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1、LIMIT 語(yǔ)句

分頁(yè)查詢是最常用的場(chǎng)景之一，但也通常也是最容易出問題的地方。比如對(duì)于下面簡(jiǎn)單的語(yǔ)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(shí)，程序員仍然會(huì)抱怨：我只取10條記錄為什么還是慢？

要知道數(shù)據(jù)庫(kù)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(jì)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(yè)，或者大數(shù)據(jù)分批導(dǎo)出等場(chǎng)景下，是可以將上一頁(yè)的最大值當(dāng)成參數(shù)作為查詢條件的。SQL 重新設(shè)計(jì)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(jì)下查詢時(shí)間基本固定，不會(huì)隨著數(shù)據(jù)量的增長(zhǎng)而發(fā)生變化。

2、隱式轉(zhuǎn)換

SQL語(yǔ)句中查詢變量和字段定義類型不匹配是另一個(gè)常見的錯(cuò)誤。比如下面的語(yǔ)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(dòng)填入的參數(shù)，而不是程序員的原意。現(xiàn)在應(yīng)用框架很多很繁雜，使用方便的同時(shí)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(duì)查詢語(yǔ)句的優(yōu)化。對(duì)于更新或刪除需要手工重寫成 JOIN。

比如下面 UPDATE 語(yǔ)句，MySQL 實(shí)際執(zhí)行的是循環(huán)/嵌套子查詢（DEPENDENT SUBQUERY)，其執(zhí)行時(shí)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(jì)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(jì)劃簡(jiǎn)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(chǎng)景，還是有機(jī)會(huì)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(jì)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(shí)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(yǔ)句

MySQL 對(duì)待 EXISTS 子句時(shí)，仍然采用嵌套子查詢的執(zhí)行方式。如下面的 SQL 語(yǔ)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(jì)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(shí)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(jì)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(yǔ)句，從執(zhí)行計(jì)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_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(yǔ)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(jì)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(qǐng)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(yǔ)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(yǔ)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(jì)劃也可以看出，最后一步估算排序記錄數(shù)為90萬(wàn)，時(shí)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(duì)最左主表，因此可以先對(duì) 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(shí)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 DESClimit 0, 15

再檢查執(zhí)行計(jì)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(wàn)，但是利用了索引以及 LIMIT 子句后，實(shí)際執(zhí)行時(shí)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(gè)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(yǔ)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(huì)導(dǎo)致整個(gè)語(yǔ)句的性能下降。

其實(shí)對(duì)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(yǔ)句如下，執(zhí)行時(shí)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(yǔ)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(gè)語(yǔ)句顯的繁雜。使用 WITH 語(yǔ)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(kù)編譯器產(chǎn)生執(zhí)行計(jì)劃，決定著SQL的實(shí)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(kù)的編譯器都不是盡善盡美的。

上述提到的多數(shù)場(chǎng)景，在其它數(shù)據(jù)庫(kù)中也存在性能問題。了解數(shù)據(jù)庫(kù)編譯器的特性，才能避規(guī)其短處，寫出高性能的SQL語(yǔ)句。

程序員在設(shè)計(jì)數(shù)據(jù)模型以及編寫SQL語(yǔ)句時(shí)，要把算法的思想或意識(shí)帶進(jìn)來。

編寫復(fù)雜SQL語(yǔ)句要養(yǎng)成使用 WITH 語(yǔ)句的習(xí)慣。簡(jiǎn)潔且思路清晰的SQL語(yǔ)句也能減小數(shù)據(jù)庫(kù)的負(fù)擔(dān) 。

文章來源：https://yq.aliyun.com/articles/72501

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