why inner join on condition does not use key - mysql

Question

I am creating a table as
create table temp_test2 (
date_id int(11) NOT NULL DEFAULT '0',
`date` date NOT NULL,
`day` int(11) NOT NULL,
PRIMARY KEY (date_id)
);
create table temp_test1 (
date_id int(11) NOT NULL DEFAULT '0',
`date` date NOT NULL,
`day` int(11) NOT NULL,
PRIMARY KEY (date_id)
);
explain select * from temp_test as t inner join temp_test2 as t2 on (t2.date_id =t.date_id) limit 3;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| 1 | SIMPLE | t | ALL | date_id | NULL | NULL | NULL | 4 | NULL |
| 1 | SIMPLE | t2 | ALL | date_id | NULL | NULL | NULL | 4 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
why the code_id key is not used in both the table, but when I use code_id=something in on condition it's using the key,
explain select * from temp_test as t inner join temp_test2 as t2 on (t2.date_id =t.date_id and t.date_id =1) limit 3;
+----+-------------+-------+-------+-------------------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-------------------------------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | t | const | PRIMARY,date_id,date_id_2,date_id_3 | PRIMARY | 4 | const | 1 | NULL |
| 1 | SIMPLE | t2 | ref | date_id,date_id_2,date_id_3 | date_id | 4 | const | 1 | NULL |
+----+-------------+-------+-------+-------------------------------------+---------+---------+-------+------+-------+
I tried (unique,composite primary,composite) key also but it is not working.
Can anyone explain why this so?

Answer 1

Because your tables contain a very small number of records, the optimiser decides that it is not worth using the index. A table scan will do just as good.
Also, you selected all fields (SELECT *), if it used the index for executing the JOIN a row scan would still be required to get the full contents.
The query would be more likely to use the index if:
you selected only the date_id field
there were more than 4 rows in temp_test