The majority of Field_xxx classes could detect "Impossible WHERE" by catching out-of-domain constants that appear in WHERE condition in the form field=const. It could avoid full table scan in such cases.

Examples:

a DATE column compared to a DATETIME constant

{code}
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a DATE);
INSERT INTO t1 VALUES ('2001-01-01'),('2001-01-02');
EXPLAIN SELECT * FROM t1 WHERE a=TIMESTAMP'2001-01-01 10:20:30';
{code}
returns:

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

It's to do the full table scan, through the condition is known to return FALSE for all records in the table.

INT compared to a constant that is out of range

{code}
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a TINYINT);
INSERT INTO t1 VALUES (10),(20),(30);
EXPLAIN SELECT * FROM t1 WHERE a=300;
{code}

INT compared to a constant with fractional digits

{code}
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a INT);
INSERT INTO t1 VALUES (10),(20),(30);
EXPLAIN SELECT * FROM t1 WHERE a=10.1
{code}

String column compared to a long constant

{code}
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a VARCHAR(5));
INSERT INTO t1 VALUES ('a'),('b'),('c');
EXPLAIN SELECT * FROM t1 WHERE a=REPEAT('a',100);
{code}