Query not using index depending on COUNT parameter - mysql

Question

First, I create a simple database with one MyISAM table with an indexed field called feature.
CREATE DATABASE test;
USE test;
CREATE TABLE data(
id INT(11) NOT NULL AUTO_INCREMENT,
feature VARCHAR(64),
PRIMARY KEY (id)
) ENGINE=MyISAM;
INSERT INTO data VALUES (1, 'a'), (2, 'b');
CREATE INDEX data_feature ON data(feature);
Then, when I test a GROUP BY query with a count, it doesn't use the index when the COUNT() is made by id (see Extra column at the end of the EXPLAIN).
mysql> EXPLAIN SELECT feature, COUNT(1) FROM data GROUP BY feature;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | data | NULL | index | data_feature | data_feature | 259 | NULL | 2 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT feature, COUNT(*) FROM data GROUP BY feature;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | data | NULL | index | data_feature | data_feature | 259 | NULL | 2 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT feature, COUNT(id) FROM data GROUP BY feature;
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
| 1 | SIMPLE | data | NULL | index | data_feature | data_feature | 259 | NULL | 2 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
I have tested it on MySQL Community 8.0.21 and MariaDB 10.3.25.

Answer 1

COUNT(id) obligates the Optimizer to check id for being NOT NULL. The standard pattern is to simply say COUNT(*).
InnoDB probably works differently. But this is because the PRIMARY KEY is implicitly tacked onto the end of any secondary index. That makes INDEX(feature) work like INDEX(feature, id), in which case it would be a "covering" index as indicated by "Using index".
(There is virtually no reason to stick with MyISAM in this decade.)