MySQL中常見的八種SQL錯誤用法示例

更新時間：2019年08月29日 10:46:34 作者：db匠

這篇文章主要給大家介紹了關(guān)于MySQL中常見的八種SQL錯誤用法示例的相關(guān)資料，文中通過示例代碼介紹的非常詳細(xì)，對大家學(xué)習(xí)或者使用MySQL具有一定的參考學(xué)習(xí)價值，需要的朋友們下面來一起學(xué)習(xí)學(xué)習(xí)吧

前言

MySQL在2016年仍然保持強(qiáng)勁的數(shù)據(jù)庫流行度增長趨勢。越來越多的客戶將自己的應(yīng)用建立在MySQL數(shù)據(jù)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數(shù)據(jù)庫的過程中遇到一些比如響應(yīng)時間慢，CPU打滿等情況。

阿里云RDS專家服務(wù)團(tuán)隊幫助云上客戶解決過很多緊急問題?，F(xiàn)將《ApsaraDB專家診斷報告》中出現(xiàn)的部分常見SQL問題總結(jié)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。

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 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 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 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 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(zhí)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(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 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 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_key
0
> 
| <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)椋?/p>

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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)解釋說明請參考文章：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 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(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
) 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  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(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  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(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  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) 。

好了，以上就是這篇文章的全部內(nèi)容了，希望本文的內(nèi)容對大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價值，謝謝大家對腳本之家的支持。

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