diff --git a/lib/db/install.xml b/lib/db/install.xml
index 8e4dd6215f0..51e9ac16601 100644
--- a/lib/db/install.xml
+++ b/lib/db/install.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<XMLDB PATH="lib/db" VERSION="20180618" COMMENT="XMLDB file for core Moodle tables"
+<XMLDB PATH="lib/db" VERSION="20190111" COMMENT="XMLDB file for core Moodle tables"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../lib/xmldb/xmldb.xsd"
 >
@@ -676,6 +676,9 @@
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
         <KEY NAME="useridto" TYPE="foreign" FIELDS="useridto" REFTABLE="user" REFFIELDS="id"/>
       </KEYS>
+      <INDEXES>
+        <INDEX NAME="useridfrom" UNIQUE="false" FIELDS="useridfrom" COMMENT="The useridfrom would normally be a foreign key to the users table. But it can also contain some extra negative values with special semantics (-10 or -20 for no-reply or support user). Therefore we can't make it a foreign key and the index must be created explicitly."/>
+      </INDEXES>
     </TABLE>
     <TABLE NAME="message_contacts" COMMENT="Maintains lists of relationships between users">
       <FIELDS>
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 682ce58bf6a..4fa1f58ec07 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2361,5 +2361,17 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2018051704.02);
     }
 
+    if ($oldversion < 2018051704.03) {
+        // Add index 'useridfrom' to the table 'notifications'.
+        $table = new xmldb_table('notifications');
+        $index = new xmldb_index('useridfrom', XMLDB_INDEX_NOTUNIQUE, ['useridfrom']);
+
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        upgrade_main_savepoint(true, 2018051704.03);
+    }
+
     return true;
 }
diff --git a/version.php b/version.php
index 48269860341..89c8802a5d7 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2018051704.02;              // 20180517      = branching date YYYYMMDD - do not modify!
+$version  = 2018051704.03;              // 20180517      = branching date YYYYMMDD - do not modify!
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.