Bug #103631 Mismatch of number systems of select number in query and hints
Submitted: 8 May 8:35 Modified: 17 May 8:39
Reporter: Frank Fan Email Updates:
Status: Verified Impact on me:
None 
Category:MySQL Server: DML Severity:S4 (Feature request)
Version:8.0.23 OS:Any
Assigned to: CPU Architecture:Any
Tags: hints

[8 May 8:35] Frank Fan
Description:
In the result of explain select ..., select number in the query is decimal while the select number in hints like JOIN_SUFFIX is hexadecimal. There seems to be no reason to use two different number systems here and the mismatch of number systems may confuse users.

How to repeat:
create table t1 (id int, score int);
create table t2 (id int, score int);
create table t3 (id int, score int);
create table t4 (id int, score int);

explain select count(*) from (
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id) union all  
  (select /*+ JOIN_SUFFIX(t3, t2) */ t1.score from t1, t2, t3, t4 where t1.id = t2.id and t2.id = t3.id and t3.id = t4.id)
) a;

show warnings \G

The result is 

*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select /*+ JOIN_SUFFIX(@`select#2` `t3`,`t2`) JOIN_SUFFIX(@`select#3` `t3`,`t2`) JOIN_SUFFIX(@`select#4`
 `t3`,`t2`) JOIN_SUFFIX(@`select#5` `t3`,`t2`) JOIN_SUFFIX(@`select#6` `t3`,`t2`) JOIN_SUFFIX(@`select#7` `t3`,`t2`) JOIN_SUFFIX
(@`select#8` `t3`,`t2`) JOIN_SUFFIX(@`select#9` `t3`,`t2`) JOIN_SUFFIX(@`select#a` `t3`,`t2`) JOIN_SUFFIX(@`select#b` `t3`,`t2`)
 JOIN_SUFFIX(@`select#c` `t3`,`t2`) JOIN_SUFFIX(@`select#d` `t3`,`t2`) */ count(0) AS `count(*)` from (/* select#2 */ select `db
1`.`t1`.`score` AS `score` from `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.
`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#3 */ select `db1`.`t1
`.`score` AS `score` from `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) 
and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#4 */ select `db1`.`t1`.`sco
re` AS `score` from `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`
db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#5 */ select `db1`.`t1`.`score` AS
 `score` from `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`
t3`.`id` = `db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#6 */ select `db1`.`t1`.`score` AS `scor
e` from `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`i
d` = `db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#7 */ select `db1`.`t1`.`score` AS `score` fro
m `db1`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `
db1`.`t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#8 */ select `db1`.`t1`.`score` AS `score` from `db1
`.`t1` join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`
t1`.`id`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#9 */ select `db1`.`t1`.`score` AS `score` from `db1`.`t1`
 join `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`i
d`) and (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#10 */ select `db1`.`t1`.`score` AS `score` from `db1`.`t1` join
 `db1`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) a
nd (`db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#11 */ select `db1`.`t1`.`score` AS `score` from `db1`.`t1` join `db1
`.`t2` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`
db1`.`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#12 */ select `db1`.`t1`.`score` AS `score` from `db1`.`t1` join `db1`.`t2
` join `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`db1`.
`t2`.`id` = `db1`.`t1`.`id`)) union all /* select#13 */ select `db1`.`t1`.`score` AS `score` from `db1`.`t1` join `db1`.`t2` joi
n `db1`.`t3` join `db1`.`t4` where ((`db1`.`t4`.`id` = `db1`.`t1`.`id`) and (`db1`.`t3`.`id` = `db1`.`t1`.`id`) and (`db1`.`t2`.
`id` = `db1`.`t1`.`id`))) `a`

Suggested fix:
I suggest to use decimal in hints. 

The code to change is in opt_hints.cc:202

Opt_hints_qb::Opt_hints_qb(Opt_hints *opt_hints_arg, MEM_ROOT *mem_root_arg,
                           uint select_number_arg)
    : Opt_hints(nullptr, opt_hints_arg, mem_root_arg),
      select_number(select_number_arg),
      subquery_hint(nullptr),
      semijoin_hint(nullptr),
      join_order_hints(mem_root_arg),
      join_order_hints_ignored(0) {
  sys_name.str = buff;
  sys_name.length =
      snprintf(buff, sizeof(buff), "%s%x", sys_qb_prefix.str, select_number);
}
[17 May 8:39] MySQL Verification Team
Hi Frank,

Thanks for the request. I changed the severity to S4 (Feature request) and verified it as it makes sense. 

Thanks
Bogdan