hi all, i have this table

RESULTS
{
integer ID //PK
integer SOURCE_ID //FK
timestamp DATE
}

with an index on SOURCE_ID.

i need a store procedure wich, given a source_id, will return the id of the row with the last DATE.
i found 2 solutions:

using 'order by' and selecting the first

SELECT FIRST 1 r.ID from RESULTS r
WHERE r.SOURCE_ID = :SourceId
ORDER BY r.DATE DESC

using the max function

SELECT FIRST 1 r.ID FROM RESULTS r
WHERE r.SOURCE_ID = :SourceId AND
r.DATE = (SELECT MAX(DATE) FROM RESULTS r2
WHERE SOURCE_ID = :SourceId)

at the moment both are too slow (the table dimension is of same milions of records) so i'm trying to find same optimization.
the solution 2 in my idea must be faster becouse it don't need to sort all the records but in my test it's about 20/30% slower, i'm doing/thinking samething wrong?
and in general do you have same suggestion to do a better query to find what i need?

thanks