title: What is Multiversion Concurrency Control (MVCC) and who supports it?
body: <p>Recently Jeff has <a href="http://blog.codinghorror.com/deadlocked/" rel="noreferrer">posted</a> regarding his trouble with database deadlocks related to reading.  <a href="https://en.m.wikipedia.org/wiki/Multiversion_concurrency_control" rel="noreferrer">Multiversion Concurrency Control (MVCC)</a> claims to solve this problem.  What is it, and what databases support it?</p>
  
  <p>updated: these support it (which others?)</p>
  
  <ul>
  <li>oracle</li>
  <li>postgresql  </li>
  </ul>