mydb').table('tab').between(
    ['s', 0],
    ['s', 99999], {index: 's-t'}).orderBy({index: r.desc('s-t')})
  </code></p>
  
  <p>Will it be executed optimally (using single read instead of reading all records and then sorting them?</p>