title: EXPLAIN SELECT shows that MySQL is not using my index
body: <p>Table definition, note the <code>UNIQUE</code> index:</p>
  
  
  
  <pre class="lang-sql prettyprint-override"><code>CREATE TABLE meta
  (
    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    type SET('tag', 'keyword') NOT NULL,
    name VARCHAR(255) NOT NULL,
    user_id INT UNSIGNED NOT NULL,
    UNIQUE (name, type, user_id),
    FOREIGN KEY (user_id) REFERENCES user (id) ON DELETE CASCADE
  );
  </code></pre>
  
  <p>So MySQL should use the index in searches like <code>WHERE name = 'tag' and type = 'cat'</code> as well as only (leftmost prefixing) <code>WHERE type = 'tag'</code>.</p>
  
  <p>I did:</p>
  
  <pre class="lang-sql prettyprint-override"><code>EXPLAIN SELECT * FROM meta WHERE type = 'tag'
  </code></pre>
  
  <p>And result is (fifth column is <code>possible_keys</code>):</p>
  
  <pre class="lang-sql prettyprint-override"><code>'1', 'SIMPLE', 'meta', 'ALL', NULL, NULL, NULL, NULL, '1', 'Using where'
  </code></pre>
  
  <p>I'm sure i'm missing something, but can't find what. Any clue?</p>