#План выполнения запроса с LEFT JOIN
mysql> EXPLAIN SELECT wp_posts.post_title FROM wp_posts LEFT JOIN wp_term_relationships on wp_term_relationships.object_id=wp_posts.ID WHERE wp_term_relationships.term_taxonomy_id=4;
+----+-------------+-----------------------+--------+--------------------------+------------------+---------+-----------------------------------------------+------+-------------+
| id | select_type | table                 | type   | possible_keys            | key              | key_len | ref                                           | rows | Extra       |
+----+-------------+-----------------------+--------+--------------------------+------------------+---------+-----------------------------------------------+------+-------------+
|  1 | SIMPLE      | wp_term_relationships | ref    | PRIMARY,term_taxonomy_id | term_taxonomy_id | 8       | const                                         |   18 | Using where |
|  1 | SIMPLE      | wp_posts              | eq_ref | PRIMARY                  | PRIMARY          | 8       | user3697_test.wp_term_relationships.object_id |    1 |             |
+----+-------------+-----------------------+--------+--------------------------+------------------+---------+-----------------------------------------------+------+-------------+
2 rows in set (0.00 sec)

#План выполнения запроса с вложенным SELECTом
mysql> EXPLAIN SELECT post_title FROM wp_posts WHERE ID in (SELECT object_id FROM wp_term_relationships WHERE term_taxonomy_id=4);
+----+--------------------+-----------------------+-----------------+--------------------------+---------+---------+------------+------+--------------------------+
| id | select_type        | table                 | type            | possible_keys            | key     | key_len | ref        | rows | Extra                    |
+----+--------------------+-----------------------+-----------------+--------------------------+---------+---------+------------+------+--------------------------+
|  1 | PRIMARY            | wp_posts              | ALL             | NULL                     | NULL    | NULL    | NULL       | 1814 | Using where              |
|  2 | DEPENDENT SUBQUERY | wp_term_relationships | unique_subquery | PRIMARY,term_taxonomy_id | PRIMARY | 16      | func,const |    1 | Using index; Using where |
+----+--------------------+-----------------------+-----------------+--------------------------+---------+---------+------------+------+--------------------------+

#План выполнения вложенного SELECTа отдельно (на выходе получаем список ID который используется ниже)
mysql> EXPLAIN SELECT object_id FROM wp_term_relationships WHERE term_taxonomy_id=4;
+----+-------------+-----------------------+------+------------------+------------------+---------+-------+------+-------+
| id | select_type | table                 | type | possible_keys    | key              | key_len | ref   | rows | Extra |
+----+-------------+-----------------------+------+------------------+------------------+---------+-------+------+-------+
|  1 | SIMPLE      | wp_term_relationships | ref  | term_taxonomy_id | term_taxonomy_id | 8       | const |   18 |       |
+----+-------------+-----------------------+------+------------------+------------------+---------+-------+------+-------+
1 row in set (0.00 sec)

#План выполнения запроса выбирающего строки с ID входящими в полученный ранее список
mysql> EXPLAIN SELECT post_title FROM wp_posts WHERE ID in (12,15,234,246,251,259,355,282,293,298,316,425,812,818,855,944,947,1472,1477);
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | wp_posts | range | PRIMARY       | PRIMARY | 8       | NULL |   19 | Using where |
+----+-------------+----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)