diff --git a/classes/db/stepdata_migration_utils.php b/classes/db/stepdata_migration_utils.php
index cbc86d9..3e7868a 100644
--- a/classes/db/stepdata_migration_utils.php
+++ b/classes/db/stepdata_migration_utils.php
@@ -70,14 +70,16 @@ public static function to_new_name(
         return qtype_matrix_question::responsekey($newrowindex, $newcolindex);
     }
 
-    public static function stepdata_sql(string $qinsql, string $stepdatanamelike): string {
+    public static function stepdata_sql(int $minqid, int $maxqid, string $stepdatanamelike): string {
+        $operator = str_contains($stepdatanamelike, '%') ? 'LIKE' : '=';
          return "SELECT qasd.id as stepdataid, q.id as questionid, qa.id as attemptid, qas.id as stepid, qasd.name, qasd.value
                 FROM {question} q
                 JOIN {question_attempts} qa ON qa.questionid = q.id
                 JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
                 JOIN {question_attempt_step_data} qasd ON qasd.attemptstepid = qas.id
-                WHERE qasd.name LIKE '" . $stepdatanamelike . "' 
-                AND q.id " . $qinsql . "
+                WHERE qasd.name ".$operator." '" . $stepdatanamelike . "'
+                AND q.qtype = 'matrix'
+                AND q.id BETWEEN ".$minqid." AND ".$maxqid." 
                 ORDER BY qasd.id
             ";
     }
@@ -87,7 +89,7 @@ public static function extract_matrixinfos(array $questionids):array {
 
         $matrixinfos = [];
 
-        [$qinsql, $qidparams] = $DB->get_in_or_equal(array_keys($questionids));
+        [$qinsql, $qidparams] = $DB->get_in_or_equal($questionids);
         // Leave out matrix questions with broken data (missing col records)
         $colssql = "
                 SELECT qmc.id as colid, qm.id as matrixid, q.id as questionid
@@ -119,9 +121,10 @@ public static function extract_matrixinfos(array $questionids):array {
         $matrixcols->close();
 
         // Collect info about attempt row order.
-        $orderstepdatasql = stepdata_migration_utils::stepdata_sql($qinsql, '_order');
-        // FIXME: Should probably be done in batches of 100.000
-        $orderdatars = $DB->get_recordset_sql($orderstepdatasql, $qidparams);
+        $minqid = min($questionids);
+        $maxqid = max($questionids);
+        $orderstepdatasql = stepdata_migration_utils::stepdata_sql($minqid, $maxqid, '_order');
+        $orderdatars = $DB->get_recordset_sql($orderstepdatasql);
         foreach ($orderdatars as $orderdata) {
             if (!$orderdata->questionid || !isset($matrixinfos[$orderdata->questionid])) {
                 continue;
@@ -137,12 +140,14 @@ public static function extract_matrixinfos(array $questionids):array {
 
     public static function migrate_stepdata(array &$matrixinfos, array $questionids):void {
         global $DB;
-        [$qinsql, $qidparams] = $DB->get_in_or_equal(array_keys($questionids));
-        $cellstepdatasql = self::stepdata_sql($qinsql, 'cell%');
-        // FIXME: Should probably be done in batches of 100.000
-
-        $celldataupdatechunksize = 10000;
-        $celldatars = $DB->get_recordset_sql($cellstepdatasql, $qidparams);
+        $minqid = min($questionids);
+        $maxqid = max($questionids);
+        $cellstepdatasql = self::stepdata_sql($minqid, $maxqid, 'cell%');
+
+        // Lower update chunks lead to lower times for the update query.
+        // This seems to be caused by using lots of CASE statements.
+        $celldataupdatechunksize = 100;
+        $celldatars = $DB->get_recordset_sql($cellstepdatasql);
         $nrprocessedchunkcelldata = 0;
         $celldataids = [];
         $sqlnamecase = '';
@@ -190,5 +195,6 @@ public static function migrate_stepdata(array &$matrixinfos, array $questionids)
             }
         }
         $celldatars->close();
+        unset($celldatars);
     }
 }
diff --git a/db/upgrade.php b/db/upgrade.php
index d575d19..e4f6282 100644
--- a/db/upgrade.php
+++ b/db/upgrade.php
@@ -162,22 +162,29 @@ function xmldb_qtype_matrix_upgrade(int $oldversion): bool {
         if ($total) {
             core_php_time_limit::raise();
             // Ensure we have a base memory limit with which to work.
+            // This should at least amount to 2GB RAM for this process.
             raise_memory_limit(MEMORY_HUGE);
             $now = time();
-            // Guessed batch size for processing questions when updating question attempt step data.
-            $questionbatchsize = 1000;
+            // Batch size for processing questions when updating question attempt step data.
+            // The given batch size led to 1.4 GB of the process being matrixinfos.
+            // This was deemed okay because the recordsets do not take much memory.
+            $questionbatchsize = 25000;
             // Show a progress bar.
             $pbar = new progress_bar('upgrade_qtype_matrix_stepdata_to_row', 500, true);
             $offset = 0;
             while ($offset < $total) {
                 $pbar->update($offset, $total, "Updating attempt data for qtype_matrix questions - $offset/$total questions.");
-                $questionids = $DB->get_records(
+                $questionids = array_keys($DB->get_records(
                     'question', ['qtype' => 'matrix'], 'id ASC', 'id', $offset, $questionbatchsize
-                );
+                ));
                 $offset += $questionbatchsize;
                 $matrixinfos = stepdata_migration_utils::extract_matrixinfos($questionids);
                 // Now migrate the cell stepdata for the question batch (also done in batches).
                 stepdata_migration_utils::migrate_stepdata($matrixinfos, $questionids);
+                // Necessary to prevent out of memory errors because PHP didn't garbage collect early enough.
+                unset($matrixinfos);
+                gc_collect_cycles();
+                gc_mem_caches();
             }
             $pbar->update($offset, $total, 'Done. Seconds: '.(time() - $now));
         }