Index: includes/session.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/session.inc,v
retrieving revision 1.40
diff -u -F^f -r1.40 session.inc
--- includes/session.inc	25 May 2007 15:04:41 -0000	1.40
+++ includes/session.inc	23 Jul 2007 01:52:28 -0000
@@ -75,8 +75,9 @@ function sess_write($key, $value) {
   else {
     db_query("UPDATE {sessions} SET uid = %d, cache = %d, hostname = '%s', session = '%s', timestamp = %d WHERE sid = '%s'", $user->uid, isset($user->cache) ? $user->cache : '', ip_address(), $value, time(), $key);
 
-    // TODO: this can be an expensive query. Perhaps only execute it every x minutes. Requires investigation into cache expiration.
-    if ($user->uid) {
+    // Last access time is updated no more frequently than once every 180 seconds.
+    // This reduces contention in the users table.
+    if ($user->uid && time() - $user->access > 180) {
       db_query("UPDATE {users} SET access = %d WHERE uid = %d", time(), $user->uid);
     }
   }