drop table if exists t1;create table t1 (id int not null, str char(10), unique(str));explain select * from t1;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	const row not foundinsert into t1 values (1, null),(2, null),(3, "foo"),(4, "bar");select * from t1 where str is null;id	str1	NULL2	NULLselect * from t1 where str="foo";id	str3	fooexplain select * from t1 where str is null;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	t1	ref	str	str	11	const	1	Using whereexplain select * from t1 where str="foo";id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	t1	const	str	str	11	const	1	explain select * from t1 ignore key (str) where str="foo";id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using whereexplain select * from t1 use key (str,str) where str="foo";id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	t1	const	str	str	11	const	1	explain select * from t1 use key (str,str,foo) where str="foo";ERROR 42000: Key column 'foo' doesn't exist in tableexplain select * from t1 ignore key (str,str,foo) where str="foo";ERROR 42000: Key column 'foo' doesn't exist in tabledrop table t1;explain select 1;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables usedcreate table t1 (a int not null);explain select count(*) from t1;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized awayinsert into t1 values(1);explain select count(*) from t1;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized awayinsert into t1 values(1);explain select count(*) from t1;id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized awaydrop table t1;set names koi8r;create table 粵