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大家好，我是大彬~

今天給大家分享幾個SQL常見的“壞毛病”及優(yōu)化技巧。

SQL語句的執(zhí)行順序：

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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(dāng) 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(dǎo)出等場景下，是可以將上一頁的最大值當(dāng)成參數(shù)作為查詢條件的。SQL 重新設(shè)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(shè)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。

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2、隱式轉(zhuǎn)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(zhuǎn)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(yīng)用程序框架自動填入的參數(shù)，而不是程序員的原意?，F(xiàn)在應(yīng)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(guān)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 idFROM   (SELECT o.id,o.statusFROM   operation oWHERE  o.group = 123AND o.status NOT IN ( 'done' )ORDER  BY o.parent,o.idLIMIT  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 oJOIN  (SELECT o.id,o.statusFROM   operation oWHERE  o.group = 123AND o.status NOT IN ( 'done' )ORDER  BY o.parent,o.idLIMIT  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(jìn)行混合排序。但在某些場景，還是有機(jī)會使用特殊方法提升性能的。

SELECT *
FROM   my_order oINNER 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 oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 0ORDER  BY appraise_time DESCLIMIT  0, 20)UNION ALL(SELECT *FROM   my_order oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 1ORDER  BY appraise_time DESCLIMIT  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 nLEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'
WHERE  n.topic_status < 4AND EXISTS(SELECT 1FROM   message_info mWHERE  n.id = m.neighbor_idAND 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 nINNER JOIN message_info mON n.id = m.neighbor_idAND m.inuser = 'xxx'LEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'
WHERE  n.topic_status < 4AND 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(fù)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   operationGROUP  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(guān)于 MySQL 外部條件不能下推的詳細(xì)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 oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON 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
) oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON 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(jié)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)：

SELECT    a.*,c.allocated
FROM      (SELECT   resourceidFROM     my_distribute dWHERE    isdelete = 0AND      cusmanagercode = '1234567'ORDER BY salecode limit 20) a
LEFT JOIN(SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM     my_resourcesGROUP 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(shù)量特別大的情況下會導(dǎo)致整個語句的性能下降。

其實對于子查詢 c，左連接最后結(jié)果集只關(guān)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   resourceidFROM     my_distribute dWHERE    isdelete = 0AND      cusmanagercode = '1234567'ORDER BY salecode limit 20) a
LEFT JOIN(SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM     my_resources r,(SELECT   resourceidFROM     my_distribute dWHERE    isdelete = 0AND      cusmanagercode = '1234567'ORDER BY salecode limit 20) aWHERE    r.resourcesid = a.resourcesidGROUP 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   resourceidFROM     my_distribute dWHERE    isdelete = 0AND      cusmanagercode = '1234567'ORDER BY salecode limit 20)
SELECT    a.*,c.allocated
FROM      a
LEFT JOIN(SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM     my_resources r,aWHERE    r.resourcesid = a.resourcesidGROUP BY resourcesid) c
ON        a.resourceid = c.resourcesid

總結(jié)

數(shù)據(jù)庫編譯器產(chǎn)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(wù)，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(shè)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(jìn)來。

編寫復(fù)雜SQL語句要養(yǎng)成使用 WITH 語句的習(xí)慣。簡潔且思路清晰的SQL語句也能減小數(shù)據(jù)庫的負(fù)擔(dān) 。

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