MySQL の EXPLAIN の読み方

EXPLAIN SELECT * FROM orders WHERE user_id = 42;

+----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | orders | NULL       | ref  | idx_user_id   | idx_user_id | 4       | const |   12 |   100.00 | NULL        |
+----+-------------+--------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+

CREATE TABLE orders (
    id         INT PRIMARY KEY AUTO_INCREMENT,
    user_id    INT NOT NULL,
    status     VARCHAR(20) NOT NULL,
    created_at DATETIME NOT NULL
);

-- No indexes yet
EXPLAIN SELECT * FROM orders WHERE status = 'shipped' ORDER BY created_at;

| type | key  | rows    | Extra                          |
|------|------|---------|--------------------------------|
| ALL  | NULL | 1000000 | Using where; Using filesort    |

CREATE INDEX idx_status_created ON orders (status, created_at);

EXPLAIN SELECT * FROM orders WHERE status = 'shipped' ORDER BY created_at;

| type | key                | rows | Extra |
|------|--------------------|------|-------|
| ref  | idx_status_created | 4200 | NULL  |

EXPLAIN ANALYZE SELECT * FROM orders WHERE user_id = 42;

-> Index lookup on orders using idx_user_id (user_id=42)
   (cost=4.50 rows=12) (actual time=0.045..0.112 rows=11 loops=1)