MDL-43176 enrol_meta: enrol_meta_sync query is inefficient

Before this change the nested select did a full table scan of
either the enrol or user_enrolments table. This meant that
on large sites the query could be very slow.

This change ensures that there are no full table scans which
makes the query much more efficient.

In my testing the all courses version of the query changed
from ~15 s to ~ 5 s; the singles course version changed
from ~15 s to > 0.1 s

enrol/meta/locallib.php

@@ -407,21 +407,43 @@ function enrol_meta_sync($courseid = NULL, $verbose = false) {
     $onecourse = $courseid ? "AND e.courseid = :courseid" : "";
     list($enabled, $params) = $DB->get_in_or_equal(explode(',', $CFG->enrol_plugins_enabled), SQL_PARAMS_NAMED, 'e');
     $params['courseid'] = $courseid;
-    $sql = "SELECT ue.userid, ue.enrolid, pue.pstatus, pue.ptimestart, pue.ptimeend
+    // The query builds a a list of all the non-meta enrolments that are on courses (the children) that are linked to by a meta
+    // enrolment, it then groups them by the course that linked to them (the parents).
+    // 
+    // It will only return results where the there is a difference between the status of the parent and the lowest status
+    // of the children (remember that 0 is active, any other status is some form of inactive), or the time the earliest non-zero
+    // start time of a child is different to the parent, or the longest effective end date has changed.
+    //
+    // The last two case statements in the HAVING clause are designed to ignore any inactive child records when calculating
+    // the start and end time.
+    $sql = "SELECT ue.userid, ue.enrolid,
+                   MIN(xpue.status + xpe.status) AS pstatus,
+                   MIN(CASE WHEN (xpue.status + xpe.status = 0) THEN xpue.timestart ELSE 9999999999 END) AS ptimestart,
+                   MAX(CASE WHEN (xpue.status + xpe.status = 0) THEN
+                                 (CASE WHEN xpue.timeend = 0 THEN 9999999999 ELSE xpue.timeend END)
+                            ELSE 0 END) AS ptimeend
               FROM {user_enrolments} ue
               JOIN {enrol} e ON (e.id = ue.enrolid AND e.enrol = 'meta' $onecourse)
-              JOIN (SELECT xpue.userid, xpe.courseid, MIN(xpue.status + xpe.status) AS pstatus,
-                      MIN(CASE WHEN (xpue.status + xpe.status = 0) THEN xpue.timestart ELSE 9999999999 END) AS ptimestart,
-                      MAX(CASE WHEN (xpue.status + xpe.status = 0) THEN
-                                (CASE WHEN xpue.timeend = 0 THEN 9999999999 ELSE xpue.timeend END)
-                                ELSE 0 END) AS ptimeend
-                      FROM {user_enrolments} xpue
-                      JOIN {enrol} xpe ON (xpe.id = xpue.enrolid AND xpe.enrol <> 'meta' AND xpe.enrol $enabled)
-                  GROUP BY xpue.userid, xpe.courseid
-                   ) pue ON (pue.courseid = e.customint1 AND pue.userid = ue.userid)
-             WHERE (pue.pstatus = 0 AND ue.status > 0) OR (pue.pstatus > 0 and ue.status = 0)
-             OR ((CASE WHEN pue.ptimestart = 9999999999 THEN 0 ELSE pue.ptimestart END) <> ue.timestart)
-             OR ((CASE WHEN pue.ptimeend = 9999999999 THEN 0 ELSE pue.ptimeend END) <> ue.timeend)";
+              JOIN {user_enrolments} xpue ON (xpue.userid = ue.userid)
+              JOIN {enrol} xpe ON (xpe.id = xpue.enrolid AND xpe.enrol <> 'meta'
+                   AND xpe.enrol $enabled AND xpe.courseid = e.customint1)
+          GROUP BY ue.userid, ue.enrolid
+            HAVING (MIN(xpue.status + xpe.status) = 0 AND MIN(ue.status) > 0)
+                   OR (MIN(xpue.status + xpe.status) > 0 AND MIN(ue.status) = 0)
+                   OR ((CASE WHEN
+                                  MIN(CASE WHEN (xpue.status + xpe.status = 0) THEN xpue.timestart ELSE 9999999999 END) = 9999999999
+                             THEN 0
+                             ELSE
+                                  MIN(CASE WHEN (xpue.status + xpe.status = 0) THEN xpue.timestart ELSE 9999999999 END)
+                              END) <> MIN(ue.timestart))
+                   OR ((CASE
+                         WHEN MAX(CASE WHEN (xpue.status + xpe.status = 0)
+                                       THEN (CASE WHEN xpue.timeend = 0 THEN 9999999999 ELSE xpue.timeend END)
+                                       ELSE 0 END) = 9999999999
+                         THEN 0 ELSE MAX(CASE WHEN (xpue.status + xpe.status = 0)
+                                              THEN (CASE WHEN xpue.timeend = 0 THEN 9999999999 ELSE xpue.timeend END)
+                                              ELSE 0 END)
+                          END) <> MAX(ue.timeend))";
     $rs = $DB->get_recordset_sql($sql, $params);
     foreach($rs as $ue) {
         if (!isset($instances[$ue->enrolid])) {