mirror_php/moodle
1
0
Fork 0
mirror of https://github.com/moodle/moodle.git synced 2025-04-18 06:58:08 +02:00
Code Issues Projects Releases Wiki Activity

MDL-47426 assign local roles: rewrite query for better performance.

Browse Source 
This is an extremely dangerous query, because it includes the user
table twice, along-side two other potentially large tables,
role_assignments and user_enrolments.

The solution is to rewrite the query so that:

1. The subquery is JOINed, not WHERE ... INed. Typically query
   optimisers handle the JOIN case better.

2. Before the join was role-assignments <-> users <-> subquery.
   That is, everything was linked to u.id.

   Now the linking is role-assignments <-> subquery <-> users,
   so the SELECT DISTINT eu1_u.id FROM {enrolled users} is central.
   That seems to send a strong hint to the query optimiser about
   a good order to execute the query.
This commit is contained in:
Tim Hunt 2014-09-25 15:52:00 +01:00
parent 6597413d41
commit 482ca72089
1 changed files with 6 additions and 5 deletions

11
admin/roles/classes/potential_assignees_below_course.php

@ -48,11 +48,12 @@ class core_role_potential_assignees_below_course extends core_role_assign_user_s
$fields = 'SELECT ' . $this->required_fields_sql('u');
$countfields = 'SELECT COUNT(u.id)';
$sql = " FROM {user} u
LEFT JOIN {role_assignments} ra ON (ra.userid = u.id AND ra.roleid = :roleid AND ra.contextid = :contextid)
WHERE u.id IN ($enrolsql)
$wherecondition
AND ra.id IS NULL";
$sql = " FROM ($enrolsql) enrolled_users_view
JOIN {user} u ON u.id = enrolled_users_view.id
LEFT JOIN {role_assignments} ra ON (ra.userid = enrolled_users_view.id AND
ra.roleid = :roleid AND ra.contextid = :contextid)
WHERE ra.id IS NULL
$wherecondition";
$params['contextid'] = $this->context->id;
$params['roleid'] = $this->roleid;