MySQL常見的8種SQL錯誤用法

Java知音發表於2019-08-22

作者:db匠

https://yq.aliyun.com/articles/72501

前言

MySQL在2016年仍然保持強勁的資料庫流行度增長趨勢。越來越多的客戶將自己的應用建立在MySQL資料庫之上,甚至是從Oracle遷移到MySQL上來。但也存在部分客戶在使用MySQL資料庫的過程中遇到一些比如響應時間慢,CPU打滿等情況。

阿里雲RDS專家服務團隊幫助雲上客戶解決過很多緊急問題。現將《ApsaraDB專家診斷報告》中出現的部分常見SQL問題總結如下,供大家參考。

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 欄位上加組合索引。這樣條件排序都能有效的利用到索引,效能迅速提升。

SELECT * 
FROM   operation 
WHERE  type = 'SQLStats' 
       AND name = 'SlowLog' 
ORDER  BY create_time 
LIMIT  1000, 10;複製程式碼

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,程式設計師仍然會抱怨:我只取10條記錄為什麼還是慢?

要知道資料庫也並不知道第1000000條記錄從什麼地方開始,即使有索引也需要從頭計算一次。出現這種效能問題,多數情形下是程式設計師偷懶了。

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;複製程式碼

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語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'複製程式碼

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後再比較。函式作用於表欄位,索引失效。

上述情況可能是應用程式框架自動填入的引數,而不是程式設計師的原意。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 實際執行的是迴圈/巢狀子查詢(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);複製程式碼

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                         |
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+複製程式碼

重寫為 JOIN 之後,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' 複製程式碼

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                         |
+----+-------------+-------+------+---------------+-------+---------+-------+------+-----------------------------------------------------+複製程式碼

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效能的。

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 複製程式碼

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           |
+----+-------------+-------+--------+---------+---------+---------+-----------------+---------+-+複製程式碼

由於 is_reply 只有0和1兩種狀態,我們按照下面的方法重寫後,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;複製程式碼

5、EXISTS語句

MySQL 對待 EXISTS 子句時,仍然採用巢狀子查詢的執行方式。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 複製程式碼

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 |
+----+--------------------+-------+------+ -----+------------------------------------------+---------+-------+---------+ -----+複製程式碼

去掉 exists 更改為 join,能夠避免巢狀子查詢,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 複製程式碼

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           |
+----+-------------+-------+--------+ -----+------------------------------------------+---------+ -----+------+ -----+複製程式碼

6、條件下推

外部查詢條件不能夠下推到複雜的檢視或子查詢的情況有:

  • 聚合子查詢;

  • 含有 LIMIT 的子查詢;

  • UNION 或 UNION ALL 子查詢;

  • 輸出欄位中的子查詢;

如下面的語句,從執行計劃可以看出其條件作用於聚合子查詢之後:

SELECT * 
FROM   (SELECT target, 
               Count(*) 
        FROM   operation 
        GROUP  BY target) t 
WHERE  target = 'rm-xxxx'複製程式碼

+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
| id | select_type | table      | type  | possible_keys | key         | key_len | ref   | rows | Extra       |
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+
|
1
| PRIMARY     |
 <derived2> 
| ref   |
 <auto_key
0
>   
| <auto_key0> |
514
| const |
2
| Using where |
|  2 | DERIVED     | operation  | index | idx_4         | idx_4       | 519     | NULL  |   20 | Using index |
+----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+複製程式碼

確定從語義上查詢條件可以直接下推後,重寫如下:

SELECT target, 
       Count(*) 
FROM   operation 
WHERE  target = 'rm-xxxx' 
GROUP  BY target複製程式碼

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 |
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+複製程式碼

關於 MySQL 外部條件不能下推的詳細解釋說明請參考文章:

http://mysql.taobao.org/monthly/2016/07/08

7、提前縮小範圍

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 複製程式碼

該SQL語句原意是:先做一系列的左連線,然後排序取前15條記錄。從執行計劃也可以看出,最後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) |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+複製程式碼

由於最後 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線。SQL 重寫後如下,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複製程式碼

再檢查執行計劃:子查詢物化後(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 子句後,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                                        |
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+複製程式碼

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(左連線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複製程式碼

那麼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效能下降。

其實對於子查詢 c,左連線最後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複製程式碼

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複製程式碼

總結

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效能問題。瞭解資料庫編譯器的特性,才能避規其短處,寫出高效能的SQL語句。

程式設計師在設計資料模型以及編寫SQL語句時,要把演算法的思想或意識帶進來。

編寫複雜SQL語句要養成使用 WITH 語句的習慣。簡潔且思路清晰的SQL語句也能減小資料庫的負擔 。

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