【編程知識】常被忽視的 SQL 錯誤用法

1、LIMIT 陳述句
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陳述句，一般 DBA 想到的辦法是在 type, name, create_time 欄位上加組合索引，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，程式員仍然會抱怨：我只取10條記錄為什么還是慢？

要知道資料庫也并不知道第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，多數情形下是程式員偷懶了，

在前端資料瀏覽翻頁，或者大資料分批匯出等場景下，是可以將上一頁的最大值當成引數作為查詢條件的，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，不會隨著資料量的增長而發生變化，

2、隱式轉換
SQL陳述句中查詢變數和欄位定義型別不匹配是另一個常見的錯誤，比如下面的陳述句：

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'

其中欄位 bpn 的定義為 varchar(20)，MySQL 的策略是將字串轉換為數字之后再比較，函式作用于表欄位，索引失效，

上述情況可能是應用程式框架自動填入的引數，而不是程式員的原意，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、洗掉
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陳述句的優化，對于更新或洗掉需要手工重寫成 JOIN，

比如下面 UPDATE 陳述句，MySQL 實際執行的是回圈/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陳述句
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 陳述句：

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：

1、聚合子查詢； 2、含有 LIMIT 的子查詢； 3、UNION 或 UNION ALL 子查詢； 4、輸出欄位中的子查詢；

如下面的陳述句，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 |+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+

確定從語意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：

[code]

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+| 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 |+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+[/code]

7、提前縮小范圍**
先上初始 SQL 陳述句：

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陳述句原意是：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 排序提前縮小資料量再做左連接，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 DESClimit 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

那么該陳述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陳述句的性能下降，

其實對于子查詢 c，左連接最后結果集只關心能和主表 resourceid 能匹配的資料，因此我們可以重寫陳述句如下，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陳述句中出現了多次，這種寫法不僅存在額外的開銷，還使得整個陳述句顯的繁雜，使用 WITH 陳述句再次重寫：

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

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資料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，所有資料庫的編譯器都不是盡善盡美的，

上述提到的多數場景，在其它資料庫中也存在性能問題，了解資料庫編譯器的特性，才能避規其短處，寫出高性能的SQL陳述句，

程式員在設計資料模型以及撰寫SQL陳述句時，要把演算法的思想或意識帶進來，

撰寫復雜SQL陳述句要養成使用 WITH 陳述句的習慣，簡潔且思路清晰的SQL陳述句也能減小資料庫的負擔 ，

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