title: how mysql `select where in` query work ? why larger amount parameter faster?
body: <p>I am doing query with SELECT WHERE IN, and found something unpredicted,</p>
  
  <ol>
  <li><code>SELECT a,b,c FROM tbl1 where a IN (a1,a2,a3...,a5000)</code> --> this query took 1.7ms</li>
  <li><code>SELECT a,b,c FROM tbl1 where a IN (a1,a2,a3...,a20)</code> --> this query took 6.4ms</li>
  </ol>
  
  <p>what is the algorithm of IN ? why larger amount of parameter faster than smaller one ?</p>