mysql> explain SELECT DISTINCT * FROM wp_posts WHERE 1=1 AND (post_type = 'post' AND (post_status = 'publish')) GROUP BY wp_posts.ID ORDER BY post_date DESC LIMIT 0, 10
-> ;
+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+
| 1 | SIMPLE | wp_posts | ALL | | | | | 3 | Using where; Using temporary; Using filesort |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------+
1 row in set (0.00 sec)
mysql> explain SELECT * FROM wp_posts WHERE 1=1 AND (post_type = 'post' AND (post_status = 'publish')) ORDER BY post_date DESC LIMIT 0, 10
-> ;
+----+-------------+----------+------+---------------+------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+-----------------------------+
| 1 | SIMPLE | wp_posts | ALL | | | | | 3 | Using where; Using filesort |
+----+-------------+----------+------+---------------+------+---------+------+------+-----------------------------+
1 row in set (0.01 sec)
mysql> create index blah on wp_posts(post_status,post_type,post_date);
Query OK, 3 rows affected (0.05 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> explain SELECT * FROM wp_posts WHERE 1=1 AND (post_type = 'post' AND (post_status = 'publish')) ORDER BY post_date DESC LIMIT 0, 10;
+----+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | wp_posts | ref | blah | blah | 103 | const,const | 1 | Using where |
+----+-------------+----------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.01 sec)