title: Pull Rows That Equal Max Date
body: <p>My table structure is below.  The ma_symbol and ma_date are primary keys for this table.</p>
  
  <pre><code>ma_symbol |    ma_date | ma
          A   2015-03-01    1
          A   2016-04-01    2
          B   2014-01-01    3
          B   2014-02-02    4
  </code></pre>
  
  <p>For each ma_symbol, I want to pull the rows where ma_date equals max(ma_date).  So the result would look like this from the above example:</p>
  
  <pre><code>ma_symbol |    ma_date | ma
          A   2016-04-01    2
          B   2015-02-02    4
  </code></pre>
  
  <p>I have tried to adapt the following example but it takes far too long to run.  This table has 5 million + records.</p>
  
  <p><a href="https://stackoverflow.com/questions/17038193/select-row-with-most-recent-date-per-user">Attempted Adapted Query</a></p>