sql語句的執行順序：

FROM


ON



?JOIN


WHERE


GROUP?BY


HAVING


SELECT

DISTINCT


ORDER?BY


LIMIT

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：

注 意

SELECT???*
FROM?????operation
WHERE????type?=?'SQLStats'
AND??????name?=?'SlowLog'
AND??????create_time?>?'2017-03-16?1400'
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)?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??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??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：

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?????|??|?ref???|????|??|?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 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.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?????|??|?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 。

編輯：黃飛

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