Extremely slow query in calendar for large sites

Set the calendar_adminseesall config setting to true Visit a calendar page Ensure that all entries are correctly shown Run the old query and the new query to ensure that the same results are always returned. For convenience, I've pasted them below: Old code: SELECT DISTINCT c.* , ctx.id AS ctxid, ctx.path AS ctxpath, ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel, ctx.instanceid AS ctxinstance FROM mdl_course c JOIN mdl_event e ON e.courseid = c.id LEFT JOIN mdl_context ctx ON (ctx.instanceid = c.id AND ctx.contextlevel = 50) ; New code: SELECT c.* , ctx.id AS ctxid, ctx.path AS ctxpath, ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel, ctx.instanceid AS ctxinstance FROM mdl_course c LEFT JOIN mdl_context ctx ON (ctx.instanceid = c.id AND ctx.contextlevel = 50) WHERE EXISTS (SELECT 1 FROM mdl_event e WHERE e.courseid = c.id) ;