select FILE_NAME, EVENT_NAME from performance_schema.file_instances
limit 1
into @fname, @ename;

====================================================================
Testing index for columns FILE_NAME
====================================================================
############ Explain for Query ####################################
explain select count(*) 
from performance_schema.file_instances 
where FILE_NAME = "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 count(*)
from performance_schema.file_instances
where FILE_NAME > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select count(*)
from performance_schema.file_instances
where FILE_NAME < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select count(*)
from performance_schema.file_instances
where FILE_NAME = @fname;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	const	PRIMARY	PRIMARY	2050	const	#	100.00	NULL
############# Explain End #########################################
flush status;
select count(*)
from performance_schema.file_instances
where FILE_NAME = @fname;
count(*)
#
OK: handler_read_key incremented

====================================================================
Testing index for columns EVENT_NAME
====================================================================
############ Explain for Query ####################################
explain select count(*) 
from performance_schema.file_instances 
where EVENT_NAME = "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ref	EVENT_NAME	EVENT_NAME	514	const	#	100.00	NULL
############ Explain for Query ####################################
explain select count(*)
from performance_schema.file_instances
where EVENT_NAME > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ALL	EVENT_NAME	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select count(*)
from performance_schema.file_instances
where EVENT_NAME < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ALL	EVENT_NAME	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select count(*)
from performance_schema.file_instances
where EVENT_NAME = @ename;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	file_instances	NULL	ref	EVENT_NAME	EVENT_NAME	514	const	#	100.00	NULL
############# Explain End #########################################
flush status;
select count(*)
from performance_schema.file_instances
where EVENT_NAME = @ename;
count(*)
#
OK: handler_read_key incremented