CREATE TABLE test (
id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
uId int(10) unsigned NOT NULL,
chatId smallint(5) unsigned NOT NULL,
tId int(11) NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY test_uId_chatId_tId (uId,chatId,tId) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8mb4;

test_uId_chatId_tId是联合索引，我理解的最左匹配原则，意思是where语句中应该有uId索引才起作用，但下面但语句，竟然也是使用了索引，这是什么原因？
mysql> explain select * from test where chatId = “1” and tId = “2”\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test
partitions: NULL
type: index
possible_keys: NULL
key: test_uId_chatId_tId
key_len: 10
ref: NULL
rows: 1
filtered: 100.00
Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)