title: How to guarantee uniqueness of hash-based ID?
body: <p>In database, uniqueness of PK is very important. And usually PK  is set as incremental integer or hash-based value. It's easy to know how to guarantee uniqueness of incremental integer, but I can't know how to guarantee uniqueness of hash-based values. </p>
  
  <p>What is the popular, usual way to guarantee uniqueness of hash-based keys? Can you recommend some articles?</p>