title: How to efficiently determine whether there is a record in millions of records?
body: <p>I want to check whether there is a record in millions of records.</p>
  
  <p>I know I can store the records in some relational database with an index.</p>
  
  <p>But I want some other resolutions, more efficiently. </p>
  
  <p>I need an IDEA.</p>
  
  <p>redis? mongodb? leveldb? cluster?</p>