Multicolumn Indexes

CREATE DATABASE practise_2;
USE practise_2;

DROP TABLE `myTable`;

CREATE TABLE `myTable` (
  `id` mediumint(8) unsigned NOT NULL auto_increment,
  `first_name` varchar(255) default NULL,
  `last_name` varchar(255) default NULL,
  `hire_date` varchar(255),
  PRIMARY KEY (`id`)
) AUTO_INCREMENT=1;

DROP INDEX my_table_hire_date_first_name_idx ON myTable;
CREATE INDEX my_table_hire_date_first_name_idx ON myTable(hire_date, first_name);

EXPLAIN
SELECT
    first_name
FROM myTable
WHERE hire_date BETWEEN '2015-01-01' AND '2015-12-31';