help with optimize a query plzzzz.....

[henri_indon]

for example:

create table test(
id int(10) unsigned auto_increment,
checkIn datetime default '0000-00-00 00:00:00',
primary key (id)
);

and i want to optimize the following query.

explain select * from visits where (checkIn <= '2005-08-18 23:59:59') and (checkIn >= '2005-08-18 00:00:00');

and it gives me:

+--------+------+---------------+------+---------+------+------+-------------+
| table | type | possible_keys | key | key_len | ref | rows | Extra |
+--------+------+---------------+------+---------+------+------+-------------+
| visits | ALL | checkIn | NULL | NULL | NULL | 30 | Using where |
+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

i add index on checkIn, but still somehow it give the same result.
but when i change the query to:
explain select * from visits where (checkIn <= '2005-08-17 23:59:59') and (checkIn >= '2005-08-17 00:00:00'); (checkIn date is different)

it gives me the "range" type.
the main difference between the 2005-08-18 records and 2005-08-17 records is that the 18th has about 10 rows in the database while 17th only have about 3 records.

so can somebody help me with this plz????