title: When running UPDATE ... datetime = NOW(); will all rows updated have the same date/time?
body: <p>When you run something similar to:</p>
  
  <pre><code>UPDATE table SET datetime = NOW();
  </code></pre>
  
  <p>on a table with 1 000 000 000 records and the query takes 10 seconds to run, will all the rows have the exact same time (minutes and seconds) or will they have different times? In other words, will the time be when the query started or when each row is updated?</p>
  
  <p>I'm running MySQL, but I'm thinking this applies to all dbs.</p>