title: Can I use index in MySQL in this way?
body: <p>The SQL is</p>
  
  <pre><code>select xxx from xxx where region in (A , B , C) 
  </code></pre>
  
  <p>explain the SQL, and I find the SQL does not use index region, when I change the SQL to</p>
  
  <p><code>select xxx from xxx where region in (B , C)</code><br>
  it can use index</p>
  
  <p>The sum of A and B and C is 2000000+, A is 1000000+</p>