title: MySQL Index sometimes not being used
body: <p>I have a table with 150k rows of data, and I have column with a <code>UNIQUE INDEX</code>, It has a type of <code>VARCHAR(10)</code> and stores 10 digit account numbers.</p>
  
  <p>Now whenever I query, like a simple one:</p>
  
  <p><code>SELECT * FROM table WHERE account_number LIKE '0103%'</code></p>
  
  <p>It results 30,000+ ROWS, and when I run a EXPLAIN on my query It shows no INDEX is used.</p>
  
  <p>But when I do:</p>
  
  <p><code>SELECT * FROM table WHERE account_number LIKE '0104%'</code></p>
  
  <p>It results 4,000+ ROWS, with the INDEX used.</p>
  
  <p>Anyone can explain this?</p>
  
  <p><code>I'm using MySQL 5.7 Percona XtraDB.</code></p>