MySQL 5.6比MySQL 8快

Question

I am running one query which is faster on MySQL5.6 than MySQL8. It takes around 4 seconds in MySQL5.6 whereas takes 32 secs in MySQL8. As per MySQL documentation MySQL8 is faster but I have seen the opposite case.

Query:

select id 
from defaultitem 
where masterId='3b1e9726-12ea-4adf-a6ef-f0950075fec4' 
and mainID in ( 
    select id 
    from campaign 
    where deleteflag=0 
    and validflag=1 
    and deptid='9d9d7e0f-7256-4ac6-9398-378ae9a0239e')

Show create table:

defaultitem - CREATE TABLE `defaultitem` (
  `id` varchar(255) NOT NULL,
  `value` varchar(50) NOT NULL,
  `masterId` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK22505D768EC27364` (`masterId`),
  CONSTRAINT `FK22505D768EC27364` FOREIGN KEY (`masterId`) REFERENCES `combomaster` (`masterid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 

campaign - CREATE TABLE `campaign` (
  `id` varchar(255) NOT NULL,
  `deptid` varchar(255) DEFAULT NULL,
  `deleteflag` tinyint(4) DEFAULT '0',
  `validflag` tinyint(4) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `FK47DDEA51F157E732` (`deptid`),
  CONSTRAINT `FK47DDEA51F157E732` FOREIGN KEY (`deptid`) REFERENCES `dept` (`deptid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |


EXPLAIN query in MySQL5.6 - 
| id | select_type | table             | type   | possible_keys              | key                | key_len | ref                               | rows   | Extra                              |
+----+-------------+-------------------+--------+----------------------------+--------------------+---------+-----------------------------------+--------+------------------------------------+
|  1 | SIMPLE      | defaultitem | ref    | FK22505D768EC27364         | FK22505D768EC27364 | 768     | const                             | 812528 | Using index condition; Using where |
|  1 | SIMPLE      | campaign      | eq_ref | PRIMARY,FK47DDEA51F157E732 | PRIMARY            | 767     | db.defaultitem.mainID |      1 | Using where                        |
+----+-------------+-------------------+--------+----------------------------+--------------------+---------+-----------------------------------+--------+------------------------------------+


EXPLAIN QUERY in MySQL8 -
| id | select_type | table             | partitions | type | possible_keys              | key                | key_len | ref   | rows   | filtered | Extra       |
+----+-------------+-------------------+------------+------+----------------------------+--------------------+---------+-------+--------+----------+-------------+
|  1 | SIMPLE      | campaign      | NULL       | ref  | PRIMARY,FK47DDEA51F157E732 | FK47DDEA51F157E732 | 768     | const |     15 |     1.00 | Using where |
|  1 | SIMPLE      | defaultitem | NULL       | ref  | FK22505D768EC27364         | FK22505D768EC27364 | 768     | const | 787517 |    10.00 | Using where |
+----+-------------+-------------------+------------+------+----------------------------+--------------------+---------+-------+--------+----------+-------------+

innodb_buffer_pool_size is same in both versions.

I assumed that MySQL 8 would be faster.

Answer 1

Your query is functionally identical to:

SELECT DISTINCT x.id  
  FROM defaultitem x
  JOIN campaign y
    ON y.id = x.mainid
   AND y.deleteflag = 0 
   AND y.validflag = 1 
   AND y.deptid = '9d9d7e0f-7256-4ac6-9398-378ae9a0239e'
 WHERE x.masterId = '3b1e9726-12ea-4adf-a6ef-f0950075fec4';

...so why not write it that way.

A key on some combination of (deptid,validflag,deleteflag) would probably help.