MySQL  EXPLAIN 关键参数实战举例

-- 分析SELECT语句
EXPLAIN SELECT * FROM users WHERE id = 1;
-- 分析DELETE/UPDATE/INSERT（MySQL 5.6+）
EXPLAIN DELETE FROM users WHERE status = 'inactive';
-- 显示更详细信息（MySQL 8.0.18+）
EXPLAIN ANALYZE SELECT * FROM users WHERE id = 1;  -- 实际执行并显示耗时
-- 输出JSON格式（便于程序解析）
EXPLAIN FORMAT=JSON SELECT * FROM users WHERE id = 1;
-- 查看连接中正在执行的SQL的执行计划
EXPLAIN FOR CONNECTION 123;  -- 123为connection_id

EXPLAIN SELECT o.order_id, u.user_name 
FROM orders o 
LEFT JOIN users u ON o.user_id = u.user_id 
WHERE o.order_status = 'pending' 
  AND o.created_at >= '2024-01-01' 
LIMIT 10\G

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: o
   partitions: NULL
         type: range
possible_keys: idx_status_created,idx_created_at
          key: idx_status_created
      key_len: 102
          ref: NULL
         rows: 185000
     filtered: 100.00
        Extra: Using index condition
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: u
   partitions: NULL
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: test.o.user_id
         rows: 1
     filtered: 100.00
        Extra: NULL

-- id相同：多表JOIN
EXPLAIN SELECT * FROM orders o, users u WHERE o.user_id = u.user_id;
-- 结果：两个id均为1
-- id不同：子查询
EXPLAIN SELECT * FROM users WHERE user_id IN (SELECT user_id FROM orders);
-- 结果：子查询id=2，外层查询id=1（子查询先执行）

-- 避免DEPENDENT SUBQUERY（子查询依赖外层）
EXPLAIN SELECT * FROM users u 
WHERE (SELECT COUNT(*) FROM orders o WHERE o.user_id = u.user_id) > 5;
-- 优化：改为JOIN + GROUP BY
-- 注意DERIVED临时表开销
EXPLAIN SELECT * FROM (SELECT * FROM orders WHERE status='pending') t;
-- 建议：直接查询，或创建视图

system > const > eq_ref > ref > range > index > ALL

-- const：最优
EXPLAIN SELECT * FROM users WHERE user_id = 1;
-- type=const
-- ref：良好
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending';
-- type=ref（假设status有索引）
-- range：可接受
EXPLAIN SELECT * FROM orders WHERE created_at >= '2024-01-01';
-- type=range（假设created_at有索引）
-- ALL：必须优化
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending';
-- type=ALL（status无索引）

-- possible_keys有值但key为NULL：索引未被使用
EXPLAIN SELECT * FROM orders WHERE DATE(created_at) = '2024-01-01';
-- possible_keys: idx_created_at, key: NULL
-- 原因：对索引字段使用了函数，索引失效
-- 实际使用了联合索引
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending' AND created_at > '2024-01-01';
-- key: idx_status_created（联合索引）

-- 联合索引：idx_status_created (order_status VARCHAR(20), created_at DATETIME)
SHOW INDEX FROM orders;  -- 查看索引定义
EXPLAIN SELECT * FROM orders 
WHERE order_status = 'pending' 
  AND created_at >= '2024-01-01';
-- key_len = 83
-- 计算：order_status(20*3+2=62) + created_at(8) + NULL标志(1) = 71? 实际83包含额外开销
-- 如果只使用第一个字段
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending';
-- key_len = 62（仅使用了order_status字段）

EXPLAIN SELECT * FROM orders o, users u 
WHERE o.user_id = u.user_id 
  AND o.order_status = 'pending';
-- table=o 的 ref: const (order_status='pending')
-- table=u 的 ref: test.o.user_id (关联到orders表的user_id)

-- 全表扫描：rows ≈ 表总行数
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending';
-- rows: 5,000,000（全表）
-- 添加索引后：rows大幅下降
EXPLAIN SELECT * FROM orders WHERE order_status = 'pending';
-- rows: 250,000（索引过滤后）

EXPLAIN SELECT * FROM orders 
WHERE order_status = 'pending' 
  AND user_id = 100;
-- rows: 250000, filtered: 1.00
-- 实际返回行数 ≈ 2500（过滤掉了99%的数据）
-- 优化：创建(status, user_id)联合索引
-- 优化后 rows: 50, filtered: 100.00

-- 问题SQL
EXPLAIN SELECT * FROM orders WHERE status='pending' ORDER BY created_at DESC;
-- Extra: Using where; Using filesort
-- 优化：添加(status, created_at)联合索引
CREATE INDEX idx_status_created ON orders(status, created_at);
-- 优化后Extra不再出现filesort

-- 问题SQL
EXPLAIN SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY COUNT(*);
-- Extra: Using temporary; Using filesort
-- 优化：拆分为两个查询，或调整GROUP BY/ORDER BY顺序

-- 覆盖索引：查询字段都在索引中
EXPLAIN SELECT order_id, user_id FROM orders WHERE order_id = 100;
-- Extra: Using index（主键索引覆盖）
-- 创建覆盖索引示例
CREATE INDEX idx_cover ON orders(status, created_at, order_id);
EXPLAIN SELECT status, created_at, order_id FROM orders WHERE status='pending';
-- Extra: Using index（无需回表）

1. 执行EXPLAIN，获取执行计划
   ↓
2. 检查type：是否为ALL或index？
   ↓ 是 → 添加索引
   ↓ 否
3. 检查key：是否为NULL？
   ↓ 是 → 分析索引失效原因
   ↓ 否
4. 检查rows：是否过大？
   ↓ 是 → 优化索引选择性
   ↓ 否
5. 检查Extra：是否有Using filesort/Using temporary？
   ↓ 是 → 优化排序/分组索引
   ↓ 否
6. 性能良好

EXPLAIN SELECT 
    DATE(o.created_at) as order_date,
    u.user_level,
    COUNT(*) as order_count,
    SUM(o.order_amount) as total_amount
FROM orders o
LEFT JOIN users u ON o.user_id = u.user_id
WHERE o.created_at >= '2024-01-01'
  AND o.created_at < '2024-02-01'
  AND o.order_status IN ('paid', 'shipped')
GROUP BY DATE(o.created_at), u.user_level
ORDER BY order_date DESC, u.user_level;

-- 1. 创建联合索引
CREATE INDEX idx_status_created ON orders(order_status, created_at);
-- 2. 改写SQL，避免DATE()函数
SELECT 
    DATE(o.created_at) as order_date,
    u.user_level,
    COUNT(*) as order_count,
    SUM(o.order_amount) as total_amount
FROM orders o
LEFT JOIN users u ON o.user_id = u.user_id
WHERE o.created_at >= '2024-01-01'
  AND o.created_at < '2024-02-01'
  AND o.order_status IN ('paid', 'shipped')
GROUP BY DATE(o.created_at), u.user_level
ORDER BY order_date DESC, u.user_level;
-- 3. 考虑使用汇总表（物化视图）预处理

EXPLAIN ANALYZE SELECT * FROM orders WHERE order_status = 'pending'\G

-> Filter: (orders.order_status = 'pending')  (cost=101.23 rows=1850) (actual time=0.123..0.456 rows=1234 loops=1)
    -> Index lookup on orders using idx_status (order_status='pending')  (cost=101.23 rows=1850) (actual time=0.098..0.234 rows=1234 loops=1)