gistfile1.txt

create table t1 (a int primary key, b int, c int, key(b));
insert into t1 select seq, floor(seq/100), seq from seq_1_to_20000;

create table t2 as select * from t1;

explain
delete from t1 
where 
  b <= 42 and a not in (select b from t2);


MariaDB [test]> explain delete from t1  where    b <= 5 and a not in (select b from t2);
+------+--------------+-------+-------+---------------+------+---------+------+-------+-------------+
| id   | select_type  | table | type  | possible_keys | key  | key_len | ref  | rows  | Extra       |
+------+--------------+-------+-------+---------------+------+---------+------+-------+-------------+
|    1 | PRIMARY      | t1    | range | b             | b    | 5       | NULL | 599   | Using where |
|    2 | MATERIALIZED | t2    | ALL   | NULL          | NULL | NULL    | NULL | 19742 |             |
+------+--------------+-------+-------+---------------+------+---------+------+-------+-------------+

this has outer_lookup_keys=19735, even if we're going to invoke the subquery
only 599 times.