Query Tunning related to function

Hi Team,

We are running the below query in PostgreSQL and its taking approx. 8 to 9 sec to run the query.

Query - 1

Select * from
(
Select payment_sid_c,
lms_app.translate_payment_status(payment_sid_c) AS paymentstatus
from
lms_app.lms_payment_check_request
group by payment_sid_c) a
where paymentstatus in ('PAID', 'MANUALLYPAID')

The explain plan and other details are placed at below link for more information. We have checked the indexes on column but in the explain plan it is showing as Seq Scan which we have to find out.

https://explain.depesz.com/s/Jsiw#stats

This query is using a function translate_payment_status on column payment_sid_c whose script is attached in this mail

Could please anyone help or suggest how to improve the query performance.

Thanks and Regards,
Mukesh Kumar