different execution plan for single- and multi-DELETE

create table t1 (id int);

insert t1 values (1),(2),(3);

create table t2 (id int, acc_id int);

insert t2 values (1,1),(2,2),(3,3);

explain delete from t1 where id in (select id from t2 where acc_id = 9999);

explain delete uc from t1 as uc where uc. id in (select p. id from t2 p where p.acc_id = 9999);