MySQL · 性能優化 · MySQL常見SQL錯誤用法

時間 2019-11-06

標籤 mysql 性能優化常見 sql 錯誤用法欄目 MySQL 简体版

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1. LIMIT 語句

分頁查詢是最經常使用的場景之一，但也一般也是最容易出問題的地方。好比對於下面簡單的語句，通常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，性能迅速提高。前端

SELECT * 
FROM   operation 
WHERE  type = 'SQLStats' 
       AND name = 'SlowLog' 
ORDER  BY create_time 
LIMIT  1000, 10; 
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好吧，可能90%以上的DBA解決該問題就到此爲止。但當 LIMIT 子句變成「LIMIT 1000000,10」時，程序員仍然會抱怨：我只取10條記錄爲何仍是慢？mysql

要知道數據庫也並不知道第1000000條記錄從什麼地方開始，即便有索引也須要從頭計算一次。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，是能夠將上一頁的最大值當成參數做爲查詢條件的。SQL從新設計以下：程序員

SELECT   * 
FROM     operation 
WHERE    type = 'SQLStats' 
AND      name = 'SlowLog' 
AND      create_time > '2017-03-16 14:00:00' 
ORDER BY create_time limit 10;
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在新設計下查詢時間基本固定，不會隨着數據量的增加而發生變化。算法

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外一個常見的錯誤。好比下面的語句：sql

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'
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其中字段bpn的定義爲varchar(20)，MySQL的策略是將字符串轉換爲數字以後再比較。函數做用於表字段，索引失效。數據庫

上述狀況多是應用程序框架自動填入的參數，而不是程序員的原意。如今應用框架不少很繁雜，使用方便的同時也當心它可能給本身挖坑。bash

3. 關聯更新、刪除

雖然MySQL5.6引入了物化特性，但須要特別注意它目前僅僅針對查詢語句的優化。對於更新或刪除須要手工重寫成JOIN。app

好比下面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。框架

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); 
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執行計劃：函數

+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
| 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                         |
+----+--------------------+-------+-------+---------------+---------+---------+-------+------+-----------------------------------------------------+
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肯定從語義上查詢條件能夠直接下推後，重寫以下：

SELECT target, 
       Count(*) 
FROM   operation 
WHERE  target = 'rm-xxxx' 
GROUP  BY target
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執行計劃變爲：

+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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 |
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
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7. 提早縮小範圍

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 
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該SQL語句原意是：先作一系列的左鏈接，而後排序取前15條記錄。從執行計劃也能夠看出，最後一步估算排序記錄數爲90萬，時間消耗爲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) |
+----+-------------+-------+--------+---------------+---------+---------+-----------------+--------+----------------------------------------------------+
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因爲最後WHERE條件以及排序均針對最左主表，所以能夠先對my_order排序提早縮小數據量再作左鏈接。SQL重寫後以下，執行時間縮小爲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
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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爲90萬，可是利用了索引以及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                                        |
+----+-------------+------------+--------+---------------+---------+---------+-------+--------+----------------------------------------------------+
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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(左鏈接中的主表優先做用查詢條件)：

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
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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，在表數量特別大的狀況下會致使整個語句的性能降低。

其實對於子查詢 c，左鏈接最後結果集只關心能和主表resourceid能匹配的數據。所以咱們能夠重寫語句以下，執行時間從原來的2秒降低到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
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可是子查詢 a 在咱們的SQL語句中出現了屢次。這種寫法不只存在額外的開銷，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
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