8種經常被忽視的SQL錯誤用法，你有沒有踩過坑？

1、LIMIT 語句

分頁查詢是最常用的場景之一，但也通常也是最容易出問題的地方。比如對于下面簡單的語句，一般 DBA 想到的辦法是在 type, name, create_time 字段上加組合索引。這樣條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 解決該問題就到此為止。但當 LIMIT 子句變成 “LIMIT 1000000,10” 時，程序員仍然會抱怨：我只取10條記錄為什么還是慢？

要知道數據庫也并不知道第1000000條記錄從什么地方開始，即使有索引也需要從頭計算一次。出現這種性能問題，多數情形下是程序員偷懶了。

在前端數據瀏覽翻頁，或者大數據分批導出等場景下，是可以將上一頁的最大值當成參數作為查詢條件的。SQL 重新設計如下：

SELECT???*?
FROM?????operation?
WHERE????type?=?'SQLStats'?
AND??????name?=?'SlowLog'?
AND??????create_time?>?'2017-03-16?14:00:00'?
ORDER?BY?create_time?limit?10;

在新設計下查詢時間基本固定，不會隨著數據量的增長而發生變化。

2、隱式轉換

SQL語句中查詢變量和字段定義類型不匹配是另一個常見的錯誤。比如下面的語句：

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 的定義為 varchar(20)，MySQL 的策略是將字符串轉換為數字之后再比較。函數作用于表字段，索引失效。

上述情況可能是應用程序框架自動填入的參數，而不是程序員的原意。現在應用框架很多很繁雜，使用方便的同時也小心它可能給自己挖坑。

3、關聯更新、刪除

雖然 MySQL5.6 引入了物化特性，但需要特別注意它目前僅僅針對查詢語句的優化。對于更新或刪除需要手工重寫成 JOIN。歡迎大家關注我的公種浩【程序員追風】，整理了2019年多家公司java面試題資料100多頁pdf文檔，文章都會在里面更新，整理的資料也會放在里面。

比如下面 UPDATE 語句，MySQL 實際執行的是循環/嵌套子查詢（DEPENDENT SUBQUERY)，其執行時間可想而知。

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);

執行計劃：

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
|?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?????????????????????????|
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+

重寫為 JOIN 之后，子查詢的選擇模式從 DEPENDENT SUBQUERY 變成 DERIVED，執行速度大大加快，從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'

執行計劃簡化為：

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+
|?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 不能利用索引進行混合排序。但在某些場景，還是有機會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

執行計劃顯示為全表掃描：

+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+
|?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1兩種狀態，我們按照下面的方法重寫后，執行時間從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語句

MySQL 對待 EXISTS 子句時，仍然采用嵌套子查詢的執行方式。如下面的 SQL 語句：

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

執行計劃為：

+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+?-----+
|?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 更改為 join，能夠避免嵌套子查詢，將執行時間從1.93秒降低為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

新的執行計劃：

+----+-------------+-------+--------+?-----+------------------------------------------+---------+?-----+------+?-----+
|?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、條件下推

外部查詢條件不能夠下推到復雜的視圖或子查詢的情況有：

• 聚合子查詢；

• 含有 LIMIT 的子查詢；

• UNION 或 UNION ALL 子查詢；

• 輸出字段中的子查詢；

如下面的語句，從執行計劃可以看出其條件作用于聚合子查詢之后

?SELECT?*?
FROM???(SELECT?target,?
???????????????Count(*)?
????????FROM???operation?
????????GROUP??BY?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?|?Using?where?|
|??2?|?DERIVED?????|?operation??|?index?|?idx_4?????????|?idx_4???????|?519?????|?NULL??|???20?|?Using?index?|
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+

確定從語義上查詢條件可以直接下推后，重寫如下：

SELECT?target,?
???????Count(*)?
FROM???operation?
WHERE??target?=?'rm-xxxx'?
GROUP??BY?target

執行計劃變為：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
|?id?|?select_type?|?table?|?type?|?possible_keys?|?key?|?key_len?|?ref?|?rows?|?Extra?|
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
|?1?|?SIMPLE?|?operation?|?ref?|?idx_4?|?idx_4?|?514?|?const?|?1?|?Using?where;?Using?index?|
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

7、提前縮小范圍

先上初始 SQL 語句：

SELECT?*?
FROM???my_order?o?
???????LEFT?JOIN?my_userinfo?u?
??????????????ON?o.uid?=?u.uid
???????LEFT?JOIN?my_productinfo?p?
??????????????ON?o.pid?=?p.pid?
WHERE??(?o.display?=?0?)?
???????AND?(?o.ostaus?=?1?)?
ORDER??BY?o.selltime?DESC?
LIMIT??0,?15

