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;复制代码好吧，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'ORDERBY create_time limit 10;
复制代码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 ISNULL ;
mysql>show warnings;
| Warning |1739| Cannot use ref access on index 'bpn' due to type orcollation conversion on field 'bpn'复制代码其中字段 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 NOTIN ( 'done' )
ORDERBY o.parent,
o.id
LIMIT 1) t);
复制代码执行计划：
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+| id | select_type |table| type | possible_keys | key | key_len |ref |rows| Extra |+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+|1|PRIMARY | o | index | |PRIMARY|8 | |24 |Usingwhere; 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 |Usingwhere; Using filesort |+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+复制代码重写为 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 NOTIN ( 'done' )
ORDERBY o.parent,
o.id
LIMIT 1) t
ON o.id = t.id
SET status = 'applying'复制代码执行计划简化为：
+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+| 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 |Usingwhere; Using filesort |+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+复制代码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
INNERJOIN my_appraise a ON a.orderid = o.id
ORDERBY a.is_reply ASC,
a.appraise_time DESC
LIMIT 0, 20复制代码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|+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+复制代码由于 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
INNERJOIN my_appraise a
ON a.orderid = o.id
AND is_reply =0ORDERBY appraise_time DESC
LIMIT 0, 20)
UNIONALL
(SELECT*FROM my_order o
INNERJOIN my_appraise a
ON a.orderid = o.id
AND is_reply =1ORDERBY appraise_time DESC
LIMIT 0, 20)) t
ORDERBY is_reply ASC,
appraisetime DESC
LIMIT 20;
复制代码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
LEFTJOIN my_neighbor_apply sra
ON n.id = sra.neighbor_id
AND sra.user_id ='xxx'WHERE n.topic_status <4ANDEXISTS(SELECT1FROM message_info m
WHERE n.id = m.neighbor_id
AND m.inuser ='xxx')
AND n.topic_type <>5复制代码执行计划为：
+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+| id | select_type |table| type | possible_keys | key | key_len |ref|rows| Extra |+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+|1|PRIMARY| n |ALL||NULL|NULL|NULL|1086041|Usingwhere||1|PRIMARY| sra |ref|| idx_user_id |123| const |1|Usingwhere||2| DEPENDENT SUBQUERY | m |ref|| idx_message_info |122| const |1|Using index condition; Usingwhere|+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+复制代码去掉 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
复制代码新的执行计划：
+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+| 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|Usingwhere||1| SIMPLE | sra |ref|| idx_user_id |123| const |1|Usingwhere|+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+复制代码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：
1、聚合子查询；
2、含有 LIMIT 的子查询；
3、UNION 或 UNION ALL 子查询；
4、输出字段中的子查询；
如下面的语句，从执行计划可以看出其条件作用于聚合子查询之后：
SELECT*FROM (SELECT target,
Count(*)
FROM operation
GROUPBY target) t
WHERE target ='rm-xxxx'+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+| id | select_type |table| type | possible_keys | key | key_len |ref|rows| Extra |+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+|1|PRIMARY|<derived2>|ref|<auto_key0>|<auto_key0>|514| const |2|Usingwhere||2| DERIVED | operation | index | idx_4 | idx_4 |519|NULL|20|Using index |+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+复制代码确定从语义上查询条件可以直接下推后，重写如下：
SELECT target,
Count(*)
FROM operation
WHERE target ='rm-xxxx'GROUPBY target
复制代码执行计划变为：
