diff --git a/lib/db/install.xml b/lib/db/install.xml
index 192df05a358..ffc0d42c33b 100644
--- a/lib/db/install.xml
+++ b/lib/db/install.xml
@@ -612,11 +612,15 @@
     <TABLE NAME="message_conversations" COMMENT="Stores all message conversations">
       <FIELDS>
         <FIELD NAME="id" TYPE="int" LENGTH="10" NOTNULL="true" SEQUENCE="true"/>
+        <FIELD NAME="convhash" TYPE="char" LENGTH="40" NOTNULL="true" SEQUENCE="false"/>
         <FIELD NAME="timecreated" TYPE="int" LENGTH="10" NOTNULL="true" SEQUENCE="false"/>
       </FIELDS>
       <KEYS>
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
       </KEYS>
+      <INDEXES>
+        <INDEX NAME="convhash" UNIQUE="true" FIELDS="convhash"/>
+      </INDEXES>
     </TABLE>
     <TABLE NAME="message_conversation_members" COMMENT="Stores all members in a conversations">
       <FIELDS>
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index e7f8cde2384..5da6ff7437e 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2131,5 +2131,25 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2018032200.01);
     }
 
+    if ($oldversion < 2018032200.04) {
+        // Define table 'message_conversations' to be updated.
+        $table = new xmldb_table('message_conversations');
+        $field = new xmldb_field('convhash', XMLDB_TYPE_CHAR, '40', null, XMLDB_NOTNULL, null, 0, 'id');
+
+        // Conditionally launch add field 'convhash'.
+        if (!$dbman->field_exists($table, $field)) {
+            $dbman->add_field($table, $field);
+        }
+
+        // Conditionally launch add index.
+        $index = new xmldb_index('convhash', XMLDB_INDEX_UNIQUE, array('convhash'));
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2018032200.04);
+    }
+
     return true;
 }
diff --git a/lib/messagelib.php b/lib/messagelib.php
index 11f20695696..0b5d300609d 100644
--- a/lib/messagelib.php
+++ b/lib/messagelib.php
@@ -156,10 +156,10 @@ function message_send($eventdata) {
     } else {
         $table = 'messages';
 
-        if (!$conversationid = \core_message\api::get_conversation_between_users($eventdata->userfrom->id,
-                $eventdata->userto->id)) {
-            $conversationid = \core_message\api::create_conversation_between_users($eventdata->userfrom->id,
-                $eventdata->userto->id);
+        if (!$conversationid = \core_message\api::get_conversation_between_users([$eventdata->userfrom->id,
+                $eventdata->userto->id])) {
+            $conversationid = \core_message\api::create_conversation_between_users([$eventdata->userfrom->id,
+                $eventdata->userto->id]);
         }
 
         $tabledata = new stdClass();
diff --git a/message/classes/api.php b/message/classes/api.php
index df1ad671d29..2ff2a59bb89 100644
--- a/message/classes/api.php
+++ b/message/classes/api.php
@@ -637,7 +637,7 @@ class api {
     public static function delete_conversation($userid, $otheruserid) {
         global $DB, $USER;
 
-        $conversationid = self::get_conversation_between_users($userid, $otheruserid);
+        $conversationid = self::get_conversation_between_users([$userid, $otheruserid]);
 
         // If there is no conversation, there is nothing to do.
         if (!$conversationid) {
@@ -1202,24 +1202,15 @@ class api {
     /**
      * Returns the conversation between two users.
      *
-     * @param int $userid1 The userid of the first user
-     * @param int $userid2 The userid of the second user
+     * @param array $userids
      * @return int|bool The id of the conversation, false if not found
      */
-    public static function get_conversation_between_users($userid1, $userid2) {
+    public static function get_conversation_between_users(array $userids) {
         global $DB;
 
-        $sql = "SELECT DISTINCT mc.id
-                  FROM {message_conversations} mc
-            INNER JOIN {message_conversation_members} mcm
-                    ON mcm.conversationid = mc.id
-            INNER JOIN {message_conversation_members} mcm2
-                    ON mcm2.conversationid = mc.id
-                 WHERE mcm.userid = :userid1
-                   AND mcm2.userid = :userid2
-                   AND mcm.id != mcm2.id";
+        $hash = helper::get_conversation_hash($userids);
 
-        if ($conversation = $DB->get_record_sql($sql, ['userid1' => $userid1, 'userid2' => $userid2])) {
+        if ($conversation = $DB->get_record('message_conversations', ['convhash' => $hash])) {
             return $conversation->id;
         }
 
@@ -1229,29 +1220,25 @@ class api {
     /**
      * Creates a conversation between two users.
      *
-     * @param int $userid1 The userid of the first user
-     * @param int $userid2 The userid of the second user
+     * @param array $userids
      * @return int The id of the conversation
      */
-    public static function create_conversation_between_users($userid1, $userid2) {
+    public static function create_conversation_between_users(array $userids) {
         global $DB;
 
