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遇到坑爹同事？手寫8種降智SQL，性能降低100倍，收藏備用

2024-10-16 11:43:30　來源: Meta

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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  WHEREtype = 'SQLStats'        ANDname = 'SlowLog' ORDERBY create_time  LIMIT1000, 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  WHEREtype = 'SQLStats' ANDname = 'SlowLog' AND      create_time > '2017-03-16 14:00:00' ORDERBY create_time limit10;

在新設計下查詢時間基本固定，不會隨著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。函數作用于表字段，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 實際執行的是循環/嵌套子查詢（DEPENDENT SUBQUERY)，其執行時間可想而知。

UPDATE operation o  SETstatus = 'applying' WHERE  o.id IN (SELECTid                 FROM   (SELECT o.id,                                 o.status                          FROM   operation o                          WHERE  o.group = 123                                AND o.status NOTIN ( 'done' )                          ORDERBY o.parent,                                    o.id                          LIMIT1) 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 NOTIN ( 'done' )                       ORDERBY o.parent,                                 o.id                       LIMIT1) t          ON o.id = t.id  SETstatus = '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         INNERJOIN my_appraise a ON a.orderid = o.id  ORDERBY a.is_reply ASC,            a.appraise_time DESC LIMIT0, 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                  INNERJOIN my_appraise a                          ON a.orderid = o.id                             AND is_reply = 0          ORDERBY appraise_time DESC          LIMIT0, 20)          UNIONALL         (SELECT *          FROM   my_order o                  INNERJOIN my_appraise a                          ON a.orderid = o.id                             AND is_reply = 1          ORDERBY appraise_time DESC          LIMIT0, 20)) t  ORDERBY  is_reply ASC,            appraisetime DESC LIMIT20;

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         LEFTJOIN my_neighbor_apply sra                ON n.id = sra.neighbor_id                   AND sra.user_id = 'xxx' WHERE  n.topic_status < 4        ANDEXISTS(SELECT1                   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         INNERJOIN message_info m                 ON n.id = m.neighbor_id                    AND m.inuser = 'xxx'        LEFTJOIN 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：

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          GROUPBY target) t  WHERE  target = 'rm-xxxx' +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+ | id | select_type | table      | type  | possible_keys | key         | key_len | ref   | rows | Extra       | +----+-------------+------------+-------+---------------+-------------+---------+-------+------+-------------+ |  1 | PRIMARY     | 
     
  | ref   |     |   | 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' GROUPBY target

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

7、提前縮小范圍

先上初始 SQL 語句：

SELECT *  FROM   my_order o         LEFTJOIN my_userinfo u                ON o.uid = u.uid        LEFTJOIN my_productinfo p                ON o.pid = p.pid  WHERE  ( o.display = 0 )         AND ( o.ostaus = 1 )  ORDERBY o.selltime DESC LIMIT0, 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 )  ORDERBY o.selltime DESC LIMIT0, 15 ) o       LEFTJOIN my_userinfo u                ON o.uid = u.uid       LEFTJOIN my_productinfo p                ON o.pid = p.pid  ORDERBY  o.selltime DESC limit0, 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     | 
     
  | 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'                    ORDERBY salecode limit20) a  LEFTJOIN           (                SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocated                FROM     my_resources                     GROUPBY 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'                    ORDERBY salecode limit20) a  LEFTJOIN           (                     SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocated                     FROM     my_resources r,                              (                                       SELECT   resourceid                                       FROM     my_distribute d                                       WHERE    isdelete = 0                                      AND      cusmanagercode = '1234567'                                      ORDERBY salecode limit20) a                     WHERE    r.resourcesid = a.resourcesid                     GROUPBY 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'          ORDERBY salecode limit20) SELECT    a.*,            c.allocated  FROM      a  LEFTJOIN           (                     SELECT   resourcesid， sum(ifnull(allocation, 0) * 12345) allocated                     FROM     my_resources r,                              a                     WHERE    r.resourcesid = a.resourcesid                     GROUPBY resourcesid) c  ON        a.resourceid = c.resourcesid

總結

數據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。

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。

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。

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