title: What is the usecase of hash index?
body: <p>When to use hash index?</p>
  <p>I know the theory of hashing: when search something by hash it's not comparable on <code>&lt;</code> or <code>&gt;</code>, it can only be equal or not. So in opposite to BTree cannot perform divide and conquer searching, rather try to be precise. But I cannot understand when it's useful?</p>