id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	PRIMARY	t2	NULL	ref	PRIMARY,idx_message_language	idx_message_language	50	const	1	100	NULL
1	PRIMARY	t1	NULL	eq_ref	PRIMARY,idx_source_message_category	PRIMARY	4	5am.t2.id	1	6.62	Using where
2	UNION	t1	NULL	ref	PRIMARY,idx_source_message_category	idx_source_message_category	768	const	21	100	NULL
2	UNION	t2	NULL	eq_ref	PRIMARY,idx_message_language	PRIMARY	54	5am.t1.id,const	1	100	NULL
2	UNION	<subquery3>	NULL	eq_ref	<auto_distinct_key>	<auto_distinct_key>	5	5am.t1.id	1	100	Using where; Not exists
3	MATERIALIZED	message	NULL	ref	PRIMARY,idx_message_language	idx_message_language	50	const	1	100	Using index