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來源 | yq.aliyun.com/articles/72501

MySQL 在近幾年仍然保持強勁的數據庫流行度增長趨勢。越來越多的客戶將自己的應用建立在 MySQL 數據庫之上，甚至是從 Oracle 遷移到 MySQL上來。但也存在部分客戶在使用 MySQL 數據庫的過程中遇到一些比如響應時間慢，CPU 打滿等情況。阿里云 RDS 專家服務團隊幫助云上客戶解決過很多緊急問題。現將《ApsaraDB專家診斷報告》中出現的部分常見 SQL 問題總結如下，供大家參考。

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。

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)?tON?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?| +----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

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 · 性能優化 · 條件下推到物化表

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

AliSQL即將推出WITH語法，敬請期待。

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。

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