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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
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，或者大數(shù)據(jù)分批導(dǎo)出等場景下，是可以將上一頁的最大值當(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(gè)常見的錯(cuò)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(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(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語句

MySQL 對待 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(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(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)語句的性能下降。

其實(shí)對于子查詢 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)

數(shù)據(jù)庫編譯器產(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。

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

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) 。

鏈接：https://juejin.cn/post/6844903998974099470

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10、被問懵了：MySQL 自增主鍵一定是連續(xù)的嗎？

8種?？油碌?SQL 寫法，性能降低100倍，不來看看？

往期熱門文章：

?2隱式轉(zhuǎn)換

?3關(guān)聯(lián)更新、刪除?

?4混合排序

5EXISTS語句

?6條件下推

?7提前縮小范圍?

?8中間結(jié)果集下推

8種?？油碌?SQL 寫法，性能降低100倍，不來看看？