MDL-32462 accesslib: fix context_user::build_paths performance

The problem was that the query was updating every row whether it needed
it or not. This turns out to be a really bad thing to do on Postgres,
because it then triggers a long expensive vacuum, which lock the context
table for a long time a really bad thing to do.

lib/accesslib.php

@@ -5853,12 +5853,24 @@ class context_user extends context {
     protected static function build_paths($force) {
         global $DB;
 
-        // first update normal users
+        // First update normal users.
+        $path = $DB->sql_concat('?', 'id');
+        $pathstart = '/' . SYSCONTEXTID . '/';
+        $params = array($pathstart);
+
+        if ($force) {
+            $where = "depth <> 2 OR path IS NULL OR path <> ({$path})";
+            $params[] = $pathstart;
+        } else {
+            $where = "depth = 0 OR path IS NULL";
+        }
+
         $sql = "UPDATE {context}
                    SET depth = 2,
-                       path = ".$DB->sql_concat("'/".SYSCONTEXTID."/'", 'id')."
-                 WHERE contextlevel=".CONTEXT_USER;
-        $DB->execute($sql);
+                       path = {$path}
+                 WHERE contextlevel = " . CONTEXT_USER . "
+                   AND ($where)";
+        $DB->execute($sql, $params);
     }
 }