EXPLAINをどのように読み、遅いクエリの最適化に使いますか。

EXPLAIN SELECT * FROM orders WHERE customer_id = 42 ORDER BY created_at;
-- +------+------+------+---------+--------+----------------+
-- | type | key  | rows | Extra                             |
-- | ALL  | NULL | 900k | Using where; Using filesort       |   <- scans everything, sorts
-- +------+------+------+---------+--------+----------------+

-- Add an index that covers the filter AND the sort order:
CREATE INDEX idx_cust_created ON orders (customer_id, created_at);

EXPLAIN SELECT * FROM orders WHERE customer_id = 42 ORDER BY created_at;
-- | type | key              | rows | Extra       |
-- | ref  | idx_cust_created |   30 | Using where |   <- seeks 30 rows, no filesort