diff --git a/lib/accesslib.php b/lib/accesslib.php
index bd400e4e351..09191fadc50 100644
--- a/lib/accesslib.php
+++ b/lib/accesslib.php
@@ -2680,6 +2680,10 @@ function get_archetype_roles($archetype) {
 /**
  * Gets all the user roles assigned in this context, or higher contexts for a list of users.
  *
+ * If you try using the combination $userids = [], $checkparentcontexts = true then this is likely
+ * to cause an out-of-memory error on large Moodle sites, so this combination is deprecated and
+ * outputs a warning, even though it is the default.
+ *
  * @param context $context
  * @param array $userids. An empty list means fetch all role assignments for the context.
  * @param bool $checkparentcontexts defaults to true
@@ -2687,7 +2691,13 @@ function get_archetype_roles($archetype) {
  * @return array
  */
 function get_users_roles(context $context, $userids = [], $checkparentcontexts = true, $order = 'c.contextlevel DESC, r.sortorder ASC') {
-    global $USER, $DB;
+    global $DB;
+
+    if (!$userids && $checkparentcontexts) {
+        debugging('Please do not call get_users_roles() with $checkparentcontexts = true ' .
+                'and $userids array not set. This combination causes large Moodle sites ' .
+                'with lots of site-wide role assignemnts to run out of memory.', DEBUG_DEVELOPER);
+    }
 
     if ($checkparentcontexts) {
         $contextids = $context->get_parent_context_ids();
diff --git a/lib/tests/accesslib_test.php b/lib/tests/accesslib_test.php
index a4d16e5d60b..b98dc82fac0 100644
--- a/lib/tests/accesslib_test.php
+++ b/lib/tests/accesslib_test.php
@@ -1612,7 +1612,7 @@ class core_accesslib_testcase extends advanced_testcase {
 
         $u2roles = get_user_roles($coursecontext, $user2->id);
 
-        $allroles = get_users_roles($coursecontext);
+        $allroles = get_users_roles($coursecontext, [], false);
         $specificuserroles = get_users_roles($coursecontext, [$user1->id, $user2->id]);
         $this->assertEquals($u1roles, $allroles[$user1->id]);
         $this->assertEquals($u1roles, $specificuserroles[$user1->id]);
diff --git a/user/classes/participants_table.php b/user/classes/participants_table.php
index f634ab13e7b..14005b1e230 100644
--- a/user/classes/participants_table.php
+++ b/user/classes/participants_table.php
@@ -233,7 +233,6 @@ class participants_table extends \table_sql {
             $this->groups = groups_get_all_groups($courseid, 0, 0, 'g.*', true);
         }
         $this->allroles = role_fix_names(get_all_roles($this->context), $this->context);
-        $this->allroleassignments = get_users_roles($this->context, [], true, 'c.contextlevel DESC, r.sortorder ASC');
         $this->assignableroles = get_assignable_roles($this->context, ROLENAME_ALIAS, false);
         $this->profileroles = get_profile_roles($this->context);
     }
@@ -441,9 +440,21 @@ class participants_table extends \table_sql {
             $sort = 'ORDER BY ' . $sort;
         }
 
-        $this->rawdata = user_get_participants($this->course->id, $this->currentgroup, $this->accesssince,
+        $rawdata = user_get_participants($this->course->id, $this->currentgroup, $this->accesssince,
             $this->roleid, $this->enrolid, $this->status, $this->search, $twhere, $tparams, $sort, $this->get_page_start(),
             $this->get_page_size());
+        $this->rawdata = [];
+        foreach ($rawdata as $user) {
+            $this->rawdata[$user->id] = $user;
+        }
+        $rawdata->close();
+
+        if ($this->rawdata) {
+            $this->allroleassignments = get_users_roles($this->context, array_keys($this->rawdata),
+                    true, 'c.contextlevel DESC, r.sortorder ASC');
+        } else {
+            $this->allroleassignments = [];
+        }
 
         // Set initial bars.
         if ($useinitialsbar) {