title: Does mysql optimize the IN clause
body: <p>When i execute this mysql query like </p>
  
  <p><code>select * from t1 where colomn1 in (select colomn1 from t2)</code> , </p>
  
  <p>what really happens?</p>
  
  <p><strong>I want to know if it executes the inner statement for every row</strong>?</p>
  
  <p>PS: I have 300,000 rows in t1 and 50,000 rows in t2 and it is taking a hell of a time.</p>