WHERE  '1' IN (field1_index, field2_index)

WHERE  '1' = field1_index OR '1' = field2_index

-- Создание тест-таблицы
mysql> create table test(f1 int, f2 int, f3 int);
Query OK, 0 rows affected (0.36 sec)

-- Создание всех мыслимых индексов keyXYZ(fX,fY,fZ) - 15 штук
-- поскипано.

-- Наполнение RAND данными (таблица digits содержит числа 0..9)

mysql> insert into test (f1,f2,f3) select 1+100*d1.digit+10*d2.digit+d3.digit, CAST(10*rand() AS UNSIGNED), CAST(10*rand() AS UNSIGNED) from digits d1, digits d2, digits d3 order by 1;
Query OK, 1000 rows affected (1.94 sec)
Records: 1000  Duplicates: 0  Warnings: 0

-- Просмотр планов запросов
mysql> explain select f1 from test where 1 in (f2,f3);
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | NULL          | key123 | 15      | NULL | 1000 | Using where; Using index |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select f1 from test where 1 = f2 or 1 = f3;
+----+-------------+-------+-------------+---------------------------------------------------------------+-----------+---------+------+------+-------------------------------------+
| id | select_type | table | type        | possible_keys                                                 | key       | key_len | ref  | rows | Extra                               |
+----+-------------+-------+-------------+---------------------------------------------------------------+-----------+---------+------+------+-------------------------------------+
|  1 | SIMPLE      | test  | index_merge | key2,key3,key23,key21,key31,key32,key213,key231,key312,key321 | key2,key3 | 5,5     | NULL |  218 | Using union(key2,key3); Using where |
+----+-------------+-------+-------------+---------------------------------------------------------------+-----------+---------+------+------+-------------------------------------+
1 row in set (0.00 sec)