title: Mysql index is not being taken while field is mentioned in join on clause
body: <pre><code>explain select * from users u join wallet w on w.userId=u.uuid where w.userId='8319611142598331610'; //Index is taken
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/F5UrQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F5UrQ.png" alt="enter image description here"></a></p>
  
  <pre><code>explain select * from users u join wallet w on w.userId=u.uuid where w.currencyId=8; //index is not taken
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/iwXW8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iwXW8.png" alt="enter image description here"></a></p>
  
  <p>As can be seen above, the index userIdIdx is used in the latter case, but not in the former.</p>
  
  <p>Following are the schema of the two tables - </p>
  
  <pre><code>CREATE TABLE `users` (
    `uuid` varchar(600) DEFAULT NULL,
    KEY `uuidIdx` (`uuid`),
  ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
  
  CREATE TABLE `wallet` (
    `Id` int(11) NOT NULL AUTO_INCREMENT,
    `userId` varchar(200) NOT NULL DEFAULT '',
    `currencyId` int(11) NOT NULL,
    PRIMARY KEY (`Id`),
    KEY `userIdIdx` (`userId`),
    KEY `currencyIdIdx` (`currencyId`)
  ) ENGINE=InnoDB AUTO_INCREMENT=279668 DEFAULT CHARSET=latin1;
  </code></pre>
  
  <p>How do I force MySql to consider the <code>userIdIdx</code> or <code>uuidIdx</code> index?</p>