         $conversation = new \stdClass();
+        $conversation->convhash = helper::get_conversation_hash($userids);
         $conversation->timecreated = time();
         $conversation->id = $DB->insert_record('message_conversations', $conversation);
 
         // Add members to this conversation.
-        $member = new \stdClass();
-        $member->conversationid = $conversation->id;
-        $member->userid = $userid1;
-        $member->timecreated = time();
-        $DB->insert_record('message_conversation_members', $member);
-
-        $member = new \stdClass();
-        $member->conversationid = $conversation->id;
-        $member->userid = $userid2;
-        $member->timecreated = time();
-        $DB->insert_record('message_conversation_members', $member);
+        foreach ($userids as $userid) {
+            $member = new \stdClass();
+            $member->conversationid = $conversation->id;
+            $member->userid = $userid;
+            $member->timecreated = time();
+            $DB->insert_record('message_conversation_members', $member);
+        }
 
         return $conversation->id;
     }
diff --git a/message/classes/helper.php b/message/classes/helper.php
index c780bd7afc0..dad58fcc32b 100644
--- a/message/classes/helper.php
+++ b/message/classes/helper.php
@@ -293,6 +293,18 @@ class helper {
         return $params;
     }
 
+    /**
+     * Returns the conversation hash between users for easy look-ups in the DB.
+     *
+     * @param array $userids
+     * @return string
+     */
+    public static function get_conversation_hash(array $userids) {
+        sort($userids);
+
+        return sha1(implode('-', $userids));
+    }
+
     /**
      * Returns the cache key for the time created value of the last message between two users.
      *
diff --git a/message/tests/api_test.php b/message/tests/api_test.php
index bdca5c3c05b..98fa368f9a2 100644
--- a/message/tests/api_test.php
+++ b/message/tests/api_test.php
@@ -1921,7 +1921,7 @@ class core_message_api_testcase extends core_message_messagelib_testcase {
         $user1 = self::getDataGenerator()->create_user();
         $user2 = self::getDataGenerator()->create_user();
 
-        $this->assertFalse(\core_message\api::get_conversation_between_users($user1->id, $user2->id));
+        $this->assertFalse(\core_message\api::get_conversation_between_users([$user1->id, $user2->id]));
     }
 
     /**
@@ -1931,9 +1931,9 @@ class core_message_api_testcase extends core_message_messagelib_testcase {
         $user1 = self::getDataGenerator()->create_user();
         $user2 = self::getDataGenerator()->create_user();
 
-        $conversationid = \core_message\api::create_conversation_between_users($user1->id, $user2->id);
+        $conversationid = \core_message\api::create_conversation_between_users([$user1->id, $user2->id]);
 
         $this->assertEquals($conversationid,
-            \core_message\api::get_conversation_between_users($user1->id, $user2->id));
+            \core_message\api::get_conversation_between_users([$user1->id, $user2->id]));
     }
 }
diff --git a/message/tests/externallib_test.php b/message/tests/externallib_test.php
index 83122b2d1da..f5475d79b8d 100644
--- a/message/tests/externallib_test.php
+++ b/message/tests/externallib_test.php
@@ -73,9 +73,9 @@ class core_message_externallib_testcase extends externallib_advanced_testcase {
             return $DB->insert_record('notifications', $record);
         }
 
-        if (!$conversationid = \core_message\api::get_conversation_between_users($userfrom->id, $userto->id)) {
-            $conversationid = \core_message\api::create_conversation_between_users($userfrom->id,
-                $userto->id);
+        if (!$conversationid = \core_message\api::get_conversation_between_users([$userfrom->id, $userto->id])) {
+            $conversationid = \core_message\api::create_conversation_between_users([$userfrom->id,
+                $userto->id]);
         }
 
         // Ok, send the message.
diff --git a/message/tests/messagelib_test.php b/message/tests/messagelib_test.php
index dc47874c79b..231149aec8c 100644
--- a/message/tests/messagelib_test.php
+++ b/message/tests/messagelib_test.php
@@ -85,9 +85,9 @@ class core_message_messagelib_testcase extends advanced_testcase {
             return $DB->insert_record('notifications', $record);
         }
 
-        if (!$conversationid = \core_message\api::get_conversation_between_users($userfrom->id, $userto->id)) {
-            $conversationid = \core_message\api::create_conversation_between_users($userfrom->id,
-                $userto->id);
+        if (!$conversationid = \core_message\api::get_conversation_between_users([$userfrom->id, $userto->id])) {
+            $conversationid = \core_message\api::create_conversation_between_users([$userfrom->id,
+                $userto->id]);
         }
 
         // Ok, send the message.
diff --git a/version.php b/version.php
index c910bfd2f1f..cdca8f9f91e 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2018032200.03;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2018032200.04;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.