Re: the where condition field after ">=" can use index? is this true?

[Bing Wu]

Hi, James. Thank you for your direction. I've read your blog. It's perfact.
I have a question about "Algorithm, Step 2a (one range)".
Your example is
"WHERE aaa >= 123 AND ccc > 'xyz' ⇒ INDEX(aaa) or INDEX(ccc) (only one range)".
I have tried it out:
First, I created a talbe like this:
`id` int(11) NOT NULL AUTO_INCREMENT,
`aaa` int(11) NOT NULL DEFAULT '0',
`ccc` int(11) NOT NULL DEFAULT '0',
`xxx` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
KEY `idx_ac` (`aaa`,`ccc`)

Then, I ran the sql to see use of index idx_ac:
explain select * from T force index (idx_ac) where aaa >= 123 and ccc > 'abc';

As you directed, I have expected the "key_len" is 4 (int is 4-byte), but in fact, the key_len is 16. I'm confused. Has MySQL used the full index(idx_ac) ? Or, is there any bug in explain ?
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------+
| 1 | SIMPLE | T | range | idx_ac | idx_ac | 16 | NULL | 1002 | Using index condition |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------+