title: Mysql effective indexes
body: <p>Currently I am creating indexes as I need them for a particular sql query.
  But they are starting to overlap each other.</p>
  
  <p>Is there any rule to define them effectively?</p>
  
  <p>For example:</p>
  
  <ul>
  <li>If I have two indexes for column1 and column2, does the composite index by column1, column2 improve select by both columns?</li>
  <li>What is there any difference between an index by column1 and column2 over index by column2 and column1?</li>
  </ul>