title: how does Joint index work in mongodb?
body: <p>In this question's comments, I know how to create index for sort operation: <a href="https://stackoverflow.com/questions/13207835/how-does-mongodb-index-works">how does Mongodb index works?</a></p>
  
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  <li><p>But I want to know, when we create a joint index on <code>a</code> &amp; <code>b</code>, how does it work different  to a  simple index?</p></li>
  <li><p>And why will we benefit in just finding <code>a</code>, but if we find <code>b</code>, we do not get any benefit from it? Whether joint index is just like concatenating <code>a</code> &amp; <code>b</code>, so we will get benefit from it for Prefix?</p></li>
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