MySQL Optimization -- which is faster, or does it matter?

random.sql

--- Option 1

SELECT      p.ID
FROM        wp_posts p
INNER JOIN  wp_postmeta pm ON p.ID = pm.post_id
WHERE       pm.key = 'key'

--- Option 2

SELECT      p.ID
FROM        wp_posts p
INNER JOIN  wp_postmeta pm ON ( p.ID = pm.post_id AND pm.key = 'key' )

--- EXPLAIN results for each:



mysql> EXPLAIN EXTENDED SELECT      p.ID
FROM        wp_posts p
INNER JOIN  wp_postmeta pm ON ( p.ID = pm.post_id AND pm.meta_key = 'key' );
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
| id | select_type | table | type   | possible_keys    | key      | key_len | ref            | rows | filtered | Extra       |
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
| 1  | SIMPLE      | pm    | ref    | post_id,meta_key | meta_key | 768     | const          | 1    | 100.00   | Using where |
| 1  | SIMPLE      | p     | eq_ref | PRIMARY          | PRIMARY  | 8       | wpb.pm.post_id | 1    | 100.00   | Using index |
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
2 rows in set (0.06 sec)

mysql> EXPLAIN EXTENDED SELECT      p.ID
FROM        wp_posts p
INNER JOIN  wp_postmeta pm ON p.ID = pm.post_id
WHERE       pm.meta_key = 'key';
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
| id | select_type | table | type   | possible_keys    | key      | key_len | ref            | rows | filtered | Extra       |
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
| 1  | SIMPLE      | pm    | ref    | post_id,meta_key | meta_key | 768     | const          | 1    | 100.00   | Using where |
| 1  | SIMPLE      | p     | eq_ref | PRIMARY          | PRIMARY  | 8       | wpb.pm.post_id | 1    | 100.00   | Using index |
+----+-------------+-------+--------+------------------+----------+---------+----------------+------+----------+-------------+
2 rows in set (0.03 sec)