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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.
QUICK INSTALL ============= For the impatient, here is a basic outline of the installation process, which normally takes me only a few minutes: 1) Move the Moodle files into your web directory. 2) Create a single database for Moodle to store all its tables in (or choose an existing database). 3) Visit your Moodle site with a browser, you should be taken to the install.php script, which will lead you through creating a config.php file and then setting up Moodle, creating an admin account etc. 4) Set up a cron task to call the file admin/cron.php every five minutes or so. For more information, see the INSTALL DOCUMENTATION: http://docs.moodle.org/en/Installing_Moodle Good luck and have fun! Martin Dougiamas, Lead Developer