From 6bce418d282bbb2d96d537fcaaf40f5ce76d34c4 Mon Sep 17 00:00:00 2001
From: Tim Hunt <T.J.Hunt@open.ac.uk>
Date: Wed, 8 Jan 2014 18:22:20 +0000
Subject: [PATCH] MDL-43246 question engine: avoid order-by id.

This was breaking with oracle master/master replication. Fortunately all
the places that needed to be changed were private to datalib.php. There
is still some ordering by id there, but only places where we want a
consitent, rather than meaningful, order, so that is OK.

The queries changed by this patch all have subqueries in aggregate
queries that pull out the latest step for a question_attempt. Those
queries used to look for MAX(id) but now they look for
MAX(sequencenumber). This is equivalent (for databases where ids always
increase with time, except for auto-saved steps. In the past,  an
auto-saved step might have been considered latest. Now the latest step
will always be one that has been properly processed. You can aruge that
this change is an improvement. Anyway, it is a moot point. All these
queries are only used in reports which are run on completed attempts,
where there will not be any autosaved data.
---
 question/engine/datalib.php | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/question/engine/datalib.php b/question/engine/datalib.php
index 110012c7675..27287f36c85 100644
--- a/question/engine/datalib.php
+++ b/question/engine/datalib.php
@@ -390,8 +390,8 @@ SELECT
     qas.userid
 
 FROM {$qubaids->from_question_attempts('qa')}
-JOIN {question_attempt_steps} qas ON
-        qas.id = {$this->latest_step_for_qa_subquery()}
+JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+        AND qas.sequencenumber = {$this->latest_step_for_qa_subquery()}
 
 WHERE
     {$qubaids->where()} AND
@@ -428,8 +428,8 @@ SELECT
     COUNT(1) AS numattempts
 
 FROM {$qubaids->from_question_attempts('qa')}
-JOIN {question_attempt_steps} qas ON
-        qas.id = {$this->latest_step_for_qa_subquery()}
+JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+        AND qas.sequencenumber = {$this->latest_step_for_qa_subquery()}
 JOIN {question} q ON q.id = qa.questionid
 
 WHERE
@@ -537,8 +537,8 @@ SELECT
     1
 
 FROM {$qubaids->from_question_attempts('qa')}
-JOIN {question_attempt_steps} qas ON
-        qas.id = {$this->latest_step_for_qa_subquery()}
+JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+        AND qas.sequencenumber = {$this->latest_step_for_qa_subquery()}
 JOIN {question} q ON q.id = qa.questionid
 
 WHERE
@@ -599,8 +599,8 @@ SELECT
     COUNT(1) AS numaveraged
 
 FROM {$qubaids->from_question_attempts('qa')}
-JOIN {question_attempt_steps} qas ON
-        qas.id = {$this->latest_step_for_qa_subquery()}
+JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+        AND qas.sequencenumber = {$this->latest_step_for_qa_subquery()}
 
 WHERE
     {$qubaids->where()}
@@ -914,10 +914,11 @@ ORDER BY
         // NULL total into a 0.
         return "SELECT COALESCE(SUM(qa.maxmark * qas.fraction), 0)
             FROM {question_attempts} qa
-            JOIN {question_attempt_steps} qas ON qas.id = (
-                SELECT MAX(summarks_qas.id)
-                  FROM {question_attempt_steps} summarks_qas
-                 WHERE summarks_qas.questionattemptid = qa.id
+            JOIN {question_attempt_steps} qas ON qas.questionattemptid = qa.id
+                    AND qas.sequencenumber = (
+                            SELECT MAX(summarks_qas.sequencenumber)
+                              FROM {question_attempt_steps} summarks_qas
+                             WHERE summarks_qas.questionattemptid = qa.id
             )
             WHERE qa.questionusageid = $qubaid
             HAVING COUNT(CASE
@@ -958,15 +959,15 @@ ORDER BY
                        {$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')}
+                  JOIN {question_attempt_steps} {$alias}qas ON {$alias}qas.questionattemptid = {$alias}qa.id
+                            AND {$alias}qas.sequencenumber = {$this->latest_step_for_qa_subquery($alias . 'qa.id')}
                  WHERE {$qubaids->where()}
             ) $alias", $qubaids->from_where_params());
     }
 
     protected function latest_step_for_qa_subquery($questionattemptid = 'qa.id') {
         return "(
-                SELECT MAX(id)
+                SELECT MAX(sequencenumber)
                 FROM {question_attempt_steps}
                 WHERE questionattemptid = $questionattemptid
             )";