MySQL中如何进行SQL调优举例详解

CREATE TABLE user (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT,
    gender CHAR(1),
    city VARCHAR(50)
);
CREATE INDEX idx_name_age_gender ON user(name, age, gender);  

CREATE TABLE user_orders (
    id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT,
    order_date DATE,
    order_amount DECIMAL(10,2),
    INDEX idx_user_date_amount (user_id, order_date, order_amount)
);

from datetime import datetime, timedelta
import random

# 生成INSERT语句
def generate_insert_statements():
    start_date = datetime(2023, 1, 1)
    statements = []
    
    for _ in range(4200):
        user_id = random.randint(1, 1000)
        days = random.randint(0, 365)
        order_date = (start_date + timedelta(days=days)).strftime('%Y-%m-%d')
        order_amount = round(random.uniform(10.0, 1000.0), 2)
        
        insert_sql = f"INSERT INTO user_orders (user_id, order_date, order_amount) VALUES ({user_id}, '{order_date}', {order_amount});"
        statements.append(insert_sql)
    
    # 将所有INSERT语句写入文件
    with open('insert_data.sql', 'w') as f:
        f.write('\n'.join(statements))
        
    print("INSERT语句已生成到 insert_data.sql 文件中")

if __name__ == "__main__":
    generate_insert_statements()

-- 完全满足最左匹配原则（使用全部索引列）
EXPLAIN SELECT * FROM user_orders 
WHERE user_id = 100 AND order_date = '2023-05-01' AND order_amount = 500;

-- 满足最左匹配原则（使用索引的前两列）
EXPLAIN SELECT * FROM user_orders 
WHERE user_id = 100 AND order_date = '2023-05-01';

-- 满足最左匹配原则（只使用第一列）
EXPLAIN SELECT * FROM user_orders 
WHERE user_id = 100;

-- 不满足最左匹配原则（跳过`user_id`）
EXPLAIN SELECT * FROM user_orders 
WHERE order_date = '2023-05-01' AND order_amount = 500;

-- 不满足最左匹配原则（只使用order_date）
EXPLAIN SELECT * FROM user_orders 
WHERE order_date = '2023-05-01';

-- 不满足最左匹配原则（只使用order_amount）
EXPLAIN SELECT * FROM user_orders 
WHERE order_amount = 500;

-- 创建测试表
CREATE TABLE worker(
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50),
    salary DECIMAL(10,2),
    department VARCHAR(50),
    INDEX idx_salary (salary)  -- 只对salary创建索引
);

-- 插入测试数据
INSERT INTO worker(name, salary, department) VALUES
('张三', 5000, '技术部'),
('李四', 6000, '市场部'),
('王五', 4500, '技术部'),
('赵六', 7000, '销售部');

-- 会使用索引排序的情况：
-- 只查询索引列
SELECT salary FROM employees ORDER BY salary;
-- 或者
SELECT id, salary FROM employees ORDER BY salary;
-- 结果显示: Using index for order by

会导致filesort的情况：
-- 特例：查询所有列（SELECT *）
SELECT * FROM employees ORDER BY salary;

-- 对无索引的department字段排序
EXPLAIN SELECT * FROM employees ORDER BY department;
-- 结果显示: Using filesort