I'm trying to improve the performance on a query that is running very slowly.

Question

SELECT p.book_id AS payment_book_id, b.status AS book_status,
       p.status AS payment_status, p.supplier_ref_no AS data_cash_ref,
       f.train_pnr_no AS train_pnr, f.train_id AS train_ref_id,
       f.service_no AS train_service_no, h.hotel_pnr_no AS hotel_pnr,
       h.hotel_id AS hotel_ref_id, h.service_no AS hotel_service_no,
       b.total_book_amt AS hotel_amt, c.car_pnr_no AS car_pnr,
       c.car_id AS car_ref_id, c.service_no AS car_service_no,
       b.book_date, b.promo_code
  FROM gk_payment p
  LEFT JOIN gk_book b ON b.book_id = p.book_id
  LEFT JOIN gk_train_itiner_det f ON p.book_id = f.book_id
  LEFT JOIN gk_hotel_itiner_det h ON p.book_id = h.book_id
  LEFT JOIN gk_car_itiner_det c ON p.book_id = c.book_id
WHERE ((b.status IN ('8') AND p.status IN ('2', '0', '1', '8', '7')) OR
       (b.status IN ('5') AND p.status IN ('3')) OR
       (b.status IN ('11') AND p.status IN ('4', '5')))
       AND p.service_no IS NOT NULL
       AND b.book_type = 1
ORDER BY train_id, car_id

Answer 1

Some small comments about the query.
1) The first left join is not needed, since the where filter "  AND b.book_type = 1" eliminates all cases where table b would be outer join (=empty).
FROM gk_payment p
  LEFT JOIN gk_book b ON b.book_id = p.book_id
2) The status comparison can be written in a different way. This will probably not change anything performancewise
instead of
WHERE ((b.status IN ('8') AND p.status IN ('2', '0', '1', '8', '7')) OR
       (b.status IN ('5') AND p.status IN ('3')) OR
       (b.status IN ('11') AND p.status IN ('4', '5')))
you can write
WHERE ((b.status,p.status))
    IN (('8','0')
        ,('8','1')
        ,('8','2')
        ,('8','7')
        ,('8','8')
        ,('5','3')
        ,('11','4')
        ,('11',53')
Which is imo much easier to read, maintain and checkable for completeness.
Furthermore: If the status fields are number columns then by all means compare them with numbers and not as strings.
3) Remove the order by condition
Is it faster without it? If yes, do you really need to order the output?