title: Can multiple threads cause duplicate updates on constrained set?
body: <p>In postgres if I run the following statement</p>
  
  <pre><code>update table set col = 1 where col = 2
  </code></pre>
  
  <p>In the default <a href="http://www.postgresql.org/docs/9.1/static/transaction-iso.html" rel="nofollow"><code>READ COMMITTED</code> isolation level</a>, from multiple concurrent sessions, am I guaranteed that:</p>
  
  <ol>
  <li>In a case of a single match only 1 thread will get a ROWCOUNT of 1 (meaning only one thread writes)</li>
  <li>In a case of a multi match that only 1 thread will get a ROWCOUNT > 0 (meaning only one thread writes the batch) </li>
  </ol>