create table t1(a int not null, b text, index(b(10))) engine=innodb
key_block_size=1;
insert into t1 values (1,1);
commit;
begin;
update t1 set b=repeat('B',100);
select a,left(b,40),b=1 is_equal from t1;
a	left(b,40)	is_equal
1	1	1
rollback;
select a,left(b,40),b=1 is_equal from t1;
a	left(b,40)	is_equal
1	1	1
drop table t1;