diff --git a/mod/forum/db/install.xml b/mod/forum/db/install.xml
index ef7545dce73..bbda97aba99 100644
--- a/mod/forum/db/install.xml
+++ b/mod/forum/db/install.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<XMLDB PATH="mod/forum/db" VERSION="20191001" COMMENT="XMLDB file for Moodle mod/forum"
+<XMLDB PATH="mod/forum/db" VERSION="20200508" COMMENT="XMLDB file for Moodle mod/forum"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../../lib/xmldb/xmldb.xsd"
 >
@@ -99,6 +99,7 @@
         <INDEX NAME="userid" UNIQUE="false" FIELDS="userid"/>
         <INDEX NAME="created" UNIQUE="false" FIELDS="created"/>
         <INDEX NAME="mailed" UNIQUE="false" FIELDS="mailed"/>
+        <INDEX NAME="privatereplyto" UNIQUE="false" FIELDS="privatereplyto" COMMENT="The field is used in certain queries (such as privacy requests) to search for private replies to the user."/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="forum_queue" COMMENT="For keeping track of posts that will be mailed in digest form">
diff --git a/mod/forum/db/upgrade.php b/mod/forum/db/upgrade.php
index 7d36e0413b4..5f709bd6bfa 100644
--- a/mod/forum/db/upgrade.php
+++ b/mod/forum/db/upgrade.php
@@ -246,5 +246,17 @@ function xmldb_forum_upgrade($oldversion) {
     // Automatically generated Moodle v3.9.0 release upgrade line.
     // Put any upgrade step following this.
 
+    if ($oldversion < 2020072100) {
+        // Add index privatereplyto (not unique) to the forum_posts table.
+        $table = new xmldb_table('forum_posts');
+        $index = new xmldb_index('privatereplyto', XMLDB_INDEX_NOTUNIQUE, ['privatereplyto']);
+
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        upgrade_mod_savepoint(true, 2020072100, 'forum');
+    }
+
     return true;
 }
diff --git a/mod/forum/version.php b/mod/forum/version.php
index 7eddcd86571..11f48d60362 100644
--- a/mod/forum/version.php
+++ b/mod/forum/version.php
@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2020061500;       // The current module version (Date: YYYYMMDDXX)
+$plugin->version   = 2020072100;       // The current module version (Date: YYYYMMDDXX)
 $plugin->requires  = 2020060900;       // Requires this Moodle version
 $plugin->component = 'mod_forum';      // Full name of the plugin (used for diagnostics)