关于 MySQL bug 91418 一些看法
导读:
郑松华,知数堂SQL 优化班老师
现任 CCmediaService DBA,主要负责数据库优化相关工作
擅长SQL优化 ,数据核对
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大家好,我是知数堂SQL 优化班老师 网名:骑龟的兔子
下面是bug链接
https://bugs.mysql.com/bug.php?id=91418
create table TEST_SUB_PROBLEM
(
UID integer PRIMARY KEY AUTO_INCREMENT
,BID VARCHAR(10)
,THING_NAME VARCHAR(100)
,OTHER_IFO VARCHAR(100)
); insert into TEST_SUB_PROBLEM(BID,THING_NAME,OTHER_IFO)
values ('thing1','name1','look a chicken')
,( 'thing1','name1','look an airplane')
,('thing2','name2','look a mouse')
,('thing3','name3','look a taperecorder')
,('thing3','name3','look an explosion')
,('thing4','name4','look at the stars')
; select
TST.UID
,TST.BID
,TST.THING_NAME
,TST.OTHER_IFO
,vw2.DIST_UID
from TEST_SUB_PROBLEM TST
join ( select uuid() as DIST_UID, vw.* from ( select DISTINCT BID, THING_NAME from TEST_SUB_PROBLEM ) vw ) vw2 on vw2.BID = TST.BID ;
bug 的问题就是
select uuid() as DIST_UID, vw.* from ( select DISTINCT BID, THING_NAME from TEST_SUB_PROBLEM ) vw +--------------------------------------+--------+------------+
| DIST_UID | BID | THING_NAME |
+--------------------------------------+--------+------------+
| 00a993b0-cc29-11e8-9fff-080027158a34 | thing1 | name1 |
| 00a993d6-cc29-11e8-9fff-080027158a34 | thing2 | name2 |
| 00a993df-cc29-11e8-9fff-080027158a34 | thing3 | name3 |
| 00a993e5-cc29-11e8-9fff-080027158a34 | thing4 | name4 |
+--------------------------------------+--------+------------+ 这里 DIST_UID 应该是4个 值 但是 原来的SQL 出现了6个不同的值
我查看了下原因 如下
root@mysql3308.sock>[test]>show warnings\G
*************************** 1. row *************************** Level: Note Code: 1003
Message: /* select#1 */ select `test`.`TST`.`UID` AS `UID`,`test`.`TST`.`BID` AS `BID`,
`test`.`TST`.`THING_NAME` AS `THING_NAME`,`test`.`TST`.`OTHER_IFO` AS `OTHER_IFO`,uuid() AS `DIST_UID`
from `test`.`TEST_SUB_PROBLEM` `TST` join
(/* select#3 */ select distinct `test`.`TEST_SUB_PROBLEM`.`BID` AS `BID`,
`test`.`TEST_SUB_PROBLEM`.`THING_NAME` AS `THING_NAME` from `test`.`TEST_SUB_PROBLEM
`) `vw` where (`vw`.`BID` = `test`.`TST`.`BID`)
1 row in set (0.00 sec)
我的环境是8.0 可以使用如下hint 就可以解决 select /*+ set_var(optimizer_switch = 'derived_merge=off' ) */
TST.UID
,TST.BID
,TST.THING_NAME
,TST.OTHER_IFO
,vw2.DIST_UID
from TEST_SUB_PROBLEM TST
join ( select uuid() as DIST_UID, vw.* from ( select DISTINCT BID, THING_NAME from TEST_SUB_PROBLEM ) vw ) vw2 on vw2.BID = TST.BID ; +-----+--------+------------+---------------------+--------------------------------------+
| UID | BID | THING_NAME | OTHER_IFO | DIST_UID |
+-----+--------+------------+---------------------+--------------------------------------+
| 1 | thing1 | name1 | look a chicken | 3d65599a-cc26-11e8-9fff-080027158a34 |
| 2 | thing1 | name1 | look an airplane | 3d65599a-cc26-11e8-9fff-080027158a34 |
| 3 | thing2 | name2 | look a mouse | 3d655a0b-cc26-11e8-9fff-080027158a34 |
| 4 | thing3 | name3 | look a taperecorder | 3d655a1a-cc26-11e8-9fff-080027158a34 |
| 5 | thing3 | name3 | look an explosion | 3d655a1a-cc26-11e8-9fff-080027158a34 |
| 6 | thing4 | name4 | look at the stars | 3d655a25-cc26-11e8-9fff-080027158a34 |
+-----+--------+------------+---------------------+--------------------------------------+
6 rows in set (0.00 sec) +----+-------------+------------------+------------+------+---------------+-------------+---------+--------------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------------+------------+------+---------------+-------------+---------+--------------+------+----------+-----------------+
