下面是关于 MySQL bug 91418 的重现
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
;
+-----+--------+------------+---------------------+--------------------------------------+
| UID | BID | THING_NAME | OTHER_IFO | DIST_UID |
+-----+--------+------------+---------------------+--------------------------------------+
| 1 | thing1 | name1 | look a chicken | 03d198cc-cc26-11e8-9fff-080027158a34 |
| 2 | thing1 | name1 | look an airplane | 03d198f0-cc26-11e8-9fff-080027158a34 |
| 3 | thing2 | name2 | look a mouse | 03d19903-cc26-11e8-9fff-080027158a34 |
| 4 | thing3 | name3 | look a taperecorder | 03d19912-cc26-11e8-9fff-080027158a34 |
| 5 | thing3 | name3 | look an explosion | 03d19920-cc26-11e8-9fff-080027158a34 |
| 6 | thing4 | name4 | look at the stars | 03d1992e-cc26-11e8-9fff-080027158a34 |
+-----+--------+------------+---------------------+--------------------------------------+
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
[email protected]>[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)
从上面的show warnings 可以发现
uuid() AS `DIST_UID` 这个部分经过MySQL 转换之后挪到了最上层 所以导致最终有六个不同的值
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！
[email protected]>[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)
***********************************************************************************
上面的问题是因为MySQL5.7 开始的视图合并功能引起的如果是MySQL5.6 角度看的话 确实是bug
因为跟5.6 结果是不同的 但是从另一个角度上说 这个也是无法解决的Bug 因为这个问题的根本原因有一方面
是视图合并 但更重要的是类似uuid() 这样的随着时间或者次数不停变化的函数特性导致这种问题必然存在
类似的函数还有sysdate()
[email protected]>[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)
[email protected]>[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)