I need an example of performance tuning insert for this query

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SQL

performance

I've been having trouble understanding with Performance Tuning previous queries. Underneath is the query that I am trying to improve by adding an index.

Thank you for your help.

Query:

select model, descr, mpg,
(
select count(*) from car_details where mpg is not null) as "Number of Cars"
from car_names, car_details
where car_details.id = car_names.id and mpg is null
for (reg_name) in ('NE', 'NW', 'SE', 'SW')
);

SQL Script File:

https://docs.google.com/document/d/1vClWrxZnU3AKhQzeWwqbMtf5du-3EcfHTuLdbgGvxU4/edit?usp=sharing

What I have tried:

I've tried making simple insert statements but it looks like it made no changes in performance.