title: Why clickhouse have both WHERE and PREWHERE constructs?
body: <p>I'm reading the documentation and just curious about next:</p>
  
  <p>PREWHERE is like an optimized form of WHERE, which reads only necessary data before filtering and then with this information reads all of the actual data for the user.</p>
  
  <p>The question is why two keywords exist? It seems irrational to use WHERE instead of PREWHERE whenever it's possible. Is there any cases when the usage of WHERE is the better choice? </p>