🚀 go-pugleaf

Trying to solve the following problem.
I have quite a large table with 2 fields, a unique field with integer 
number, call this F1 and
a field with datetime values, call this F2.
Both fields will appear in ascending order when you do a select.
F1 is indexed, but F2 is not.

To show a small sample:

F1                    F2
--------------------------------------
10303521     01/04/2006 19:50
10303525     01/04/2006 19:50
10303529     01/04/2006 19:50
10303535     01/04/2006 19:51
10303536     02/04/2006 07:43   <<<
10303540     02/04/2006 07:43
10303541     02/04/2006 07:43
10303546     02/04/2006 07:43

Now what I need to do is fetch all the rows that are the first row for the 
particular day, so
in the above example I need the row indicated with <<<

Now maybe I could do this with a GROUP BY and CAST(F2 AS INTEGER) but I 
can't work it out.
Any suggestions how to tackle this? As this is a large table (over 20 
million rows) and as F2 is not
indexed maybe this will too slow in SQL and maybe I need an algorithm for 
this in my application.

RBS