Check key_len in the explain to determine which index is used

Check the key_len in the explain to determine which index is used?

There are multiple indexes in a table, and the conditions in our where field have multiple index values, so which one should we use?

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We can use explain to view, the key_len field can be seen

For example, the following sql

explain select * from ent_calendar_diary where email='xxxx' and diary_id=1784;
 
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+
| id | select_type | table              | partitions | type  | possible_keys           | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | ent_calendar_diary | NULL       | const | PRIMARY,idx_email_stime | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------------------+------------+-------+-------------------------+---------+---------+-------+------+----------+-------+

There are two index fields in possible_keys, but key_len is 4 bytes

Remarks, key_len only indicates The index column selected for conditional filtering in WHERE does not contain ORDER BY/GROUP BY

int type and not null is 4 bytes, so the above sql uses the primary key index

explain select * from ent_calendar_diary where email='xxxx';
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table              | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | ent_calendar_diary | NULL       | ref  | idx_email_stime | idx_email_stime | 767     | const |  111 |   100.00 | NULL  |
+----+-------------+--------------------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+

This is 767 bytes, varchar(255) not null 255 * 3 2 exactly matches, so it is the ordinary index of email used

CREATE TABLE `ent_calendar_diary` (
`diary_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`email` varchar(255) NOT NULL,
`title` varchar(100) NOT NULL,
`summary` varchar(500) NOT NULL DEFAULT '',
`stime` bigint(11) NOT NULL DEFAULT '0',
`ctime` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`diary_id`),
KEY `idx_email_stime` (`email`,`stime`)
) ENGINE=InnoDB AUTO_INCREMENT=1809 DEFAULT CHARSET=utf8

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