MySQL去重该使用distinct还是group by？

show variables like '%query_cache%';

set global query_cache_size = 0;

drop table if exists t0;
create table t0(
id bigint primary key auto_increment,
a varchar(255) not null
) engine=InnoDB default charset=utf8mb4 collate=utf8mb4_bin;
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drop procedure insert_t0_simple_category_data_sp;
delimiter //
create procedure insert_t0_simple_category_data_sp(IN num int)
begin
set @i = 0;
while @i < num do
	insert into t0(a) value(truncate(@i/1000, 0));
 set @i = @i + 1;
end while;
end
//
call insert_t0_simple_category_data_sp(100000);

drop table if exists t1;
create table t1 like t0;
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drop procedure insert_t1_complex_category_data_sp;
delimiter //
create procedure insert_t1_complex_category_data_sp(IN num int)
begin
set @i = 0;
while @i < num do
	insert into t1(a) value(truncate(@i/10, 0));
 set @i = @i + 1;
end while;
end
//
call insert_t1_complex_category_data_sp(10000);

drop table if exists t2;
create table t2 like t1;
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drop procedure insert_t2_complex_category_data_sp;
delimiter //
create procedure insert_t2_complex_category_data_sp(IN num int)
begin
set @i = 0;
while @i < num do
	insert into t1(a) value(truncate(@i/10, 0));
 set @i = @i + 1;
end while;
end
//
call insert_t2_complex_category_data_sp(5000000);

set profiling = 1;
select distinct a from t0;
show profiles;
select a from t0 group by a;
show profiles;
alter table t0 add index `a_t0_index`(a);

alter table t0 add index `a_t0_index`(a);

alter table t1 add index `a_t1_index`(a);

SELECT count(1) FROM t2;

alter table t2 add index `a_t2_index`(a);