MDL-51241 cohorts: Update to cohort query.

This improves the speed of returning a list of
cohorts. Thanks to François Gannaz for providing
this patch.

cohort/lib.php

@@ -256,15 +256,17 @@ function cohort_get_available_cohorts($currentcontext, $withmembers = 0, $offset
     $groupbysql = '';
     $havingsql = '';
     if ($withmembers) {
-        $groupbysql = " GROUP BY $fieldssql";
+        $fieldssql .= ', s.memberscnt';
+        $subfields = "c.id, COUNT(DISTINCT cm.userid) AS memberscnt";
+        $groupbysql = " GROUP BY c.id";
         $fromsql = " LEFT JOIN {cohort_members} cm ON cm.cohortid = c.id ";
-        $fieldssql .= ', COUNT(DISTINCT cm.userid) AS memberscnt';
         if (in_array($withmembers,
                 array(COHORT_COUNT_ENROLLED_MEMBERS, COHORT_WITH_ENROLLED_MEMBERS_ONLY, COHORT_WITH_NOTENROLLED_MEMBERS_ONLY))) {
             list($esql, $params2) = get_enrolled_sql($currentcontext);
             $fromsql .= " LEFT JOIN ($esql) u ON u.id = cm.userid ";
             $params = array_merge($params2, $params);
-            $fieldssql .= ', COUNT(DISTINCT u.id) AS enrolledcnt';
+            $fieldssql .= ', s.enrolledcnt';
+            $subfields .= ', COUNT(DISTINCT u.id) AS enrolledcnt';
         }
         if ($withmembers == COHORT_WITH_MEMBERS_ONLY) {
             $havingsql = " HAVING COUNT(DISTINCT cm.userid) > 0";
@@ -280,13 +282,20 @@ function cohort_get_available_cohorts($currentcontext, $withmembers = 0, $offset
         $params = array_merge($params, $searchparams);
     }
 
-    $sql = "SELECT $fieldssql
-              FROM {cohort} c
-              $fromsql
-             WHERE $wheresql
-             $groupbysql
-             $havingsql
-          ORDER BY c.name, c.idnumber";
+    if ($withmembers) {
+        $sql = "SELECT " . str_replace('c.', 'cohort.', $fieldssql) . "
+                  FROM {cohort} cohort
+                  JOIN (SELECT $subfields
+                          FROM {cohort} c $fromsql
+                         WHERE $wheresql $groupbysql $havingsql
+                        ) s ON cohort.id = s.id
+              ORDER BY cohort.name, cohort.idnumber";
+    } else {
+        $sql = "SELECT $fieldssql
+                  FROM {cohort} c $fromsql
+                 WHERE $wheresql
+              ORDER BY c.name, c.idnumber";
+    }
 
     return $DB->get_records_sql($sql, $params, $offset, $limit);
 }