1. not in 和 <> 的优化 –>left join

SELECT customer_id, first_name, last_name, email 
FROM customer
WHERE customer_id 
NOT IN (SELECT customer_id FROM payment);

-- 优化后
SELECT a.customer_id, a.first_name, a.last_name, a.email
FROM customer a
LEFT JOIN payment b ON a.customer_id=b.customer_id
WHERE b.customer_id IS NULL;

2. 大表的数据批改要分批解决

比方 1000 万行记录在表中要删除, 或者更新 100 万行记录;

优化计划: 一次只删除 / 删除 5000 条, 而后 sleep 几秒(暂停几秒)–> 给主从同步缓冲工夫;

3. 汇总查问的优化 select count(*)-> 减少汇总表

减少一个汇总表, 每天更新一次汇总值;

而后每次查问时, 汇总表 + 实时表 (time>date(now())) 即可;

SELECT COUNT(*) FROM product_comment WHERE product_id=999;

-- 减少汇总表 product_comment_cnt
CREATE TABLE product_comment_cnt(product_id INT, cnt INT);

-- 查问时:
SELECT SUM(cnt) FROM(
    SELECT cnt FROM product_comment_cnt WHERE product_id = 999
    UNION ALL
    SELECT count(*) FROM product_comment WHERE product_id = 999 AND timestr>DATE(now())
);

--
mysql> select DATE(now());
+-------------+
| DATE(now()) |
+-------------+
| 2020-09-12  |
+-------------+
1 row in set (0.01 sec)