From 94c0ec215925dd52a5c8ea8ba59e62ef7c236c5d Mon Sep 17 00:00:00 2001
From: Tim Hunt <T.J.Hunt@open.ac.uk>
Date: Tue, 8 Nov 2011 16:53:15 +0000
Subject: [PATCH] MDL-30185 question engine reporting: add redundant where to
 query for perf.

Without this, MySQL fails to cope.
---
 mod/quiz/report/attemptsreport.php | 10 ++++-
 question/engine/datalib.php        | 63 +++++++++++++++++-------------
 2 files changed, 45 insertions(+), 28 deletions(-)

diff --git a/mod/quiz/report/attemptsreport.php b/mod/quiz/report/attemptsreport.php
index 89bfefc1fc5ee..f26b2ec707127 100644
--- a/mod/quiz/report/attemptsreport.php
+++ b/mod/quiz/report/attemptsreport.php
@@ -621,13 +621,21 @@ protected function add_latest_state_join($slot) {
             return;
         }
 
+        // This condition roughly filters the list of attempts to be considered.
+        // It is only used in a subselect to help crappy databases (see MDL-30122)
+        // therefore, it is better to use a very simple join, which may include
+        // too many records, than to do a super-accurate join.
+        $qubaids = new qubaid_join("{quiz_attempts} {$alias}quiza", "{$alias}quiza.uniqueid",
+                "{$alias}quiza.quiz = :{$alias}quizid", array("{$alias}quizid" => $this->sql->params['quizid']));
+
         $dm = new question_engine_data_mapper();
-        $inlineview = $dm->question_attempt_latest_state_view($alias);
+        list($inlineview, $viewparams) = $dm->question_attempt_latest_state_view($alias, $qubaids);
 
         $this->sql->fields .= ",\n$fields";
         $this->sql->from .= "\nLEFT JOIN $inlineview ON " .
                 "$alias.questionusageid = quiza.uniqueid AND $alias.slot = :{$alias}slot";
         $this->sql->params[$alias . 'slot'] = $slot;
+        $this->sql->params = array_merge($this->sql->params, $viewparams);
     }
 
     /**
diff --git a/question/engine/datalib.php b/question/engine/datalib.php
index cdcaf8f07da0f..2f41b12d7ff72 100644
--- a/question/engine/datalib.php
+++ b/question/engine/datalib.php
@@ -865,33 +865,42 @@ public function sum_usage_marks_subquery($qubaid) {
             END) = 0";
     }
 
-    public function question_attempt_latest_state_view($alias) {
-        return "(
-                SELECT
-                    {$alias}qa.id AS questionattemptid,
-                    {$alias}qa.questionusageid,
-                    {$alias}qa.slot,
-                    {$alias}qa.behaviour,
-                    {$alias}qa.questionid,
-                    {$alias}qa.variant,
-                    {$alias}qa.maxmark,
-                    {$alias}qa.minfraction,
-                    {$alias}qa.flagged,
-                    {$alias}qa.questionsummary,
-                    {$alias}qa.rightanswer,
-                    {$alias}qa.responsesummary,
-                    {$alias}qa.timemodified,
-                    {$alias}qas.id AS attemptstepid,
-                    {$alias}qas.sequencenumber,
-                    {$alias}qas.state,
-                    {$alias}qas.fraction,
-                    {$alias}qas.timecreated,
-                    {$alias}qas.userid
-
-                FROM {question_attempts} {$alias}qa
-                JOIN {question_attempt_steps} {$alias}qas ON
-                        {$alias}qas.id = {$this->latest_step_for_qa_subquery($alias . 'qa.id')}
-            ) $alias";
+    /**
+     * Get a subquery that returns the latest step of every qa in some qubas.
+     * Currently, this is only used by the quiz reports. See
+     * {@link quiz_attempt_report_table::add_latest_state_join()}.
+     * @param string $alias alias to use for this inline-view.
+     * @param qubaid_condition $qubaids restriction on which question_usages we
+     *      are interested in. This is important for performance.
+     * @return array with two elements, the SQL fragment and any params requried.
+     */
+    public function question_attempt_latest_state_view($alias, qubaid_condition $qubaids) {
+        return array("(
+                SELECT {$alias}qa.id AS questionattemptid,
+                       {$alias}qa.questionusageid,
+                       {$alias}qa.slot,
+                       {$alias}qa.behaviour,
+                       {$alias}qa.questionid,
+                       {$alias}qa.variant,
+                       {$alias}qa.maxmark,
+                       {$alias}qa.minfraction,
+                       {$alias}qa.flagged,
+                       {$alias}qa.questionsummary,
+                       {$alias}qa.rightanswer,
+                       {$alias}qa.responsesummary,
+                       {$alias}qa.timemodified,
+                       {$alias}qas.id AS attemptstepid,
+                       {$alias}qas.sequencenumber,
+                       {$alias}qas.state,
+                       {$alias}qas.fraction,
+                       {$alias}qas.timecreated,
+                       {$alias}qas.userid
+
+                  FROM {$qubaids->from_question_attempts($alias . 'qa')}
+                  JOIN {question_attempt_steps} {$alias}qas ON
+                           {$alias}qas.id = {$this->latest_step_for_qa_subquery($alias . 'qa.id')}
+                 WHERE {$qubaids->where()}
+            ) $alias", $qubaids->from_where_params());
     }
 
     protected function latest_step_for_qa_subquery($questionattemptid = 'qa.id') {