[MDEV-32192] @@character_set_collations to resolve IMPLICIT collation on JOINs - Jira

If character_set_collations is for resolving implicit collation types then in can be used in a JOIN too.

join.test
create table t520 (t varchar(30) character set utf8mb4 collate utf8mb4_unicode_520_ci);
create table t1400 (t varchar(30) character set utf8mb4 collate utf8mb4_uca1400_ai_ci);
insert into t520 values ('bob'),('jack'), ('jane');
insert into t1400 values ('bob'),('jack'), ('jane');
--error ER_CANT_AGGREGATE_2COLLATIONS
select * from t1400 join t520 on t1400.t = t520.t;
ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_uca1400_ai_ci,IMPLICIT) and (utf8mb4_unicode_520_ci,IMPLICIT) for operation '='

SET @@character_set_collations = 'utf8mb4=uca1400_ai_ci';
select * from t1400 join t520 on t1400.t = t520.t;

ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_uca1400_ai_ci,IMPLICIT) and (utf8mb4_unicode_520_ci,IMPLICIT) for operation '='

Last case shouldn't be an error as the resolution mechanism is possible.

relates to

MDEV-30164 System variable for default collations

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