diff --git a/lib/db/install.xml b/lib/db/install.xml
index a78c07b9b99..bcc3e18afce 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="20200911" COMMENT="XMLDB file for core Moodle tables"
+<XMLDB PATH="lib/db" VERSION="20201007" COMMENT="XMLDB file for core Moodle tables"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../lib/xmldb/xmldb.xsd"
 >
@@ -464,8 +464,8 @@
         <INDEX NAME="type-timesort" UNIQUE="false" FIELDS="type, timesort"/>
         <INDEX NAME="groupid-courseid-categoryid-visible-userid" UNIQUE="false" FIELDS="groupid, courseid, categoryid, visible, userid" COMMENT="used for calendar view"/>
         <INDEX NAME="eventtype" UNIQUE="false" FIELDS="eventtype"/>
-        <INDEX NAME="modulename-instance" UNIQUE="false" FIELDS="modulename, instance"/>
         <INDEX NAME="component" UNIQUE="false" FIELDS="component, eventtype, instance"/>
+        <INDEX NAME="modulename-instance-eventtype" UNIQUE="false" FIELDS="modulename, instance, eventtype" COMMENT="Improve performance on assign adhoc tasks."/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="cache_filters" COMMENT="For keeping information about cached data">
@@ -4320,4 +4320,4 @@
       </INDEXES>
     </TABLE>
   </TABLES>
-</XMLDB>
\ No newline at end of file
+</XMLDB>
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 2a27ce60697..f7665dff8bc 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2738,5 +2738,29 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2021052500.19);
     }
 
+    if ($oldversion < 2021052500.20) {
+
+        // Define index modulename-instance-eventtype (not unique) to be added to event.
+        $table = new xmldb_table('event');
+        $index = new xmldb_index('modulename-instance-eventtype', XMLDB_INDEX_NOTUNIQUE, ['modulename', 'instance', 'eventtype']);
+
+        // Conditionally launch add index modulename-instance-eventtype.
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        // Define index modulename-instance (not unique) to be dropped form event.
+        $table = new xmldb_table('event');
+        $index = new xmldb_index('modulename-instance', XMLDB_INDEX_NOTUNIQUE, ['modulename', 'instance']);
+
+        // Conditionally launch drop index modulename-instance.
+        if ($dbman->index_exists($table, $index)) {
+            $dbman->drop_index($table, $index);
+        }
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2021052500.20);
+    }
+
     return true;
 }
diff --git a/version.php b/version.php
index 2e3cda2f0ea..7163a2ff4b4 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2021052500.19;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2021052500.20;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.
 $release  = '4.0dev (Build: 20201006)'; // Human-friendly version name