Derived table for hash join is created with a key

create table ten(a int primary key);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int primary key);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t1 (a varchar(750), b varchar(750));

insert into t1 select a,a from one_k;

create table t2 as select a from t1 limit 10;

create ALGORITHM=TEMPTABLE view v1 as select a,b from t1;

set optimizer_switch='derived_with_keys=on';

set join_cache_level=4;

explain select * from t1, v1 where t1.a=v1.a;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	1000	Using where

1	PRIMARY	<derived2>	hash_ALL	key0	#hash#key0	753	test.t1.a	1000	Using join buffer (flat, BNLH join)

2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	1000