create user foo@localhost;
grant ALL on *.* to foo@localhost;

====================================================================
Testing index for columns HOST
====================================================================
############ Explain for Query ####################################
explain select TOTAL_CONNECTIONS 
from performance_schema.hosts 
where HOST = "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	#	NULL	no matching row in const table
############ Explain for Query ####################################
explain select TOTAL_CONNECTIONS
from performance_schema.hosts
where HOST > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	hosts	NULL	ALL	HOST	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select TOTAL_CONNECTIONS
from performance_schema.hosts
where HOST < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	hosts	NULL	ALL	HOST	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select TOTAL_CONNECTIONS
from performance_schema.hosts
where HOST = "localhost";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	hosts	NULL	const	HOST	HOST	256	const	#	100.00	NULL
############# Explain End #########################################
flush status;
select TOTAL_CONNECTIONS
from performance_schema.hosts
where HOST = "localhost";
TOTAL_CONNECTIONS
#
OK: handler_read_key incremented
drop user foo@localhost;