8種經(jīng)常被忽視的SQL錯(cuò)誤用法，你有沒(méi)有踩過(guò)坑？

1、LIMIT 語(yǔ)句

分頁(yè)查詢是最常用的場(chǎng)景之一，但也通常也是最容易出問(wèn)題的地方。比如對(duì)于下面簡(jiǎn)單的語(yǔ)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(wèn)題就到此為止。但當(dāng) LIMIT 子句變成 “LIMIT 1000000,10”
時(shí)，程序員仍然會(huì)抱怨：我只取10條記錄為什么還是慢？
要知道數(shù)據(jù)庫(kù)也并不知道第1000000條記錄從什么地方開(kāi)始，即使有索引也需要從頭計(jì)算一次。出現(xiàn)這種性能問(wèn)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(yè)，或者大數(shù)據(jù)分批導(dǎo)出等場(chǎng)景下，是可以將上一頁(yè)的最大值當(dāng)成參數(shù)作為查詢條件的。SQL 重新設(shè)計(jì)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(jì)下查詢時(shí)間基本固定，不會(huì)隨著數(shù)據(jù)量的增長(zhǎng)而發(fā)生變化。

2、隱式轉(zhuǎn)換

SQL語(yǔ)句中查詢變量和字段定義類型不匹配是另一個(gè)常見(jiàn)的錯(cuò)誤。比如下面的語(yǔ)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(dòng)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(shí)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(duì)查詢語(yǔ)句的優(yōu)化。對(duì)于更新或刪除需要手工重寫(xiě)成
JOIN。歡迎大家關(guān)注我的公種浩【程序員追風(fēng)】，整理了2019年多家公司java面試題資料100多頁(yè)pdf文檔，文章都會(huì)在里面更新，整理的資料也會(huì)放在里面。
比如下面 UPDATE 語(yǔ)句，MySQL 實(shí)際執(zhí)行的是循環(huán)/嵌套子查詢（DEPENDENT SUBQUERY)，其執(zhí)行時(shí)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(jì)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(xiě)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) t ON o.id
= t.id SET status = 'applying'
執(zhí)行計(jì)劃簡(jiǎn)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(chǎng)景，還是有機(jī)會(huì)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(jì)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(xiě)后，執(zhí)行時(shí)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(yǔ)句

MySQL 對(duì)待 EXISTS 子句時(shí)，仍然采用嵌套子查詢的執(zhí)行方式。如下面的 SQL 語(yǔ)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(jì)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(shí)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(jì)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：
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(yǔ)句，從執(zhí)行計(jì)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(yǔ)義上查詢條件可以直接下推后，重寫(xiě)如下：
SELECT target, Count(*) FROM operation WHERE target = 'rm-xxxx' GROUP BY target
執(zhí)行計(jì)劃變?yōu)椋?br>
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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(yǔ)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(yǔ)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(jì)劃也可以看出，最后一步估算排序記錄數(shù)為90萬(wàn)，時(shí)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(duì)最左主表，因此可以先對(duì) my_order 排序提前縮小數(shù)據(jù)量再做左連接。SQL 重寫(xiě)后如下，執(zhí)行時(shí)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(jì)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(wàn)，但是利用了索引以及 LIMIT
子句后，實(shí)際執(zhí)行時(shí)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(lái)看下面這個(gè)已經(jīng)初步優(yōu)化過(guò)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 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(yǔ)句還存在其它問(wèn)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(huì)導(dǎo)致整個(gè)語(yǔ)句的性能下降。
其實(shí)對(duì)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(xiě)語(yǔ)句如下，執(zhí)行時(shí)間從原來(lái)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(yǔ)句中出現(xiàn)了多次。這種寫(xiě)法不僅存在額外的開(kāi)銷，還使得整個(gè)語(yǔ)句顯的繁雜。使用 WITH 語(yǔ)句再次重寫(xiě)：
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(jié)

數(shù)據(jù)庫(kù)編譯器產(chǎn)生執(zhí)行計(jì)劃，決定著SQL的實(shí)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(kù)的編譯器都不是盡善盡美的。
上述提到的多數(shù)場(chǎng)景，在其它數(shù)據(jù)庫(kù)中也存在性能問(wèn)題。了解數(shù)據(jù)庫(kù)編譯器的特性，才能避規(guī)其短處，寫(xiě)出高性能的SQL語(yǔ)句。
程序員在設(shè)計(jì)數(shù)據(jù)模型以及編寫(xiě)SQL語(yǔ)句時(shí)，要把算法的思想或意識(shí)帶進(jìn)來(lái)。
編寫(xiě)復(fù)雜SQL語(yǔ)句要養(yǎng)成使用 WITH 語(yǔ)句的習(xí)慣。簡(jiǎn)潔且思路清晰的SQL語(yǔ)句也能減小數(shù)據(jù)庫(kù)的負(fù)擔(dān) 。

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