From fe76a2562e1ff4ec18cc8a8f218706703550df92 Mon Sep 17 00:00:00 2001
From: Neill Magill <neill.magill@nottingham.ac.uk>
Date: Mon, 24 Apr 2023 10:26:35 +0100
Subject: [PATCH] MDL-77965 grade: Improve data export performance

The existing query was inefficient using an index scan and
index merges. This change splits the query up into many individually
much more efficient queries.

This should significantly improve performance on large sites.
---
 grade/classes/privacy/provider.php | 93 +++++++++++++++++++++---------
 1 file changed, 67 insertions(+), 26 deletions(-)

diff --git a/grade/classes/privacy/provider.php b/grade/classes/privacy/provider.php
index 070af6fde0a..aad1f0d62f6 100644
--- a/grade/classes/privacy/provider.php
+++ b/grade/classes/privacy/provider.php
@@ -170,39 +170,80 @@ class provider implements
         $sql = "
             SELECT DISTINCT ctx.id
               FROM {context} ctx
-         LEFT JOIN {grade_outcomes_history} goh ON goh.loggeduser = :userid1 AND (
-                   (goh.courseid > 0 AND goh.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel1)
-                OR ((goh.courseid IS NULL OR goh.courseid < 1) AND ctx.id = :syscontextid1)
-            )
-         LEFT JOIN {grade_categories_history} gch ON gch.loggeduser = :userid2 AND (
-                   gch.courseid = ctx.instanceid
-               AND ctx.contextlevel = :courselevel2
-            )
-         LEFT JOIN {grade_items_history} gih ON gih.loggeduser = :userid3 AND (
-                   gih.courseid = ctx.instanceid
-               AND ctx.contextlevel = :courselevel3
-            )
-         LEFT JOIN {scale_history} sh
-                ON (sh.userid = :userid4 OR sh.loggeduser = :userid5)
-               AND (
-                       (sh.courseid > 0 AND sh.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel4)
-                    OR (sh.courseid = 0 AND ctx.id = :syscontextid2)
-            )
-             WHERE goh.id IS NOT NULL
-                OR gch.id IS NOT NULL
-                OR gih.id IS NOT NULL
-                OR sh.id IS NOT NULL";
+              JOIN {grade_outcomes_history} goh ON goh.loggeduser = :userid1 AND goh.courseid > 0
+               AND goh.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel1";
+        $params = [
+            'courselevel1' => CONTEXT_COURSE,
+            'userid1' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {grade_outcomes_history} goh ON goh.loggeduser = :userid1
+               AND (goh.courseid IS NULL OR goh.courseid < 1) AND ctx.id = :syscontextid1";
         $params = [
             'syscontextid1' => SYSCONTEXTID,
-            'syscontextid2' => SYSCONTEXTID,
             'courselevel1' => CONTEXT_COURSE,
-            'courselevel2' => CONTEXT_COURSE,
-            'courselevel3' => CONTEXT_COURSE,
-            'courselevel4' => CONTEXT_COURSE,
             'userid1' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {grade_categories_history} gch ON gch.loggeduser = :userid2
+               AND gch.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel2";
+        $params = [
+            'courselevel2' => CONTEXT_COURSE,
             'userid2' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {grade_items_history} gih ON gih.loggeduser = :userid3
+               AND gih.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel3";
+        $params = [
+            'courselevel3' => CONTEXT_COURSE,
             'userid3' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {scale_history} sh ON sh.userid = :userid4
+               AND sh.courseid > 0 AND sh.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel4";
+        $params = [
+            'courselevel4' => CONTEXT_COURSE,
             'userid4' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {scale_history} sh ON sh.loggeduser = :userid5
+               AND sh.courseid > 0 AND sh.courseid = ctx.instanceid AND ctx.contextlevel = :courselevel4";
+        $params = [
+            'courselevel4' => CONTEXT_COURSE,
+            'userid5' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {scale_history} sh ON sh.userid = :userid4 AND sh.courseid = 0 AND ctx.id = :syscontextid2";
+        $params = [
+            'syscontextid2' => SYSCONTEXTID,
+            'userid4' => $userid,
+        ];
+        $contextlist->add_from_sql($sql, $params);
+        $sql = "
+            SELECT DISTINCT ctx.id
+              FROM {context} ctx
+              JOIN {scale_history} sh ON sh.loggeduser = :userid5 AND sh.courseid = 0 AND ctx.id = :syscontextid2";
+        $params = [
+            'syscontextid2' => SYSCONTEXTID,
             'userid5' => $userid,
         ];
         $contextlist->add_from_sql($sql, $params);