8種常見的SQL錯誤用法

常見SQL錯誤用法

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。比如對於下面簡單的語句，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？

要知道數據庫也並不知道第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 > '2019-10-19 14:00:00' 
ORDER BY create_time limit 10;

在新設計下查詢時間基本固定，不會隨着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。函數作用於表字段，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。對於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); 

執行計划：

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
| 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' 

執行計划簡化為：

+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+
| 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 

執行計划顯示為全表掃描：

+----+-------------+-------+--------+-------------+---------+---------+---------------+---------+-+
| 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 |
+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+

由於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 

執行計划為：

+----+--------------------+-------+------+-----+------------------------------------------+---------+-------+---------+ -----+
| 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 

新的執行計划：

+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+
| 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，從執行計划可以看出其條件作用於聚合子查詢之后：

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

執行計划變為：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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 |
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+

關於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.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。從執行計划也可以看出，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) |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+

由於最后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

再檢查執行計划：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。

其實對於子查詢 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

1. 數據庫編譯器產生執行計划，決定着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。
2. 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。
3. 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 ^^。

---

關注微信公眾號【程序員的夢想】，專注於Java，SpringBoot，SpringCloud，微服務，Docker以及前后端分離等全棧技術。