CREATE TABLE foo (
a INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY (a)
);

mysql> EXPLAIN * FROM foo WHERE a IN (160000, 160001, 160002)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: foo
type: range
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: NULL
rows: 3
Extra: Using where
1 row in set (0.06 sec)
mysql> EXPLAIN SELECT * FROM foo WHERE a IN (NULL, 160000, 160001, 160002)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: foo
type: ALL
possible_keys: PRIMARY
key: NULL
key_len: NULL
ref: NULL
rows: 327680
Extra: Using where
1 row in set (0.00 sec)