select connection_id() into @conid;
select thread_id from performance_schema.threads
where PROCESSLIST_ID = @conid
into @tid;
set @a=1;
set @b=2;
set @c=3;

====================================================================
Testing index for columns THREAD_ID, VARIABLE_NAME
====================================================================
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ref	PRIMARY	PRIMARY	8	const	#	100.00	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ref	PRIMARY	PRIMARY	8	const	#	100.00	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = "impossible"
    and VARIABLE_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 VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid
and VARIABLE_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 VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid
and VARIABLE_NAME > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ref	PRIMARY	PRIMARY	8	const	#	33.33	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid
and VARIABLE_NAME < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	ref	PRIMARY	PRIMARY	8	const	#	33.33	Using where
############ Explain for Query ####################################
explain select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid
and VARIABLE_NAME = "a";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	user_variables_by_thread	NULL	const	PRIMARY	PRIMARY	266	const,const	#	100.00	NULL
############# Explain End #########################################
flush status;
select VARIABLE_VALUE
from performance_schema.user_variables_by_thread
where THREAD_ID = @tid
and VARIABLE_NAME = "a";
VARIABLE_VALUE
#
OK: handler_read_key incremented