diff --git a/lib/grade/grade_category.php b/lib/grade/grade_category.php
index 9764dc2a32721..e7f50af71ccd1 100644
--- a/lib/grade/grade_category.php
+++ b/lib/grade/grade_category.php
@@ -793,15 +793,28 @@ private function set_usedinaggregation($userid, $usedweights, $novalue, $dropped
 
         // Reset aggregation to unknown and 0 for all grade items for this user and category.
         $params = array('categoryid' => $this->id, 'userid' => $userid);
-        $itemssql = "SELECT id
-                       FROM {grade_items}
-                      WHERE categoryid = :categoryid";
-
-        $sql = "UPDATE {grade_grades}
-                   SET aggregationstatus = 'unknown',
-                       aggregationweight = 0
-                 WHERE userid = :userid
-                   AND itemid IN ($itemssql)";
+
+        switch ($DB->get_dbfamily()) {
+            case 'mysql':
+                // Optimize the query for MySQL by using a join rather than a sub-query.
+                $sql = "UPDATE {grade_grades} g
+                          JOIN {grade_items} gi ON (g.itemid = gi.id)
+                           SET g.aggregationstatus = 'unknown',
+                               g.aggregationweight = 0
+                         WHERE g.userid = :userid
+                           AND gi.categoryid = :categoryid";
+                break;
+            default:
+                $itemssql = "SELECT id
+                               FROM {grade_items}
+                              WHERE categoryid = :categoryid";
+
+                $sql = "UPDATE {grade_grades}
+                           SET aggregationstatus = 'unknown',
+                               aggregationweight = 0
+                         WHERE userid = :userid
+                           AND itemid IN ($itemssql)";
+        }
 
         $DB->execute($sql, $params);