| 1 | PRIMARY | TST | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 100.00 | Using where |
| 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 33 | test.TST.BID | 2 | 100.00 | NULL |
| 2 | DERIVED | <derived3> | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 100.00 | NULL |
| 3 | DERIVED | TEST_SUB_PROBLEM | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 100.00 | Using temporary |
+----+-------------+------------------+------------+------+---------------+-------------+---------+--------------+------+----------+-----------------+ 从 show warnings 部分可以看出 uuid() AS `DIST_UID 在id =2 部分 运行出结果了 所以没有结果有4个值! root@mysql3308.sock>[test]>show warnings\G
*************************** 1. row *************************** Level: Note Code: 1003
Message: /* select#1 */ select /*+ SET_VAR(optimizer_switch='derived_merge=off') */ `test`.`TST`.`UID` AS `UID`,
`test`.`TST`.`BID` AS `BID`,`test`.`TST`.`THING_NAME` AS `THING_NAME`,`test`.`TST`.`OTHER_IFO` AS `OTHER_IFO`,
`vw2`.`DIST_UID` AS `DIST_UID` from `test`.`TEST_SUB_PROBLEM` `TST`
join (/* select#2 */ select uuid() AS `DIST_UID`,`vw`.`BID` AS `BID`,`vw`.`THING_NAME` AS `THING_NAME`
from (/* select#3 */ select distinct `test`.`TEST_SUB_PROBLEM`.`BID` AS `BID`,
`test`.`TEST_SUB_PROBLEM`.`THING_NAME` AS `THING_NAME` from `test`.`TEST_SUB_PROBLEM`) `vw`) `vw2` where (`vw2`.`BID` = `test`.`TST`.`BID`)
1 row in set (0.00 sec) s/82991850
类似的函数还有sysdate()
root@mysql3308.sock>[test]>select -> TST.UID -> ,TST.BID -> ,TST.THING_NAME -> ,TST.OTHER_IFO -> ,vw2.DIST_UID -> from TEST_SUB_PROBLEM TST -> join ( -> select sysdate() as DIST_UID, vw.* ,sleep(1) s -> from ( -> select DISTINCT BID, THING_NAME from TEST_SUB_PROBLEM -> ) vw -> ) vw2 on vw2.BID = TST.BID -> ;
+-----+--------+------------+---------------------+---------------------+
| UID | BID | THING_NAME | OTHER_IFO | DIST_UID |
+-----+--------+------------+---------------------+---------------------+
| 1 | thing1 | name1 | look a chicken | 2018-10-10 10:21:42 |
| 2 | thing1 | name1 | look an airplane | 2018-10-10 10:21:42 |
| 3 | thing2 | name2 | look a mouse | 2018-10-10 10:21:42 |
| 4 | thing3 | name3 | look a taperecorder | 2018-10-10 10:21:42 |
| 5 | thing3 | name3 | look an explosion | 2018-10-10 10:21:42 |
| 6 | thing4 | name4 | look at the stars | 2018-10-10 10:21:42 |
+-----+--------+------------+---------------------+---------------------+
6 rows in set (0.00 sec) root@mysql3308.sock>[test]>select /*+ set_var(optimizer_switch = 'derived_merge=off' ) */ -> TST.UID -> ,TST.BID -> ,TST.THING_NAME -> ,TST.OTHER_IFO -> ,vw2.DIST_UID -> from TEST_SUB_PROBLEM TST -> join ( -> select sysdate() as DIST_UID, vw.* ,sleep(1) s -> from ( -> select DISTINCT BID, THING_NAME from TEST_SUB_PROBLEM -> ) vw -> ) vw2 on vw2.BID = TST.BID -> ; +-----+--------+------------+---------------------+---------------------+
| UID | BID | THING_NAME | OTHER_IFO | DIST_UID |
+-----+--------+------------+---------------------+---------------------+
| 1 | thing1 | name1 | look a chicken | 2018-10-10 10:22:04 |
| 2 | thing1 | name1 | look an airplane | 2018-10-10 10:22:04 |
| 3 | thing2 | name2 | look a mouse | 2018-10-10 10:22:05 |
| 4 | thing3 | name3 | look a taperecorder | 2018-10-10 10:22:06 |
| 5 | thing3 | name3 | look an explosion | 2018-10-10 10:22:06 |
| 6 | thing4 | name4 | look at the stars | 2018-10-10 10:22:07 |
+-----+--------+------------+---------------------+---------------------+
6 rows in set (4.00 sec)
谢谢大家~ 欢迎转发
如有关于SQL优化方面疑问需要交流的,请加入QQ群(579036588),并@骑兔子的龟 就可与我联系
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