SELECT_TYPE：
1） SIMPLE：简单的SELECT，不实用UNION或者子查询
mysql> explain select * from t2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 100 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
2） PRIMARY：最外层SELECT。
mysql> explain select * from (select * from t2 where id2=2) b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+----
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