EXPLAIN UPDATE mdl_grade_grades g
          JOIN mdl_grade_items gi ON (g.itemid = gi.id)
           SET g.aggregationstatus = 'unknown', g.aggregationweight = 0
         WHERE g.userid = '6935' AND gi.categoryid = '23052'

1	SIMPLE	g	ref	mdl_gradgrad_useite_uix,mdl_gradgrad_ite_ix,mdl_gradgrad_use_ix	mdl_gradgrad_useite_uix	8	const	1	Using where
1	SIMPLE	gi	eq_ref	PRIMARY,mdl_graditem_cat_ix	PRIMARY	8	gradebookIssues.g.itemid	1	Using where

EXPLAIN UPDATE mdl_grade_grades
		   SET aggregationstatus = 'unknown', aggregationweight = 0
		 WHERE userid = '6935' AND itemid IN (SELECT id FROM mdl_grade_items WHERE categoryid = '23052')

1	PRIMARY	mdl_grade_grades	range	mdl_gradgrad_useite_uix,mdl_gradgrad_use_ix	mdl_gradgrad_useite_uix	8	const	1	Using where
2	DEPENDENT SUBQUERY	mdl_grade_items	unique_subquery	PRIMARY,mdl_graditem_cat_ix	PRIMARY	8	func	1	Using where