MySQL Not Using Index when Specifying Non-Indexed Columns - mysql

Question

When trying to run a query specifying a non-indexed column or *, the index isn't used. Using the indexed column or count(*) does use the index. Any idea why specifying any non-indexed column in the output would case the index to not be used? I've noticed that as the number of rows increases, my queries have been slowing down.
Here are three identical queries where one uses count(*), the other just uses * and one uses an indexed column.
Optimized query
MariaDB [db]> explain select count(*) from data_history where id=18 and time > date_sub(now(), interval 7 day)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: data_history
type: index
possible_keys: PRIMARY,ID
key: ID
key_len: 26
ref: NULL
rows: 1176921
Extra: Using where; Using index
1 row in set (0.00 sec)
Non-optimized query
MariaDB [db]> explain select * from data_history where id=18 and time > date_sub(now(), interval 7 day)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: data_history
type: ALL
possible_keys: PRIMARY,ID
key: NULL
key_len: NULL
ref: NULL
rows: 1176929
Extra: Using where
1 row in set (0.00 sec)
Optimized query
MariaDB [db]> explain select id from data_history where id=18 and time > date_sub(now(), interval 7 day)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: data_history
type: index
possible_keys: PRIMARY,ID
key: ID
key_len: 26
ref: NULL
rows: 1176935
Extra: Using where; Using index
1 row in set (0.00 sec)
Table structure
CREATE TABLE `data_history` (
`id` varchar(20) NOT NULL DEFAULT '',
`time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`b1` int(3) DEFAULT NULL,
PRIMARY KEY (`id`,`time`),
UNIQUE KEY `ID` (`id`,`time`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;