Index on VARCHAR column not used to lookup INT constant

MariaDB [test]> CREATE TABLE t (a VARCHAR(10), b VARCHAR(10), INDEX idx_a (a));

Query OK, 0 rows affected (0.02 sec)

MariaDB [test]> INSERT INTO t VALUES ('1', '1'), ('2', '2'), ('3', '3'), ('4', '4'), ('5', '5'), ('1', '5');

Query OK, 6 rows affected (0.01 sec)

Records: 6  Duplicates: 0  Warnings: 0

MariaDB [test]> EXPLAIN SELECT * FROM t WHERE a = 3 \G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: t

         type: ALL

possible_keys: idx_a

          key: NULL

      key_len: NULL

          ref: NULL

         rows: 6

        Extra: Using where

1 row in set (0.00 sec)

MariaDB [test]> CREATE TABLE t (a INT UNSIGNED, b INT UNSIGNED, INDEX idx_a (a));

Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> INSERT INTO t VALUES (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (1, 5);

Query OK, 6 rows affected (0.00 sec)

Records: 6  Duplicates: 0  Warnings: 0

MariaDB [test]> EXPLAIN SELECT * FROM t WHERE a = '3' \G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: t

         type: ref

possible_keys: idx_a

          key: idx_a

      key_len: 5

          ref: const

         rows: 1

        Extra: 

1 row in set (0.00 sec)