MDL-45642 questions: add required index to question_attempts.

It is important to be able to tell which behaviours are in use, and
without this index, it requires a complete scan of this table, which can
be very big, to verify that a behaviour is unused.

lib/db/install.xml

@@ -1405,6 +1405,7 @@
       </KEYS>
       <INDEXES>
         <INDEX NAME="questionusageid-slot" UNIQUE="true" FIELDS="questionusageid, slot"/>
+        <INDEX NAME="behaviour" UNIQUE="false" FIELDS="behaviour" COMMENT="Required because we need to be able to determine efficiently which behaviours are in use."/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="question_attempt_steps" COMMENT="Stores one step in in a question attempt. As well as the data here, the step will have some data in the question_attempt_step_data table.">

lib/db/upgrade.php

@@ -3003,5 +3003,20 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2013111804.03);
     }
 
+    if ($oldversion < 2013111804.06) {
+
+        // Define index behaviour (not unique) to be added to question_attempts.
+        $table = new xmldb_table('question_attempts');
+        $index = new xmldb_index('behaviour', XMLDB_INDEX_NOTUNIQUE, array('behaviour'));
+
+        // Conditionally launch add index behaviour.
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2013111804.06);
+    }
+
     return true;
 }

version.php

@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2013111804.05;              // 20131118      = branching date YYYYMMDD - do not modify!
+$version  = 2013111804.06;              // 20131118      = branching date YYYYMMDD - do not modify!
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.