Performance issue and indexing doesn't help

Question

I created a view the SQL is basically simple but I need to group data based on a value returned from a function. I think this is slowing the performance. I first added a regular index on this, then added a function based index but neither helps. I get the data I need, but it takes too long. I hope someone can give me some ideas about how to optimize the performance of my SQL.
The base table has 1318408 rows. I need to select only a few columns and group and count the rows based on a date value in one of the columns. However the date in the table contains only an end of week value and I need to report based on quarters. The report needs both a date value and a text value for the quarter. So I created two functions that accept a date value and return a date value for the quarter start date and a text value for the quarter the date falls within respectively; my SQL is like this:
select
GLC_DATE2CAL_QRT_STARTDATE_FN(s.work_week_end_date) cyquarter_start_date,
GLC_DATE2CAL_QRTYR_FN(s.work_week_end_date) cyquarter_text,
s.ethnicity ethnicity_code,
et.description ethnicity_desc,
count( unique employee_id ) number_employees
from cpr_employees_snapshot s, ct_vendor_ethnicities et
and trim(s.ethnicity) = et.ethnicity_id
group by GLC_DATE2CAL_QRT_STARTDATE_FN(s.work_week_end_date), GLC_DATE2CAL_QRTYR_FN(s.work_week_end_date), s.ethnicity, et.description
this takes about 1 1/2 minutes to retrieve the data
when I do not use the functions and run this SQL:
select
s.work_week_end_date,
s.ethnicity ethnicity_code,
et.description ethnicity_desc,
count( unique employee_id ) number_employees
from cpr_employees_snapshot s, ct_vendor_ethnicities et
and trim(s.ethnicity) = et.ethnicity_id
group by s.work_week_end_date, s.ethnicity, et.description
it takes 7 seconds.

Answer 1

Well I was successful in writing a case statement that works as a select and it reduces the retrieval time to 5 seconds, the problem now is that when I create a view with it the view is created but with compilation errors; if the select work without errors I don;t know why that is happening. Here is the create view SQL:
CREATE OR REPLACE FORCE VIEW GLC_WORKER_ETHNICITY_VW
cyquarter_start_date,
cyquarter_text,
cyquarter_end_date,
ethnicity_code,
ethnicity_desc,
number_employees
AS
select
case to_number(to_char(s.work_week_end_date, 'mm'))
when 1 then to_date('1/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 2 then to_date('1/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 3 then to_date('1/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 4 then to_date('4/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 5 then to_date('4/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 6 then to_date('4/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 7 then to_date('7/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 8 then to_date('7/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 9 then to_date('7/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 10 then to_date('10/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 11 then to_date('10/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
when 12 then to_date('10/1/' || to_char(s.work_week_end_date, 'yyyy'),'mm/dd/yyyy')
end cyquarter_start_date,
'Q' || to_char(s.work_week_end_date, 'Q') || ' CY ' || to_char(s.work_week_end_date, 'yyyy') cyquarter_text,
s.ethnicity ethnicity_code,
et.description ethnicity_desc,
count( unique employee_id ) number_employees
from cpr_employees_snapshot s, ct_vendor_ethnicities et
where package_id = 727260
and trim(s.ethnicity) = et.ethnicity_id
group by s.work_week_end_date, s.ethnicity, et.description