create table t1(a blob , unique(a) using hash);
show keys from t1;;
Table	t1
Non_unique	0
Key_name	a
Seq_in_index	1
Column_name	a
Collation	A
Cardinality	NULL
Sub_part	NULL
Packed	NULL
Null	YES
Index_type	HASH
Comment	
Index_comment	
drop table t1;
create table t1(a blob , unique(a) using btree);
ERROR 42000: Specified key was too long; max key length is 1000 bytes
create table t1(a int , unique(a) using hash);
show keys from t1;;
Table	t1
Non_unique	0
Key_name	a
Seq_in_index	1
Column_name	a
Collation	A
Cardinality	NULL
Sub_part	NULL
Packed	NULL
Null	YES
Index_type	HASH
Comment	
Index_comment	
drop table t1;
create table t1(a int ,b int , c int,  unique(a, b, c) using hash);
show keys from t1;
Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment
t1	0	a	1	a	A	NULL	NULL	NULL	YES	HASH		
t1	0	a	2	b	A	NULL	NULL	NULL	YES	HASH		
t1	0	a	3	c	A	NULL	NULL	NULL	YES	HASH		
insert into t1 values(1,1,1);
insert into t1 values(1,1,1);
ERROR 23000: Duplicate entry '1-1-1' for key 'a'
drop table t1;
create table t1(a int ,b int , c int,  unique(a, b, c) using hash) engine=memory;
show keys from t1;
Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment
t1	0	a	1	a	NULL	NULL	NULL	NULL	YES	HASH		
t1	0	a	2	b	NULL	NULL	NULL	NULL	YES	HASH		
t1	0	a	3	c	NULL	0	NULL	NULL	YES	HASH		
insert into t1 values(1,1,1);
insert into t1 values(1,1,1);
ERROR 23000: Duplicate entry '1-1-1' for key 'a'
drop table t1;