Ich habe diese kleinen Tische,item undcategory:

CREATE TABLE `item` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(150) NOT NULL,
  `category_id` mediumint(8) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `name` (`name`),
  KEY `category_id` (`category_id`)
) CHARSET=utf8

CREATE TABLE `category` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(150) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `name` (`name`)
) CHARSET=utf8

Ich habe 100 Kategorien und 1000 Artikel eingefügt.

Wenn ich das laufen lasse:

EXPLAIN SELECT item.id,category.name AS category_name FROM item JOIN category ON item.category_id=category.id;

Wenn die Engine der Tabellen InnoDB ist, erhalte ich:

+----+-------------+----------+-------+---------------+-------------+---------+--------------------+------+-------------+
| id | select_type | table    | type  | possible_keys | key         | key_len | ref                | rows | Extra       |
+----+-------------+----------+-------+---------------+-------------+---------+--------------------+------+-------------+
|  1 | SIMPLE      | category | index | PRIMARY       | name        | 452     | NULL               |  103 | Using index |
|  1 | SIMPLE      | item     | ref   | category_id   | category_id | 3       | dbname.category.id |    5 | Using index |
+----+-------------+----------+-------+---------------+-------------+---------+--------------------+------+-------------+

Wenn ich auf MyISAM umsteige (mitalter table engine=myisam) Ich bekomme:

+----+-------------+----------+--------+---------------+---------+---------+-------------------------+------+-------+
| id | select_type | table    | type   | possible_keys | key     | key_len | ref                     | rows | Extra |
+----+-------------+----------+--------+---------------+---------+---------+-------------------------+------+-------+
|  1 | SIMPLE      | item     | ALL    | category_id   | NULL    | NULL    | NULL                    | 1003 |       |
|  1 | SIMPLE      | category | eq_ref | PRIMARY       | PRIMARY | 3       | dbname.item.category_id |    1 |       |
+----+-------------+----------+--------+---------------+---------+---------+-------------------------+------+-------+

Meine Frage ist, warum dieser Unterschied in der Art und Weise, wie Indizes behandelt werden?