SELECT SQL_NO_CACHE SQL_CALC_FOUND_ROWS field1, field2, field3
FROM tablename
WHERE field1 = 'some_value' OR field2 = 'some_value'
ORDER BY field3;

SELECT SQL_NO_CACHE SQL_CALC_FOUND_ROWS *
FROM (
SELECT field1, field2, field3
FROM tablename
WHERE field1 = 'some_value') a1
UNION (
SELECT field1, field2, field3
FROM tablename
WHERE field2 = 'some_value')
ORDER BY field3;