Index: modules/dblog/dblog.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/dblog/dblog.module,v
retrieving revision 1.55
diff -u -p -r1.55 dblog.module
--- modules/dblog/dblog.module	1 Sep 2010 03:03:05 -0000	1.55
+++ modules/dblog/dblog.module	22 Sep 2010 14:57:28 -0000
@@ -101,12 +101,28 @@ function dblog_init() {
  * Remove expired log messages and flood control events.
  */
 function dblog_cron() {
-  // Cleanup the watchdog table
-  if (variable_get('dblog_row_limit', 1000) > 0) {
-    $max = Database::getConnection('default', 'default')->query('SELECT MAX(wid) FROM {watchdog}')->fetchField();
-    Database::getConnection('default', 'default')->delete('watchdog')
-      ->condition('wid', $max - variable_get('dblog_row_limit', 1000), '<=')
+  // Cleanup the watchdog table.
+  // Get the row limit set for the watchdog table (default 1000).
+  $row_limit = variable_get('dblog_row_limit', 1000);
+ 
+  // For row limit n, get the wid of the nth row in descending wid order.
+  // Counting the most recent n rows avoids issues with wid number sequences,
+  // e.g. auto_increment value > 1 or rows deleted directly from the table. 
+  if ($row_limit > 0) {
+    $result = db_select('watchdog', 'w')
+      ->fields('w', array('wid'))
+      ->orderBy('wid', 'DESC')
+      ->range($row_limit - 1, 1)
       ->execute();
+
+    // Delete all table entries older than the nth row, if nth row was found.
+    if ($result) {
+      foreach ($result as $min_row) {
+        $num_deleted = db_delete('watchdog')
+          ->condition('wid', $min_row->wid, '<')
+          ->execute();
+      }
+    }
   }
 }