Inconsistent result of the same queries, related to index merge, null

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `87f3b2f7` (
  `12db5ecc` set('nwbk','r5','1ad3u','van','ir1z','y','9m','f1','z','e6yd','wfev') NOT NULL DEFAULT 'ir1z,f1,e6yd',
  `0a968b42` enum('soo2','4s4j','qi9om','8ue','i71o','qon','3','3feh','6o1i','5yebx','d') NOT NULL DEFAULT '8ue',
  `af528200` varchar(66) DEFAULT '13mdezixgcn',
  PRIMARY KEY (`12db5ecc`,`0a968b42`) /*T![clustered_index] CLUSTERED */,
  UNIQUE KEY `e521f911` (`0a968b42`),
  KEY `17d3c2c4` (`12db5ecc`)
) ENGINE=InnoDB DEFAULT CHARSET=ascii COLLATE=ascii_bin COMMENT='9302fd19';

INSERT INTO `87f3b2f7` VALUES('ir1z,f1,e6yd','i71o','13mdezixgcn'),('ir1z,f1,e6yd','d','13mdezixgcn'),('nwbk','8ue','13mdezixgcn');

select     trim( `87f3b2f7`.`12db5ecc` ) as r0 , var_samp( `87f3b2f7`.`0a968b42` ) as r1 from `87f3b2f7` where `87f3b2f7`.`12db5ecc` between 'e6yd' and 'z' or `87f3b2f7`.`0a968b42` <> '8ue' group by `87f3b2f7`.`12db5ecc`  order by r0,r1;

select  /*+ use_index_merge( `87f3b2f7` ) */ /*+ agg_to_cop() stream_agg() */  trim( `87f3b2f7`.`12db5ecc` ) as r0 , var_samp( `87f3b2f7`.`0a968b42` ) as r1 from `87f3b2f7` where `87f3b2f7`.`12db5ecc` between 'e6yd' and 'z' or `87f3b2f7`.`0a968b42` <> '8ue' group by `87f3b2f7`.`12db5ecc`  order by r0,r1;

2. What did you expect to see? (Required)

Two queries got the same results.

3. What did you see instead (Required)

mysql> select  /*+ use_index_merge( `87f3b2f7` ) */ /*+ agg_to_cop() stream_agg() */  trim( `87f3b2f7`.`12db5ecc` ) as r0 , var_samp( `87f3b2f7`.`0a968b42` ) as r1 from `87f3b2f7` where `87f3b2f7`.`12db5ecc` between 'e6yd' and 'z' or `87f3b2f7`.`0a968b42` <> '8ue' group by `87f3b2f7`.`12db5ecc`  order by r0,r1;
+--------------+------+
| r0           | r1   |
+--------------+------+
| ir1z,f1,e6yd |   18 |
+--------------+------+
1 row in set (0.01 sec)

mysql> select     trim( `87f3b2f7`.`12db5ecc` ) as r0 , var_samp( `87f3b2f7`.`0a968b42` ) as r1 from `87f3b2f7` where `87f3b2f7`.`12db5ecc` between 'e6yd' and 'z' or `87f3b2f7`.`0a968b42` <> '8ue' group by `87f3b2f7`.`12db5ecc`  order by r0,r1;
+--------------+------+
| r0           | r1   |
+--------------+------+
| ir1z,f1,e6yd |   18 |
| nwbk         | NULL |
+--------------+------+
2 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

master