select PROCESSLIST_ID from performance_schema.session_connect_attrs limit 1
into @plid;

====================================================================
Testing index for columns PROCESSLIST_ID, ATTR_NAME
====================================================================
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ref	PRIMARY	PRIMARY	8	const	#	100.00	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ALL	PRIMARY	NULL	NULL	NULL	#	33.33	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ref	PRIMARY	PRIMARY	8	const	#	100.00	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = "impossible"
    and ATTR_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 ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid
and ATTR_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 ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid
and ATTR_NAME > "impossible";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ref	PRIMARY	PRIMARY	8	const	#	33.33	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid
and ATTR_NAME < "2";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	ref	PRIMARY	PRIMARY	8	const	#	33.33	Using where
############ Explain for Query ####################################
explain select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid
and ATTR_NAME = "_os";
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	session_connect_attrs	NULL	const	PRIMARY	PRIMARY	138	const,const	#	100.00	NULL
############# Explain End #########################################
flush status;
select ORDINAL_POSITION
from performance_schema.session_connect_attrs
where PROCESSLIST_ID = @plid
and ATTR_NAME = "_os";
ORDINAL_POSITION
#
OK: handler_read_key incremented