MySQL optimisation needs explaination - mysql

Question

I needed to get values from the "latest" (i.e. highest record id) record for each value of a field (server_name in this case).
I had already added a server_name_id index on server_name and id.
My first attempt took minutes to run.
SELECT server_name, state
FROM replication_client as a
WHERE id = (
SELECT MAX(id)
FROM replication_client
WHERE server_name = a.server_name)
ORDER BY server_name
My second attempt took 0.001s to run.
SELECT rep.server_name, state FROM (
SELECT server_name, MAX(id) AS max_id
FROM replication_client
GROUP BY server_name) AS newest,
replication_client AS rep
WHERE rep.id = newest.max_id
ORDER BY server_name
What is the principle behind this optimisation? (I'd like to be able to write optimised queries without trial and error.)
P.S. Explained below:
mysql> EXPLAIN
->
-> SELECT server_name, state
-> FROM replication_client as a
-> WHERE id = (SELECT MAX(id) FROM replication_client WHERE server_name = a.server_name)
-> ORDER BY server_name
-> ;
+----+--------------------+--------------------+------+----------------+----------------+---------+-------------------+--------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+--------------------+------+----------------+----------------+---------+-------------------+--------+-----------------------------+
| 1 | PRIMARY | a | ALL | NULL | NULL | NULL | NULL | 630711 | Using where; Using filesort |
| 2 | DEPENDENT SUBQUERY | replication_client | ref | server_name_id | server_name_id | 18 | mrg.a.server_name | 45050 | Using index |
+----+--------------------+--------------------+------+----------------+----------------+---------+-------------------+--------+-----------------------------+
mysql> explain
-> SELECT rep.server_name, state FROM (
-> SELECT server_name, MAX(id) AS max_id
-> FROM replication_client
-> GROUP BY server_name) AS newest,
-> replication_client AS rep
-> WHERE rep.id = newest.max_id
-> ORDER BY server_name
-> ;
+----+-------------+--------------------+--------+---------------+----------------+---------+---------------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------------+--------+---------------+----------------+---------+---------------+------+---------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 2 | Using temporary; Using filesort |
| 1 | PRIMARY | rep | eq_ref | PRIMARY | PRIMARY | 4 | newest.max_id | 1 | |
| 2 | DERIVED | replication_client | range | NULL | server_name_id | 18 | NULL | 15 | Using index for group-by |
+----+-------------+--------------------+--------+---------------+----------------+---------+---------------+------+---------------------------------+

Answer 1

Well, the whole thing is quite self-explaining, when you look at two words in your first explain plan: DEPENDENT SUBQUERY
This means, that for every row, your where condition examines, the subquery is executed. Of course this can be slow as hell.
Also note, that there's an order of operations when executing a query.
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
ORDER BY clause
SELECT clause
When you can filter in FROM clause, it's better than filtering in WHERE clause...