title: Select only most recent record
body: <p>There is a  MariaDB table</p>
  
  <pre><code>calldate timestamp,
  channel  varchar(30) 
  </code></pre>
  
  <p>Values are:</p>
  
  <pre><code>2019-12-01 SIP/91953838-084ec34
  2019-12-02 SIP/91953838-023ec35
  2019-12-03 SIP/91953839-03e4523
  2019-12-04 SIP/91953839-0365hee
  </code></pre>
  
  <p>How to select only most recent record for each  channel? </p>
  
  <p>So i need result as:</p>
  
  <pre><code>2019-12-02 SIP/91953838
  2019-12-04 SIP/91953839
  </code></pre>
  
  <p>'91953838' or '91953839' are actually channel numbers (about 50 unique values)  whereas '084ec34' just system code assigned to each calls randomly so there is no two records like  that:</p>
  
  <pre><code>    2019-12-01 SIP/91953838-084ec34
      2019-12-02 SIP/91953838-084ec34
  </code></pre>
  
  <p>Table contains 10+ millions rows, so i need most effective query.
  Mysql Ver 15.1 Distrib 5.5.60-MariaDB,</p>