Я думаю, это ответ
mysql> create table test(id int primary key, s varchar(20));
Query OK, 0 rows affected (0.06 sec)
mysql> show indexes from test \G
*************************** 1. row ***************************
Table: test
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
1 row in set (0.00 sec)