MySQL慢查询开启与优化指南

-- 查看当前慢查询状态
SHOW VARIABLES LIKE '%slow_query%';
SHOW VARIABLES LIKE '%long_query_time%';
-- 开启慢查询日志（全局，立即生效，重启失效）
SET GLOBAL slow_query_log = ON;
-- 设置慢查询阈值（秒），建议设为0.1~2秒之间
SET GLOBAL long_query_time = 1;
-- 设置日志文件路径（可选，默认在数据目录下）
SET GLOBAL slow_query_log_file = '/var/lib/mysql/slow-query.log';
-- 设置未使用索引的SQL也记录
SET GLOBAL log_queries_not_using_indexes = ON;

[mysqld]
# 开启慢查询日志
slow_query_log = 1
# 日志文件路径
slow_query_log_file = /var/lib/mysql/slow-query.log
# 慢查询阈值（秒）
long_query_time = 1
# 记录未使用索引的查询
log_queries_not_using_indexes = 1
# 日志输出格式（FILE或TABLE，默认FILE）
# log_output = FILE

# systemctl
sudo systemctl restart mysqld
# service
sudo service mysql restart

# 基本用法
mysqldumpslow /var/lib/mysql/slow-query.log
# 常用参数
mysqldumpslow -s t -t 10 /var/lib/mysql/slow-query.log   # 按查询时间排序，取前10条
mysqldumpslow -s c -t 10 /var/lib/mysql/slow-query.log   # 按执行次数排序
mysqldumpslow -s r -t 10 /var/lib/mysql/slow-query.log   # 按返回行数排序
mysqldumpslow -a /var/lib/mysql/slow-query.log           # 不抽象数字，显示具体SQL

# 安装percona-toolkit
# Ubuntu/Debian
sudo apt-get install percona-toolkit
# CentOS/RHEL
sudo yum install percona-toolkit
# 分析慢查询日志
pt-query-digest /var/lib/mysql/slow-query.log > slow_report.txt
# 分析当前运行的查询（实时）
pt-query-digest --processlist h=localhost,u=root,p=password

-- 临时开启慢查询记录阈值0.5秒
SET GLOBAL slow_query_log = ON;
SET GLOBAL long_query_time = 0.5;
SET GLOBAL log_queries_not_using_indexes = ON;
-- 确认日志文件位置
SHOW VARIABLES LIKE 'slow_query_log_file';
-- 结果：/var/lib/mysql/slow-query.log

SELECT 
    o.order_id,
    o.user_id,
    o.order_amount,
    o.order_status,
    o.created_at,
    u.user_name,
    u.phone
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'
  AND o.created_at < '2024-02-01'
ORDER BY o.created_at DESC
LIMIT 20;

# 查看慢查询日志
mysqldumpslow -s t -t 5 /var/lib/mysql/slow-query.log

Count: 156  Time=3.52s (549s)  Lock=0.01s (1.56s)  Rows_sent=20.0 (3120), Rows_examined=5234567.0 (816M), root[root]@localhost
SELECT o.order_id, o.user_id, o.order_amount, o.order_status, o.created_at, u.user_name, u.phone 
FROM orders o 
LEFT JOIN users u ON o.user_id = u.user_id 
WHERE o.order_status = 'S' 
  AND o.created_at >= 'YYYY-MM-DD' 
  AND o.created_at < 'YYYY-MM-DD' 
ORDER BY o.created_at DESC 
LIMIT N

EXPLAIN SELECT 
    o.order_id,
    o.user_id,
    o.order_amount,
    o.order_status,
    o.created_at,
    u.user_name,
    u.phone
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'
  AND o.created_at < '2024-02-01'
ORDER BY o.created_at DESC
LIMIT 20\G

-- 查看orders表现有索引
SHOW INDEX FROM orders;

-- 创建联合索引，将等值查询字段放前面，范围查询放后面
CREATE INDEX idx_status_created ON orders (order_status, created_at);
-- 验证索引效果
EXPLAIN SELECT ...（同原SQL）\G

-- 创建覆盖索引，避免回表查询
-- 注意：
-- 创建索引需要在线上业务停止时进行，避免死锁
-- 覆盖索引需要包含所有查询字段
-- 重建索引可能需要很长时间，可能破坏数据，建议先备份数据
CREATE INDEX idx_status_created_cover ON orders (order_status, created_at, order_id, user_id, order_amount);

-- 但orders表字段较多，覆盖索引可能过大，需权衡

-- 使用子查询先筛选出订单ID，再关联用户表
SELECT 
    o.order_id,
    o.user_id,
    o.order_amount,
    o.order_status,
    o.created_at,
    u.user_name,
    u.phone
FROM (
    SELECT order_id, user_id, order_amount, order_status, created_at
    FROM orders
    WHERE order_status = 'pending'
      AND created_at >= '2024-01-01'
      AND created_at < '2024-02-01'
    ORDER BY created_at DESC
    LIMIT 20
) o
LEFT JOIN users u ON o.user_id = u.user_id;

mysqldumpslow -s t -t 5 /var/lib/mysql/slow-query.log

-- phone字段定义为varchar(20)，但传入数字类型
SELECT * FROM users WHERE phone = 13800138000;

-- 正确写法，传入字符串
SELECT * FROM users WHERE phone = '13800138000';

SELECT * FROM orders WHERE DATE(created_at) = '2024-01-15';

SELECT * FROM orders 
WHERE created_at >= '2024-01-15' 
  AND created_at < '2024-01-16';

-- 第10000页，每页20条
SELECT * FROM orders ORDER BY order_id LIMIT 200000, 20;

SELECT * FROM orders o
INNER JOIN (
    SELECT order_id FROM orders 
    ORDER BY order_id 
    LIMIT 200000, 20
) t ON o.order_id = t.order_id;

-- 将日志输出到mysql.slow_log表
SET GLOBAL log_output = 'TABLE';

-- 查询慢日志表
SELECT * FROM mysql.slow_log 
WHERE query_time > 2 
ORDER BY start_time DESC 
LIMIT 10;

开启慢查询 → 收集日志 → 分析TOP慢SQL → EXPLAIN执行计划 → 定位问题
    ↑                                                      ↓
监控告警 ← 验证效果 ← 上线变更 ← 制定优化方案 ← 索引失效/扫描行数多