Understanding MySQL key_len in Explain Statement

According to MySQL website, the key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key column says NULL. Note that the value of key_len enables you to determine how many parts of a multiple-part key MySQL actually uses.

Using an example from my previous question, I have an EXPLAIN SELECT statement that shows MySQL using an Index with key_len: 6. Below shows the composition of the index and columns used.

`Type` char(1) NOT NULL,
`tn` char(1) NOT NULL DEFAULT 'l',
`act` tinyint(1) unsigned NOT NULL DEFAULT '0',
`flA` mediumint(6) unsigned NOT NULL DEFAULT '0',
KEY `Index` (`Type`, `tn开发者_如何学编程`, `act`, `flA`)

So how does the value of key_len allows me to determine that my query uses the first three parts of a multiple-part key?

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The key_len specifies the number of bytes that MySQL uses from the key.
Indexes are always used left_to_right. i.e. only the left-most part is used.

The length of your fields is as follows:

1 byte             `Type` char(1) NOT NULL,
1 byte             tn char(1) NOT NULL DEFAULT 'l',
1 byte             act tinyint(1) unsigned NOT NULL DEFAULT '0',
3 bytes            flA mediumint(6) unsigned NOT NULL DEFAULT '0',
1+1+1+3 = 6 bytes  KEY `Index` (`Type`, `tn`, `act`, `flA`)
 key usage always starts here ---^^^^^

If the key_len = 3 then it's using type+tn+act.
Note that Key_len = 4 is impossible in this configuration.