Mysql two table don't use the index-CodePudding

Two tables test_a, test_b structure and index information as follows, by primary key inner join associations, appearance index of why don't you go?
 
The create table test_a (
Id int, 
Birthday date is not null, 
The comment varchar (50) not null, 
Primary key test_a_pk (id), 
The index test_a_index (birthday) 
) engine=innodb default charset=utf8; 

The create table test_b (
Id int, 
Salary is double the not null, 
Struct varchar (50) not null, 
Primary key test_b_pk (id), 
The index test_b_index (salary) 
) engine=innodb default charset=utf8; 

mysql> Show the index from test_a \ G 
* * * * * * * * * * * * * * * * * * * * * * * * * * * 1 row * * * * * * * * * * * * * * * * * * * * * * * * * * * 
Table: test_a 
Non_unique: 0 
Key_name: PRIMARY 
Seq_in_index: 1 
Column_name: id 
Collation: A 
Cardinality: 5375084 
Sub_part: NULL 
The Packed: NULL 
Null: 
Index_type: BTREE 
Comment: 
Index_comment: 
* * * * * * * * * * * * * * * * * * * * * * * * * * * 2. Row * * * * * * * * * * * * * * * * * * * * * * * * * * * 
Table: test_a 
Non_unique: 1 
Key_name: test_a_index 
Seq_in_index: 1 
Column_name: birthday 
Collation: A 
Cardinality: 2728 
Sub_part: NULL 
The Packed: NULL 
Null: 
Index_type: BTREE 
Comment: 
Index_comment: 
2 rows in the set (0.00 SEC) 

mysql> Show the index from test_b \ G 
* * * * * * * * * * * * * * * * * * * * * * * * * * * 1 row * * * * * * * * * * * * * * * * * * * * * * * * * * * 
Table: test_b 
Non_unique: 0 
Key_name: PRIMARY 
Seq_in_index: 1 
Column_name: id 
Collation: A 
Cardinality: 5291561 
Sub_part: NULL 
The Packed: NULL 
Null: 
Index_type: BTREE 
Comment: 
Index_comment: 
* * * * * * * * * * * * * * * * * * * * * * * * * * * 2. Row * * * * * * * * * * * * * * * * * * * * * * * * * * * 
Table: test_b 
Non_unique: 1 
Key_name: test_b_index 
Seq_in_index: 1 
Column_name: salary 
Collation: A 
Cardinality: 943640 
Sub_part: NULL 
The Packed: NULL 
Null: 
Index_type: BTREE 
Comment: 
Index_comment: 
2 rows in the set (0.00 SEC)