diff --git a/lib/cronlib.php b/lib/cronlib.php
index c39b0c6d538..3c6c0d3e07c 100644
--- a/lib/cronlib.php
+++ b/lib/cronlib.php
@@ -373,6 +373,13 @@ function cron_run() {
     }
 
 
+    // Run question bank clean-up.
+    mtrace("Starting the question bank cron...", '');
+    require_once($CFG->libdir . '/questionlib.php');
+    question_bank::cron();
+    mtrace('done.');
+
+
     //Run registration updated cron
     mtrace(get_string('siteupdatesstart', 'hub'));
     require_once($CFG->dirroot . '/' . $CFG->admin . '/registration/lib.php');
diff --git a/question/engine/bank.php b/question/engine/bank.php
index 5a3118b98ee..d55e7758a66 100644
--- a/question/engine/bank.php
+++ b/question/engine/bank.php
@@ -398,6 +398,17 @@ abstract class question_bank {
         self::ensure_fraction_options_initialised();
         return self::$fractionoptionsfull;
     }
+
+    /**
+     * Perform scheduled maintenance tasks relating to the question bank.
+     */
+    public static function cron() {
+        global $CFG;
+
+        // Delete any old question preview that got left in the database.
+        require_once($CFG->dirroot . '/question/previewlib.php');
+        question_preview_cron();
+    }
 }
 
 
diff --git a/question/engine/datalib.php b/question/engine/datalib.php
index 41283044f49..61165989b5e 100644
--- a/question/engine/datalib.php
+++ b/question/engine/datalib.php
@@ -758,6 +758,14 @@ ORDER BY
      * @param qubaid_condition $qubaids identifies which question useages to delete.
      */
     protected function delete_usage_records_for_mysql(qubaid_condition $qubaids) {
+        $qubaidtest = $qubaids->usage_id_in();
+        if (strpos($qubaidtest, 'question_usages') !== false &&
+                strpos($qubaidtest, 'IN (SELECT') === 0) {
+            // This horrible hack is required by MDL-29847. It comes from
+            // http://www.xaprb.com/blog/2006/06/23/how-to-select-from-an-update-target-in-mysql/
+            $qubaidtest = 'IN (SELECT * FROM ' . substr($qubaidtest, 3) . ' AS hack_subquery_alias)';
+        }
+
         // TODO once MDL-29589 is fixed, eliminate this method, and instead use the new $DB API.
         $this->db->execute('
                 DELETE qu, qa, qas, qasd
@@ -765,7 +773,7 @@ ORDER BY
                   JOIN {question_attempts}          qa   ON qa.questionusageid = qu.id
              LEFT JOIN {question_attempt_steps}     qas  ON qas.questionattemptid = qa.id
              LEFT JOIN {question_attempt_step_data} qasd ON qasd.attemptstepid = qas.id
-                 WHERE qu.id ' . $qubaids->usage_id_in(),
+                 WHERE qu.id ' . $qubaidtest,
                 $qubaids->usage_id_in_params());
     }
 
diff --git a/question/previewlib.php b/question/previewlib.php
index 5d40879868d..4f11ab1eaeb 100644
--- a/question/previewlib.php
+++ b/question/previewlib.php
@@ -313,3 +313,30 @@ function restart_preview($previewid, $questionid, $displayoptions, $context) {
     redirect(question_preview_url($questionid, $displayoptions->behaviour,
             $displayoptions->maxmark, $displayoptions, $displayoptions->variant, $context));
 }
+
+/**
+ * Scheduled tasks relating to question preview. Specifically, delete any old
+ * previews that are left over in the database.
+ */
+function question_preview_cron() {
+    $maxage = 24*60*60; // We delete previews that have not been touched for 24 hours.
+    $lastmodifiedcutoff = time() - $maxage;
+
+    mtrace("\n  Cleaning up old question previews...", '');
+    $oldpreviews = new qubaid_join('{question_usages} quba', 'quba.id',
+            'quba.component = :qubacomponent
+                    AND NOT EXISTS (
+                        SELECT 1
+                          FROM {question_attempts} qa
+                          JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+                         WHERE qa.questionusageid = quba.id
+                           AND (qa.timemodified > :qamodifiedcutoff
+                                    OR qas.timecreated > :stepcreatedcutoff)
+                    )
+            ',
+            array('qubacomponent' => 'core_question_preview',
+                'qamodifiedcutoff' => $lastmodifiedcutoff, 'stepcreatedcutoff' => $lastmodifiedcutoff));
+
+    question_engine::delete_questions_usage_by_activities($oldpreviews);
+    mtrace('done.');
+}