The EXPLAIN command is an important tool to review how a SQL query is executed and in this example includes what indexes are used.
By adding a covering index I ended up with the following EXPLAIN plan I was unable to explain. The end result was a boost in server performance which was the ultimate goal.
mysql> explain select max(md) from e_r where email = 'xxxx@gmail.com' and id = '36981'; +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ 1 row in set (0.00 sec)
The queries still produced the expected results.