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_UIDfrom TEST_SUB_PROBLEM TSTjoin ( 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: 1003Message: /* 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_UIDfrom TEST_SUB_PROBLEM TSTjoin ( 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: 1003Message: /* 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
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()
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)