mysql> EXPLAIN SELECT  SQL_CALC_FOUND_ROWS wp_posts.ID
    ->     FROM  wp_posts
    ->     LEFT JOIN  wp_term_relationships
    ->         ON (wp_posts.ID = wp_term_relationships.object_id)
    ->     WHERE  1=1
    ->       AND  ( wp_term_relationships.term_taxonomy_id IN (3) )
    ->       AND  wp_posts.post_type = 'post'
    ->       AND  ((wp_posts.post_status = 'publish'))
    ->     GROUP BY  wp_posts.ID
    ->     ORDER BY  wp_posts.post_date DESC
    ->     LIMIT  0, 6;
+----+-------------+-----------------------+----------------------+--------+---------------------------------------------------------------------------------+----------------------+---------+-------------------------------------+-------+----------+-----------------------------------------------------------+
| id | select_type | table                 | partitions           | type   | possible_keys                                                                   | key                  | key_len | ref                                 | rows  | filtered | Extra                                                     |
+----+-------------+-----------------------+----------------------+--------+---------------------------------------------------------------------------------+----------------------+---------+-------------------------------------+-------+----------+-----------------------------------------------------------+
|  1 | SIMPLE      | wp_posts              | NULL                 | ref    | PRIMARY,post_name,type_status_date,post_parent,post_author,type_status_date_ndx | type_status_date_ndx | 164     | const,const                         | 39756 |   100.00 | Using where; Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | wp_term_relationships | p0,p1,p2,p3,p4,p5,p6 | eq_ref | PRIMARY,term_taxonomy_id                                                        | PRIMARY              | 16      | xxxx.wp_posts.ID,const |     1 |   100.00 | Using index                                               |
+----+-------------+-----------------------+----------------------+--------+---------------------------------------------------------------------------------+----------------------+---------+-------------------------------------+-------+----------+-----------------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT  SQL_CALC_FOUND_ROWS wp_posts.ID
    ->     FROM  wp_posts
    ->     WHERE  wp_posts.post_type = 'post'
    ->       AND  wp_posts.post_status = 'publish'
    ->     ORDER BY  wp_posts.post_date DESC
    ->     LIMIT  0, 6;
+----+-------------+----------+------------+------+---------------------------------------+----------------------+---------+-------------+-------+----------+--------------------------+
| id | select_type | table    | partitions | type | possible_keys                         | key                  | key_len | ref         | rows  | filtered | Extra                    |
+----+-------------+----------+------------+------+---------------------------------------+----------------------+---------+-------------+-------+----------+--------------------------+
|  1 | SIMPLE      | wp_posts | NULL       | ref  | type_status_date,type_status_date_ndx | type_status_date_ndx | 164     | const,const | 39756 |   100.00 | Using where; Using index |
+----+-------------+----------+------------+------+---------------------------------------+----------------------+---------+-------------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)