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8 种常被忽视的 SQL 错误用法

發(fā)布時間：2025/3/21 31 豆豆

生活随笔收集整理的這篇文章主要介紹了 8 种常被忽视的 SQL 错误用法小編覺得挺不錯的,現(xiàn)在分享給大家,幫大家做個參考.

sql語句的執(zhí)行順序：

FROM? <left_table>ON? <join_condition><join_type>JOIN? <right_table>WHERE? <where_condition>GROUP?BY? <group_by_list>HAVING? <having_condition>SELECTDISTINCT? <select_list>ORDER?BY? <order_by_condition>LIMIT? <limit_number>

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(shù)據(jù)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(xiàn)這種性能問題，多數(shù)情形下是程序員偷懶了。

在前端數(shù)據(jù)瀏覽翻頁，或者大數(shù)據(jù)分批導出等場景下，是可以將上一頁的最大值當成參數(shù)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(shù)據(jù)量的增長而發(fā)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(shù)字之后再比較。函數(shù)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(shù)，而不是程序員的原意。現(xiàn)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(lián)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(yōu)化。對于更新或刪除需要手工重寫成 JOIN。

比如下面 UPDATE 語句，MySQL 實際執(zhí)行的是循環(huán)/嵌套子查詢（DEPENDENT SUBQUERY)，其執(zhí)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(zhí)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(zhí)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)?tON?o.id?=?t.id? SET????status?=?'applying'?

執(zhí)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(zhí)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(tài)，我們按照下面的方法重寫后，執(zhí)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(zhí)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(zhí)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(zhí)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(zhí)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：

1、聚合子查詢； 2、含有 LIMIT 的子查詢； 3、UNION 或 UNION ALL 子查詢； 4、輸出字段中的子查詢；

如下面的語句，從執(zhí)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(zhí)行計劃變?yōu)?#xff1a;

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

關于 MySQL 外部條件不能下推的詳細解釋說明請參考以前文章：MySQL · 性能優(yōu)化 · 條件下推到物化表 http://mysql.taobao.org/monthly/2016/07/08

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.uidLEFT?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(zhí)行計劃也可以看出，最后一步估算排序記錄數(shù)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(shù)據(jù)量再做左連接。SQL 重寫后如下，執(zhí)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(zhí)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(zhí)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(jīng)初步優(yōu)化過的例子(左連接中的主表優(yōu)先作用查詢條件)：

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(shù)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(shù)據(jù)。因此我們可以重寫語句如下，執(zhí)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(xiàn)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(shù)據(jù)庫編譯器產生執(zhí)行計劃，決定著SQL的實際執(zhí)行方式。但是編譯器只是盡力服務，所有數(shù)據(jù)庫的編譯器都不是盡善盡美的。

上述提到的多數(shù)場景，在其它數(shù)據(jù)庫中也存在性能問題。了解數(shù)據(jù)庫編譯器的特性，才能避規(guī)其短處，寫出高性能的SQL語句。

程序員在設計數(shù)據(jù)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(yǎng)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(shù)據(jù)庫的負擔。

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