🚀 go-pugleaf

I have one table which has 90.000 records and it has all accounts to receive
Each record has a Contract_ID and the same Contract can have up to 20
Accounts to receive
But, I want to show only the first record of each Contract which was not
paid yet or if it was paid between two dates.

Select P.Name R.* from Receive R inner join Customer P on
R.Customer_ID = P.Customer_ID where R.Receive_ID = (select min(Receive_ID)
from Receive
where (PayDate is Null or PayDate between '12/01/02' and '12/31/02') and
Contract_ID=R.Contract_ID group by Contract_ID ) and R.DueDate between
'12/01/02' and '12/31/02'

Example:
ID Contract_ID DueDate PayDate
1    1                 09/01/02 09/01/02
2    1                 10/01/02
3    1                 11/01/02
4    2                 10/01/02 10/01/02
5    2                 11/01/02
6    3                 10/01/02
7    3                 11/01/02
8    4                 12/01/02 12/01/02
9    4                 01/01/03
ID is Primary Key
Contract_ID is Foreign Key
DueDate is indexed
PayDate is indexed too.

The result I want and it results is:
2    1                 10/01/02
5    2                 11/01/02
6    3                 10/01/02
8    4                 12/01/02 12/01/02

So, the result is OK but my problem is the speed.
It takes up to 11 seconds to return using IBConsole.
and, if I use:
select * from Receive where DueDate between '12/01/02' and '12/31/02'
It takes just 0.006 Seconds to return

Then, What do need I do to run that first query faster, like this last one ?