Question

I have following query which take more than a minute to execute, how can I optimize it. Its slow because of order by o.id desc, if I remove it query executes it few ms.

select o.*, per.email, p.name
from order o 
inner join product p 
on o.product_id=p.id 
inner join person per 
on o.person_id=per.id 
order by o.id desc 
limit 100;

Following is the result of explain

1   SIMPLE  p   index   PRIMARY FK2EFC6C1E5DE2FC    8   NULL    6886    Using index; Using temporary; Using filesort
1   SIMPLE  o   ref FK67E9050121C383DB,FK67E90501FC44A17C   FK67E90501FC44A17C  8   dev.p.id    58  
1   SIMPLE  per eq_ref  PRIMARY PRIMARY 8   dev.o.person_id 1   Using index

All the tables are InnoDB and joins are on Primary and Foreign keys. Other than that indexes are on email column in Person and status column in Order

Number of records in each table

Person : 1,300,000
Product: 7,000
Order : 70,000

Answer 1

The planner, most probably, is not using the limit hint to eliminate rows from order table before the join. So the server has to do the join for all rows and then return just a few.

Try this:

select o.* from
(select * order order by id desc limit 100) o
inner join product p 
on o.product_id=p.id 
inner join person per 
on o.person_id=per.id 
order by o.id desc limit 100;

EDIT: This will work only if there is a constraint guaranteeing that corresponding rows are present in Product and Person tables.