Client Dataset Sorting

Hi All,

I have a table which holds more than 60000 records of inventory data.
I want my users to be able to sort the table by item code and by qty.

I write a code so that my users can sort the table by clicking the
title in dbgrid. I took approx 1 secs to sort
qty(ascending/descending) and item code(ascending). the problem is
when i tried to sort item code field in descending order, it took more
than 1 minute.

can any one tell me what went wrong and give me suggestions to improve
the sorting/filtering performance?

i use delphi 7 and dbexpress. my database is firebird 2.

Thanks,

Reynaldi