title: Is it efficient to query on a sub-index
body: <p>I want to query on a table which has an index for columns a,b,c,d all together. 
  If in the where clause I filter on columns a, b and c, is the index good enough? Or should I create a new index just for the tree fields (a, b and c) ?</p>
  
  <pre><code>alter table myTable add index (a,b,c,d)
  </code></pre>
  
  <p>...</p>
  
  <pre><code>SELECT * 
  FROM myTable
  WHERE a = ? AND b = ? AND c = ?
  </code></pre>