1、LIMIT 语句 分页查询是最常用的场景之一,但也通常也是最容易出问题的地方。比如对于下面简单的语句,一般 DBA 想到的办法是在 type, name, create_time 字段上加组合索引。这样条件排序都能有效的利用到索引,性能迅速提升。 SELECT * FROM operation WHERE type = ‘SQLStats‘ AND name = ‘SlowLog‘ ORDER BY create_time LIMIT 1000, 10; 好吧,可能90%以上的 DBA 解决该问题就到此为止。但当 LIMIT 子句变成 “LIMIT 1000000,10” 时,程序员仍然会抱怨:我只取10条记录为什么还是慢? 要知道数据库也并不知道第1000000条记录从什么地方开始,即使有索引也需要从头计算一次。出现这种性能问题,多数情形下是程序员偷懒了。 在前端数据浏览翻页,或者大数据分批导出等场景下,是可以将上一页的最大值当成参数作为查询条件的。SQL 重新设计如下: SELECT * FROM operation WHERE type = ‘SQLStats‘ AND name = ‘SlowLog‘ AND create_time > ‘2017-03-16 14:00:00‘ ORDER BY create_time limit 10; 在新设计下查询时间基本固定,不会随着数据量的增长而发生变化。 2、隐式转换 SQL语句中查询变量和字段定义类型不匹配是另一个常见的错误。比如下面的语句: mysql> explain extended SELECT * > FROM my_balance b > WHERE b.bpn = 14000000123 > AND b.isverified IS NULL ; mysql> show warnings; | Warning | 1739 | Cannot use ref access on index ‘bpn‘ due to type or collation conversion on field ‘bpn‘ 其中字段 bpn 的定义为 varchar(20),MySQL 的策略是将字符串转换为数字之后再比较。函数作用于表字段,索引失效。 上述情况可能是应用程序框架自动填入的参数,而不是程序员的原意。现在应用框架很多很繁杂,使用方便的同时也小心它可能给自己挖坑。 3、关联更新、删除 虽然 MySQL5.6 引入了物化特性,但需要特别注意它目前仅仅针对查询语句的优化。对于更新或删除需要手工重写成 JOIN。 比如下面 UPDATE 语句,MySQL 实际执行的是循环/嵌套子查询(DEPENDENT SUBQUERY),其执行时间可想而知。 UPDATE operation o SET status = ‘applying‘ WHERE o.id IN (SELECT id FROM (SELECT o.id, o.status FROM operation o WHERE o.group = 123 AND o.status NOT IN ( ‘done‘ ) ORDER BY o.parent, o.id LIMIT 1) t); 执行计划: ---- -------------------- ------- ------- --------------- --------- --------- ------- ------ ----------------------------------------------------- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | ---- -------------------- ------- ------- --------------- --------- --------- ------- ------ ----------------------------------------------------- | 1 | PRIMARY | o | index | | PRIMARY | 8 | | 24 | Using where; Using temporary | | 2 | DEPENDENT SUBQUERY | | | | | | | | Impossible WHERE noticed after reading const tables | | 3 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort | ---- -------------------- ------- ------- --------------- --------- --------- ------- ------ ----------------------------------------------------- 重写为 JOIN 之后,子查询的选择模式从 DEPENDENT SUBQUERY 变成 DERIVED,执行速度大大加快,从7秒降低到2毫秒。 UPDATE operation o JOIN (SELECT o.id, o.status FROM operation o WHERE o.group = 123 AND o.status NOT IN ( ‘done‘ ) ORDER BY o.parent, o.id LIMIT 1) t ON o.id = t.id SET status = ‘applying‘ 执行计划简化为: ---- ------------- ------- ------ --------------- ------- --------- ------- ------ ----------------------------------------------------- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | ---- ------------- ------- ------ --------------- ------- --------- ------- ------ ----------------------------------------------------- | 1 | PRIMARY | | | | | | | | Impossible WHERE noticed after reading const tables | | 2 | DERIVED | o | ref | idx_2,idx_5 | idx_5 | 8 | const | 1 | Using where; Using filesort | ---- ------------- ------- ------ --------------- ------- --------- ------- ------ ----------------------------------------------------- 4、混合排序 MySQL 不能利用索引进行混合排序。