Mysql query optimisation - Very slow - mysql

Question

I need to optimise my query which is running very slow, but don't know how to do it. It contains a subquery which is making it very slow. If I remove the inline query then it runs very well.
The query is:
EXPLAIN
SELECT t.service_date,
t.service_time,
(SELECT js.modified_date FROM rej_job_status js WHERE js.booking_id=b.booking_id ORDER BY id DESC LIMIT 1) `cancel_datetime`,
b.booking_id,
b.ref_booking_id,
b.phone, b.city,
b.booking_time,
CONCAT(rc.firstname," ",rc.lastname) customer_name,
rc.phone_no,
rs.service_id,
rs.service_name,
rct.city_name
FROM rej_job_details t
JOIN rej_booking b ON t.booking_id = b.booking_id
JOIN rej_customer rc ON rc.customer_id = b.customer
JOIN rej_service rs ON t.service_id = rs.service_id
JOIN rej_city rct ON rct.city_id=b.city
WHERE t.act_status = 0 AND DATE(b.booking_time) >= '2016-06-01'
AND DATE(b.booking_time) <= '2016-06-14'
ORDER BY b.booking_time DESC
LIMIT 0 , 50
The explain plan shows this:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY b ALL PRIMARY NULL NULL NULL 32357 Using where; Using filesort
1 PRIMARY rct eq_ref PRIMARY PRIMARY 4 crmdb.b.city 1 NULL
1 PRIMARY t ref booking_id booking_id 4 crmdb.b.booking_id 1 Using where
1 PRIMARY rs eq_ref PRIMARY,service_id PRIMARY 4 crmdb.t.service_id 1 NULL
1 PRIMARY rc eq_ref PRIMARY PRIMARY 4 crmdb.b.customer 1 Using where
2 DEPENDENT SUBQUERY js index NULL PRIMARY 4 NULL 1 Using where
a) How to read this explain plan and know what it means?
b) How can I optimize this query?

Answer 1

booking_time is hiding inside a function, so INDEX(booking_time) cannot be used. That leads to a costly table scan.
AND DATE(b.booking_time) >= '2016-06-01'
AND DATE(b.booking_time) <= '2016-06-14'
-->
AND b.booking_time >= '2016-06-01'
AND b.booking_time < '2016-06-15' -- note 3 differences in this line
Or, this might be simpler (by avoiding second date calculation):
AND b.booking_time >= '2016-06-01'
AND b.booking_time < '2016-06-01' + INTREVAL 2 WEEK
In the EXPLAIN, I expect the 'ALL' to become 'range', and 'Filesort' to vanish.

Answer 2

To understand the full explain-plan, you should read the documentation, but the most important information it includes is the indexes mysql uses, or, usually more revealing, which it doesn't use.
For your DEPENDENT SUBQUERY (that is your "inline query"), it doesn't use a good index, which makes your query slow, so you need to add the index rej_job_status(booking_id) on your table rej_job_status.
Create it, test it and check your explain plan again, it should then list that new index under key for your DEPENDENT SUBQUERY.
Another optimization might be to add an index rej_booking(booking_time) for your table rej_booking. It depends on your data if it improves the query, but you should try it, since right now, mysql doesn't use an index for that selection.