Two possible indexes in simple select but none is used

[Ozkan Mehmedali]

I have a table with two indexes: one primary on column ID and one non-unique on column UNIQUE_ID. The second index is with specified prefix length 9.

When running select from the table with OR in where clause, MySQL doesn't use any of the indexes. Why?

mysql> explain select * from FOO where id = 0 or unique_id = 'blah';
+----+-------------+--------+------+------------------------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+------------------------------+------+---------+------+--------+-------------+
| 1 | SIMPLE | FOO | ALL | PRIMARY,FOO_unique_id_idx | NULL | NULL | NULL | 149920 | Using where |
+----+-------------+--------+------+------------------------------+------+---------+------+--------+-------------+
1 row in set, 1 warning (0.00 sec)

MySQL version is 4.1.16