I was surprised to find yesterday when using MySQL 5.1.26-rc with a client I’m recommending 5.1 to, some information not seen in the EXPLAIN plan before while reviewing SQL Statements.

Using join buffer

+----+-------------+-------+--------+---------------+--------------+---------+------------------------+-------+----------------------------------------------+
| id | select_type | table | type   | possible_keys | key          | key_len | ref                    | rows  | Extra                                        |
+----+-------------+-------+--------+---------------+--------------+---------+------------------------+-------+----------------------------------------------+
|  1 | SIMPLE      | lr    | ALL    | NULL          | NULL         | NULL    | NULL                   |  1084 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | ca    | ref    | update_check  | update_check | 4       | XXXXXXXXXXXXXXXXX      |     4 | Using where; Using index                     |
|  1 | SIMPLE      | ce    | ALL    | NULL          | NULL         | NULL    | NULL                   | 13319 | Using where; Using join buffer               |
|  1 | SIMPLE      | co    | eq_ref | PRIMARY       | PRIMARY      | 4       | XXXXXXXXXXXXXXXXX      |     1 | Using where                                  |
+----+-------------+-------+--------+---------------+--------------+---------+------------------------+-------+----------------------------------------------+
4 rows in set (0.00 sec)
mysql> select version();
+-----------+
| version() |
+-----------+
| 5.1.26-rc |
+-----------+
1 row in set (0.00 sec)

Sergey Petrunia of the MySQL Optimizer team writes about this in Use of join buffer is now visible in EXPLAIN.