title: SQL query plan sequence
body: <p><strong>Context</strong> (not really necessary to answer the question)<br>
  We are trying to optimize the data structure for SPARQL on SQL queries and want to avoid unneccesary comparisons for join operations using a data format other than the N-Triples. We want to filter out data that is not relevant for the result/joins. Crucial for the runtime is the size of the join, so we have to know if the expression in the WHERE clause gets evaluated before the actual join. Assumably it is true, but we need to know for sure.</p>
  
  <p><strong>tl;dr</strong><br>
  When is the WHERE clause evaluated? Does the standard specify this at all, or is it left to the implementation? I read about using the ON clause to force it. I'd apreciate if someone could reference this with proper sources.</p>