但在某些场景,还是有机会使用特殊方法提升性能的。 SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id ORDER BY a.is_reply ASC, a.appraise_time DESC LIMIT 0, 20 执行计划显示为全表扫描: ---- ------------- ------- -------- ------------- --------- --------- --------------- --------- - | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra ---- ------------- ------- -------- ------------- --------- --------- --------------- --------- - | 1 | SIMPLE | a | ALL | idx_orderid | NULL | NULL | NULL | 1967647 | Using filesort | | 1 | SIMPLE | o | eq_ref | PRIMARY | PRIMARY | 122 | a.orderid | 1 | NULL | ---- ------------- ------- -------- --------- --------- --------- ----------------- --------- - 由于 is_reply 只有0和1两种状态,我们按照下面的方法重写后,执行时间从1.58秒降低到2毫秒。 SELECT * FROM ((SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id AND is_reply = 0 ORDER BY appraise_time DESC LIMIT 0, 20) UNION ALL (SELECT * FROM my_order o INNER JOIN my_appraise a ON a.orderid = o.id AND is_reply = 1 ORDER BY appraise_time DESC LIMIT 0, 20)) t ORDER BY is_reply ASC, appraisetime DESC LIMIT 20; 5、EXISTS语句 MySQL 对待 EXISTS 子句时,仍然采用嵌套子查询的执行方式。如下面的 SQL 语句: SELECT * FROM my_neighbor n LEFT JOIN my_neighbor_apply sra ON n.id = sra.neighbor_id AND sra.user_id = ‘xxx‘ WHERE n.topic_status < 4 AND EXISTS(SELECT 1 FROM message_info m WHERE n.id = m.neighbor_id AND m.inuser = ‘xxx‘) AND n.topic_type <> 5 执行计划为: ---- -------------------- ------- ------ ----- ------------------------------------------ --------- ------- --------- ----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | ---- -------------------- ------- ------ ----- ------------------------------------------ --------- ------- --------- ----- | 1 | PRIMARY | n | ALL | | NULL | NULL | NULL | 1086041 | Using where | | 1 | PRIMARY | sra | ref | | idx_user_id | 123 | const | 1 | Using where | | 2 | DEPENDENT SUBQUERY | m | ref | | idx_message_info | 122 | const | 1 | Using index condition; Using where | ---- -------------------- ------- ------ ----- ------------------------------------------ --------- ------- --------- ----- 去掉 exists 更改为 join,能够避免嵌套子查询,将执行时间从1.93秒降低为1毫秒。 SELECT * FROM my_neighbor n INNER JOIN message_info m ON n.id = m.neighbor_id AND m.inuser = ‘xxx‘ LEFT JOIN my_neighbor_apply sra ON n.id = sra.neighbor_id AND sra.user_id = ‘xxx‘ WHERE n.topic_status < 4 AND n.topic_type <> 5 新的执行计划: ---- ------------- ------- -------- ----- ------------------------------------------ --------- ----- ------ ----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | ---- ------------- ------- -------- ----- ------------------------------------------ --------- ----- ------ ----- | 1 | SIMPLE | m | ref | | idx_message_info | 122 | const | 1 | Using index condition | | 1 | SIMPLE | n | eq_ref | | PRIMARY | 122 | ighbor_id | 1 | Using where | | 1 | SIMPLE | sra | ref | | idx_user_id | 123 | const | 1 | Using where | ---- ------------- ------- -------- ----- ------------------------------------------ --------- ----- ------ ----- 6、条件下推 外部查询条件不能够下推到复杂的视图或子查询的情况有:
- 聚合子查询;
- 含有 LIMIT 的子查询;
- UNION 或 UNION ALL 子查询;
- 输出字段中的子查询;