title: Fast indexing for chat SQL table
body: <p>I'm building a MySQL table that will hold all the data for many one-on-one chat sessions.  </p>
  
  <p>Are insertions to an indexed table still slow when insertions are always sequential?</p>
  
  <p>With a chat system, I expect frequent insertions as well as frequent searches. </p>
  
  <p>Each chat session will keep track of it's own session ID, and the latest post ID.  So to get new posts I could run a query like this:</p>
  
  <pre><code>SELECT * FROM chat WHERE sessionID = xxxx AND postID &gt; xxx
  </code></pre>
  
  <p>Is there a way to have multi-dimensional indexing?  It seems like the fastest way to index would be primarily by sessionID, then by postID. </p>