title: Performance issue full index scan in mysql database
body: <p>I have a database with a table called QuizMatches. The table has the following structure:</p>
  
  <pre><code>CREATE TABLE `QuizMatches` (
    `QuizMatchesGuid` binary(16) NOT NULL,
    `DateStarted` datetime NOT NULL,
    `LatestChanged` datetime NOT NULL,
    `HostFBUserToken` varchar(250) NOT NULL,
    `GuestFBUserToken` varchar(250) NOT NULL,
    `ArrayOfQuestionIDs` varchar(200) NOT NULL,
    `ArrayOfQuestionResponseTimesAndAnswersHost` varchar(900) NOT NULL,
    `ArrayOfQuestionResponseTimesAndAnswersGuest` varchar(900) NOT NULL,
    `MatchFinished` int(1) NOT NULL DEFAULT '0',
    `Category` varchar(45) NOT NULL,
    `JsonQuestions` varchar(4000) NOT NULL DEFAULT '[]',
    `DateFinished` datetime NOT NULL,
    `LatestPushSentDate` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
    PRIMARY KEY (`QuizMatchesGuid`),
    KEY `HostFBUserTokenIX` (`HostFBUserToken`),
    KEY `GuestFBUserTokenIX` (`GuestFBUserToken`),
    KEY `MatchFinishedIX` (`MatchFinished`),
    KEY `LatestChangedIX` (`LatestChanged`),
    KEY `LatestPushSentDateIX` (`LatestPushSentDate`),
    KEY `DateFinishedIX` (`LatestChanged`,`HostFBUserToken`,`GuestFBUserToken`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  </code></pre>
  
  <p>There is a large number of rows in this table and it is heavily used by multiple clients especially queries like the following are executed:</p>
  
  <pre><code>SELECT HEX(QuizMatchesGuid) AS QuizMatchesGuid, DateStarted,
      LatestChanged, HostFBUserToken, GuestFBUserToken,
      ArrayOfQuestionIDs, ArrayOfQuestionResponseTimesAndAnswersHost,
      ArrayOfQuestionResponseTimesAndAnswersGuest, JsonQuestions
  FROM CrystalDBQuiz.QuizMatches
  ORDER BY LatestChanged DESC
  LIMIT 10 
  </code></pre>
  
  <p>The main problem seem to be that the database performs a full index scan. I have tried with different combinations of indexes but to no success. </p>
  
  <p>If I run EXPLAIN on the above SELECT query I receive the following:</p>
  
  <pre><code>           id: 1
    select_type: SIMPLE
          table: 'QuizMatches'
           type: index
  possible_keys: NULL
            key: 'LatestChangedIX'
        key_len: 8
            ref: NULL
           rows: 10
          Extra: 
  </code></pre>
  
  <p>Is there a way I can optimize SELECTS as the above example towards this database table?</p>
  
  <p><img src="https://i.stack.imgur.com/KupqQ.png" alt="Neor profiler screenshot"></p>