該SQL語句原意是：先做一系列的左連接，然后排序取前15條記錄。從執行計劃也可以看出，最后一步估算排序記錄數為90萬，時間消耗為12秒。

+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+
|?id?|?select_type?|?table?|?type???|?possible_keys?|?key?????|?key_len?|?ref?????????????|?rows???|?Extra??????????????????????????????????????????????|
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+
|??1?|?SIMPLE??????|?o?????|?ALL????|?NULL??????????|?NULL????|?NULL????|?NULL????????????|?909119?|?Using?where;?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?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

由于最后 WHERE 條件以及排序均針對最左主表，因此可以先對 my_order 排序提前縮小數據量再做左連接。SQL 重寫后如下，執行時間縮小為1毫秒左右。

SELECT?*?
FROM?(
SELECT?*?
FROM???my_order?o?
WHERE??(?o.display?=?0?)?
???????AND?(?o.ostaus?=?1?)?
ORDER??BY?o.selltime?DESC?
LIMIT??0,?15
)?o?
?????LEFT?JOIN?my_userinfo?u?
??????????????ON?o.uid?=?u.uid?
?????LEFT?JOIN?my_productinfo?p?
??????????????ON?o.pid?=?p.pid?
ORDER?BY??o.selltime?DESC
limit?0,?15

再檢查執行計劃：子查詢物化后（select_type=DERIVED)參與 JOIN。雖然估算行掃描仍然為90萬，但是利用了索引以及 LIMIT 子句后，實際執行時間變得很小。

+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
|?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?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|
|??2?|?DERIVED?????|?o??????????|?index??|?NULL??????????|?idx_1???|?5???????|?NULL??|?909112?|?Using?where????????????????????????????????????????|
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+

8、中間結果集下推

再來看下面這個已經初步優化過的例子(左連接中的主表優先作用查詢條件)：

SELECT????a.*,?
??????????c.allocated?
FROM??????(?
??????????????SELECT???resourceid?
??????????????FROM?????my_distribute?d?
???????????????????WHERE????isdelete?=?0?
???????????????????AND??????cusmanagercode?=?'1234567'?
???????????????????ORDER?BY?salecode?limit?20)?a?
LEFT?JOIN?
??????????(?
??????????????SELECT???resourcesid，?sum(ifnull(allocation,?0)?*?12345)?allocated?
??????????????FROM?????my_resources?
???????????????????GROUP?BY?resourcesid)?c?
ON????????a.resourceid?=?c.resourcesid

那么該語句還存在其它問題嗎？不難看出子查詢 c 是全表聚合查詢，在表數量特別大的情況下會導致整個語句的性能下降。

其實對于子查詢 c，左連接最后結果集只關心能和主表 resourceid 能匹配的數據。因此我們可以重寫語句如下，執行時間從原來的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'?
???????????????????ORDER?BY?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'?
?????????????????????????????????????ORDER?BY?salecode?limit?20)?a?
???????????????????WHERE????r.resourcesid?=?a.resourcesid?
???????????????????GROUP?BY?resourcesid)?c?
ON????????a.resourceid?=?c.resourcesid

但是子查詢 a 在我們的SQL語句中出現了多次。這種寫法不僅存在額外的開銷，還使得整個語句顯的繁雜。使用 WITH 語句再次重寫：

WITH?a?AS?
(?
?????????SELECT???resourceid?
?????????FROM?????my_distribute?d?
?????????WHERE????isdelete?=?0?
?????????AND??????cusmanagercode?=?'1234567'?
?????????ORDER?BY?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?
???????????????????GROUP?BY?resourcesid)?c?
ON????????a.resourceid?=?c.resourcesid

總結

數據庫編譯器產生執行計劃，決定著SQL的實際執行方式。但是編譯器只是盡力服務，所有數據庫的編譯器都不是盡善盡美的。

上述提到的多數場景，在其它數據庫中也存在性能問題。了解數據庫編譯器的特性，才能避規其短處，寫出高性能的SQL語句。

程序員在設計數據模型以及編寫SQL語句時，要把算法的思想或意識帶進來。

編寫復雜SQL語句要養成使用 WITH 語句的習慣。簡潔且思路清晰的SQL語句也能減小數據庫的負擔 。

最后

歡迎大家一起交流，喜歡文章記得點個贊喲，感謝支持！