used as the    |
|                                      | key value.                           |
+--------------------------------------+--------------------------------------+
| rows                                 | An estimate of how many rows we      |
|                                      | will find in the table for each key  |
|                                      | lookup.                              |
+--------------------------------------+--------------------------------------+
| Extra                                | Extra information about this join.   |
+--------------------------------------+--------------------------------------+

Here are descriptions of the values for some of the more complex columns in
EXPLAIN ... SELECT:

"Select_type" Column
--------------------

The select_type column can have the following values:

+-----------------+-----------------------------------+-----------------------+
| Value           | Description                       | Comment               |
+-----------------+-----------------------------------+-----------------------+
| DEPENDENT       | The SUBQUERY is DEPENDENT.        |                       |
| SUBQUERY        |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| DEPENDENT UNION | The UNION is DEPENDENT.           |                       |
+-----------------+-----------------------------------+-----------------------+
| DERIVED         | The SELECT is DERIVED from the    |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-----------------------------------+-----------------------+
| MATERIALIZED    | The SUBQUERY is MATERIALIZED.     | Materialized tables   |
|                 |                                   | will be populated at  |
|                 |                                   | first access and      |
|                 |                                   | will be accessed by   |
|                 |                                   | the primary key (=    |
|                 |                                   | one key lookup).      |
|                 |                                   | Number of rows in     |
|                 |                                   | EXPLAIN shows the     |
|                 |                                   | cost of populating    |
|                 |                                   | the table             |
+-----------------+-----------------------------------+-----------------------+
| PRIMARY         | The SELECT is in the outermost    |                       |
|                 | query, but there is also a        |                       |
|                 | SUBQUERY within it.               |                       |
+-----------------+-----------------------------------+-----------------------+
| SIMPLE          | It is a simple SELECT query       |                       |
|                 | without any SUBQUERY or UNION.    |                       |
+-----------------+-----------------------------------+-----------------------+
| SUBQUERY        | The SELECT is a SUBQUERY of the   |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-----------------------------------+-----------------------+
| UNCACHEABLE     | The SUBQUERY is UNCACHEABLE.      |                       |
| SUBQUERY        |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| UNCACHEABLE     | The UNION is UNCACHEABLE.         |                       |
| UNION           |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| UNION           | The SELECT is a UNION of the      |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-------------------@߯