Indexing a table for better performance by a query

What index/indexes would you recommend to be created on the开发者_Python百科 table for making the following type of query more efficient:

SELECT MAX(val1)
FROM table
WHERE val2 = 'x' OR val3 = 'y'

x and y are of course variable. val2 and val3 are almost always unique, some duplicates may occur.

have an index on val2+val1 and another on val3+val1 and write query like:

SELECT MAX(val1)
FROM (SELECT max(val1) FROM table where val2 = 'x'
      UNION ALL
      SELECT max(val1) FROM table val3 = 'y'
     ) dt