title: Mass updating mysql rows: better performance to exclude those already having the value?
body: <p>Given the following query: </p>
  
  <pre><code>UPDATE large_table SET column_b='' WHERE column_timestamp &lt; 1434365125;
  </code></pre>
  
  <p>The timestamp is a moving target and will change for each day. </p>
  
  <p>On subsequent runs, will the query benefit from having "AND column_b != ''" added to the where clause or is mysql smart enough to sort of add that automatically in a way in the background? Is this documented somewhere?</p>
  
  <p>And as an aside; would it be better performance wise to set it to NULL instead of a blank string or doesn't that matter? Tables are InnoDB. </p>