Wrong result - count(distinct), Using index for group-by (scanning)

--source include/have_innodb.inc

CREATE TABLE tmp (

  id int NOT NULL AUTO_INCREMENT,

  a int NOT NULL,

  b int NOT NULL,

  PRIMARY KEY (id),

  UNIQUE KEY ba (b, a)

) ENGINE=InnoDB;

INSERT INTO tmp (a, b) VALUES(1,101),(1,102),(1,103),(1,104),(1,105),(1,106),(1,107),(1,108),(1,109),(1,110);

SELECT COUNT(DISTINCT b) FROM tmp WHERE a = 1;

DROP TABLE tmp;