title: Index is not used in MySQL even if the query contains an index hint
body: <ul>
  <li>MySQL 5.6</li>
  </ul>
  
  <p>Consider the following table:</p>
  
  <pre><code>create table mytbl (
      id long primary key,
      digest varchar(32)
  );
  </code></pre>
  
  <p>The <code>digest</code> column contains MD5 digest and I created index on <code>digest</code> by:</p>
  
  <pre><code>create fulltext index digest on mytbl (digest);
  </code></pre>
  
  <p>I subsequently check a query:</p>
  
  <pre><code>explain select count(id) from mytbl where digest = 'abcde12345ffhhg';  -- a digest in the table
  </code></pre>
  
  <p>The plan is (the table has 1000 rows):</p>
  
  <pre><code>id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra
  1  | SIMPLE      | mytbl | ALL  | digest        | NULL| NULL    | NULL| 1000 | Using where
  </code></pre>
  
  <p>I then called the query</p>
  
  <pre><code>explain select count(id) from mytbl use index (digest) where digest = 'abcde12345ffhhg';
  </code></pre>
  
  <p>But the plan remained the same, i.e. the index <code>digest</code> is not used.</p>
  
  <p>Why does MySQL ignore the index set on <code>digest</code>?</p>
  
  <hr>
  
  <h2>EDIT</h2>
  
  <p>I thought I had a misconception about the option <code>[fulltext | spatial | unique]</code> in <code>CREATE INDEX</code> statement. I dropped the <code>fulltext index</code> and then created a default index (<code>create index digest ...</code>). Consequently, MySQL used the <code>digest</code> index in the query plan.</p>