1、誤用LIMIT 語(yǔ)句
分頁(yè)查詢是避坑最常用的場(chǎng)景之一，但也通常也是指南種常最容易出問(wèn)題的地方。比如對(duì)于下面簡(jiǎn)單的誤用語(yǔ)句，一般 DBA 想到的避坑辦法是在 type, name, create_time 字段上加組合索引。這樣條件排序都能有效的指南種常利用到索引，性能迅速提升。誤用
SELECT *FROM operationWHERE type = 'SQLStats'AND name = 'SlowLog'ORDER BY create_timeLIMIT 1000, 10;
好吧，避坑可能90%以上的指南種常 DBA 解決該問(wèn)題就到此為止。但當(dāng) LIMIT 子句變成 “LIMIT 1000000,10” 時(shí)，誤用程序員仍然會(huì)抱怨：我只取10條記錄為什么還是避坑慢？
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。指南種常出現(xiàn)這種性能問(wèn)題，誤用多數(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 operationWHERE 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(jiàn)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?，F(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 oSET status = 'applying'WHERE o.id IN (SELECT idFROM (SELECT o.id,o.statusFROM operation oWHERE o.group = 123AND o.status NOT IN ( 'done' )ORDER BY o.parent,o.idLIMIT 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 oJOIN (SELECT o.id,o.statusFROM operation oWHERE o.group = 123AND o.status NOT IN ( 'done' )ORDER BY o.parent,o.idLIMIT 1) tON o.id = t.idSET 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 oINNER JOIN my_appraise a ON a.orderid = o.idORDER BY a.is_reply ASC,a.appraise_time DESCLIMIT 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 oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 0ORDER BY appraise_time DESCLIMIT 0, 20)UNION ALL(SELECT *FROM my_order oINNER JOIN my_appraise aON a.orderid = o.idAND is_reply = 1ORDER BY appraise_time DESCLIMIT 0, 20)) tORDER BY is_reply ASC,appraisetime DESCLIMIT 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 nLEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'WHERE n.topic_status < 4AND EXISTS(SELECT 1FROM message_info mWHERE n.id = m.neighbor_idAND 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 nINNER JOIN message_info mON n.id = m.neighbor_idAND m.inuser = 'xxx'LEFT JOIN my_neighbor_apply sraON n.id = sra.neighbor_idAND sra.user_id = 'xxx'WHERE n.topic_status < 4AND 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 operationGROUP BY target) tWHERE 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(yǔ)義上查詢條件可以直接下推后，重寫如下：
SELECT target,Count(*)FROM operationWHERE 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(shuō)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 oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON o.pid = p.pidWHERE ( o.display = 0 )AND ( o.ostaus = 1 )ORDER BY o.selltime DESCLIMIT 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 oWHERE ( o.display = 0 )AND ( o.ostaus = 1 )ORDER BY o.selltime DESCLIMIT 0, 15) oLEFT JOIN my_userinfo uON o.uid = u.uidLEFT JOIN my_productinfo pON o.pid = p.pidORDER 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 || 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(lái)看下面這個(gè)已經(jīng)初步優(yōu)化過(guò)的例子(左連接中的主表優(yōu)先作用查詢條件)：
SELECT a.*,c.allocatedFROM (SELECT resourceidFROM my_distribute dWHERE isdelete = 0AND cusmanagercode = '1234567'ORDER BY salecode limit 20) aLEFT JOIN(SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM my_resourcesGROUP BY resourcesid) cON a.resourceid = c.resourcesid
那么該語(yǔ)句還存在其它問(wèn)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(lái)的2秒下降到2毫秒。
SELECT a.*,c.allocatedFROM (SELECT resourceidFROM my_distribute dWHERE isdelete = 0AND cusmanagercode = '1234567'ORDER BY salecode limit 20) aLEFT JOIN(SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM my_resources r,(SELECT resourceidFROM my_distribute dWHERE isdelete = 0AND cusmanagercode = '1234567'ORDER BY salecode limit 20) aWHERE r.resourcesid = a.resourcesidGROUP BY resourcesid) cON 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 resourceidFROM my_distribute dWHERE isdelete = 0AND cusmanagercode = '1234567'ORDER BY salecode limit 20)SELECT a.*,c.allocatedFROM aLEFT JOIN(SELECT resourcesid， sum(ifnull(allocation, 0) * 12345) allocatedFROM my_resources r,aWHERE r.resourcesid = a.resourcesidGROUP BY resourcesid) cON 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(wèn)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(lái)。
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) 。
作者：db匠
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