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+| id | select_type |table| type | possible_keys | key | key_len |ref|rows| Extra |+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+|1| SIMPLE | operation |ref| idx_4 | idx_4 |514| const |1|Usingwhere; Using index |+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+复制代码关于 MySQL 外部条件不能下推的详细解释说明请参考以前文章：MySQL · 性能优化 · 条件下推到物化表 mysql.taobao.org/monthly/201…
7、提前缩小范围
先上初始 SQL 语句：
SELECT*FROM my_order o
LEFTJOIN my_userinfo u
ON o.uid = u.uid
LEFTJOIN my_productinfo p
ON o.pid = p.pid
WHERE ( o.display =0 )
AND ( o.ostaus =1 )
ORDERBY o.selltime DESC
LIMIT 0, 15复制代码该SQL语句原意是：先做一系列的左连接，然后排序取前15条记录。从执行计划也可以看出，最后一步估算排序记录数为90万，时间消耗为12秒。
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+| id | select_type |table| type | possible_keys | key | key_len |ref|rows| Extra |+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+|1| SIMPLE | o |ALL|NULL|NULL|NULL|NULL|909119|Usingwhere; Using temporary; Using filesort ||1| SIMPLE | u | eq_ref |PRIMARY|PRIMARY|4| o.uid |1|NULL||1| SIMPLE | p |ALL|PRIMARY|NULL|NULL|NULL|6|Usingwhere; Usingjoin buffer (Block Nested Loop) |+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+复制代码由于最后 WHERE 条件以及排序均针对最左主表，因此可以先对 my_order 排序提前缩小数据量再做左连接。SQL 重写后如下，执行时间缩小为1毫秒左右。
SELECT*FROM (
SELECT*FROM my_order o
WHERE ( o.display =0 )
AND ( o.ostaus =1 )
ORDERBY o.selltime DESC
LIMIT 0, 15
) o
LEFTJOIN my_userinfo u
ON o.uid = u.uid
LEFTJOIN my_productinfo p
ON o.pid = p.pid
ORDERBY o.selltime DESC
limit 0, 15复制代码再检查执行计划：子查询物化后（select_type=DERIVED)参与 JOIN。虽然估算行扫描仍然为90万，但是利用了索引以及 LIMIT 子句后，实际执行时间变得很小。
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+| id | select_type |table| type | possible_keys | key | key_len |ref|rows| Extra |+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+|1|PRIMARY|<derived2>|ALL|NULL|NULL|NULL|NULL|15|Using temporary; Using filesort ||1|PRIMARY| u | eq_ref |PRIMARY|PRIMARY|4| o.uid |1|NULL||1|PRIMARY| p |ALL|PRIMARY|NULL|NULL|NULL|6|Usingwhere; Usingjoin buffer (Block Nested Loop) ||2| DERIVED | o | index |NULL| idx_1 |5|NULL|909112|Usingwhere|+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+复制代码8、中间结果集下推
再来看下面这个已经初步优化过的例子(左连接中的主表优先作用查询条件)：
SELECT a.*,
c.allocated
FROM (
SELECT resourceid
FROM my_distribute d
WHERE isdelete = 0
AND cusmanagercode = '1234567'
ORDERBY salecode limit 20) a
LEFT JOIN
(
SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated
FROM my_resources
GROUPBY resourcesid) c
ON a.resourceid = c.resourcesid
复制代码那么该语句还存在其它问题吗？不难看出子查询 c 是全表聚合查询，在表数量特别大的情况下会导致整个语句的性能下降。
其实对于子查询 c，左连接最后结果集只关心能和主表 resourceid 能匹配的数据。因此我们可以重写语句如下，执行时间从原来的2秒下降到2毫秒。
SELECT a.*,
c.allocated
FROM (
SELECT resourceid
FROM my_distribute d
WHERE isdelete = 0
AND cusmanagercode = '1234567'
ORDERBY salecode limit 20) a
LEFT JOIN
(
SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated
FROM my_resources r,
(
SELECT resourceid
FROM my_distribute d
WHERE isdelete = 0
AND cusmanagercode = '1234567'
ORDERBY salecode limit 20) a
WHERE r.resourcesid = a.resourcesid
GROUPBY resourcesid) c
ON a.resourceid = c.resourcesid
复制代码但是子查询 a 在我们的SQL语句中出现了多次。这种写法不仅存在额外的开销，还使得整个语句显的繁杂。使用 WITH 语句再次重写：
WITH a AS
(
SELECT resourceid
FROM my_distribute d
WHERE isdelete = 0
AND cusmanagercode = '1234567'
ORDERBY salecode limit 20)
SELECT a.*,
c.allocated
FROM a
LEFT JOIN
(
SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocated
FROM my_resources r,
a
WHERE r.resourcesid = a.resourcesid
GROUPBY resourcesid) c
ON a.resourceid = c.resourcesid
复制代码总结
数据库编译器产生执行计划，决定着SQL的实际执行方式。但是编译器只是尽力服务，所有数据库的编译器都不是尽善尽美的。
上述提到的多数场景，在其它数据库中也存在性能问题。了解数据库编译器的特性，才能避规其短处，写出高性能的SQL语句。
程序员在设计数据模型以及编写SQL语句时，要把算法的思想或意识带进来。
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