8種專坑同事的 SQL 寫法，性能降低100倍，不來看看？
往期熱門文章：
1、最近火起的 Bean Searcher 與 MyBatis Plus 到底有啥區(qū)別？ 2、別再重復(fù)造輪子了，一個(gè) Spring 注解輕松搞定循環(huán)重試功能！ 3、2023最新互聯(lián)網(wǎng)公司時(shí)長(zhǎng)排行榜出爐！ 4、棄用 Nginx 后！它成為了最受歡迎 Web 服務(wù)器。。。 5、計(jì)算機(jī)會(huì)成為下一個(gè)土木嗎？
1、LIMIT 語(yǔ)句
往期熱門文章：
1、最近火起的 Bean Searcher 與 MyBatis Plus 到底有啥區(qū)別？ 2、別再重復(fù)造輪子了，一個(gè) Spring 注解輕松搞定循環(huán)重試功能！ 3、2023最新互聯(lián)網(wǎng)公司時(shí)長(zhǎng)排行榜出爐！ 4、棄用 Nginx 后！它成為了最受歡迎 Web 服務(wù)器。。。 5、計(jì)算機(jī)會(huì)成為下一個(gè)土木嗎？
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
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 |??| ref |?0>?| | ?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?/span>
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+
| 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 |
+----+-------------+-----------+------+---------------+-------+---------+-------+------+--------------------+復(fù)制代碼?
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，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?DESC
limit?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，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?
9
總結(jié)
鏈接：https://juejin.cn/post/6844903998974099470
往期熱門文章：
1、干掉Maven和Gradle！推出更強(qiáng)更快更牛逼的新一代構(gòu)建工具，炸裂！ 2、大公司為什么禁止SpringBoot項(xiàng)目使用Tomcat？ 3、快速交付神器：阿里巴巴官方低代碼引擎開源了！ 4、為什么 Spring和IDEA 都不推薦使用 @Autowired 注解 5、程序員的悲哀是什么？ 6、被問懵了：MySQL 自增主鍵一定是連續(xù)的嗎？ 7、點(diǎn)一下詳情系統(tǒng)掛了，CPU100% 8、我說用count(*)統(tǒng)計(jì)行數(shù)，面試官讓我回去等消息... 9、世界第三大瀏覽器正在消亡 10、被問懵了：MySQL 自增主鍵一定是連續(xù)的嗎？
評(píng)論
圖片
表情