性能優(yōu)化 | MySQL常見SQL錯誤用法
作者：佚名
MySQL在2016年仍然保持強勁的數(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?，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。
前言
MySQL在2016年仍然保持強勁的數(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?，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。
常見SQL錯誤用法
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***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
- 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_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)解釋說明請參考以前文章：MySQL · 性能優(yōu)化 · 條件下推到物化表
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，***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***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***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)
1.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。
2.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來。
3.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) ^^。
責(zé)任編輯：龐桂玉
來源：
數(shù)據(jù)庫技術(shù)大會
