How to make subquery fast - subquery

Question

for an author overview we are looking for a query which will show all the authors including their best book. The problem with this query is that it lacks speed. There are only about 1500 authors and the query do generate the overview is currently taking 20 seconds.
The main problem seems te be generating the average rating of all the books per person.
By selecting the following query, it is still rather fast
select
person.id as pers_id,
person.firstname,
person.suffix,
person.lastname,
thriller.title,
year(thriller.orig_pubdate) as year,
thriller.id as thrill_id,
count(user_rating.id) as nr,
AVG(user_rating.rating) as avgrating
from
thriller
inner join
thriller_form
on thriller_form.thriller_id = thriller.id
inner join
thriller_person
on thriller_person.thriller_id = thriller.id
and thriller_person.person_type_id = 1
inner join
person
on person.id = thriller_person.person_id
left outer join
user_rating
on user_rating.thriller_id = thriller.id
and user_rating.rating_type_id = 1
where thriller.id in
(select top 1 B.id from thriller as B
inner join thriller_person as C on B.id=C.thriller_id
and person.id=C.person_id)
group by
person.firstname,
person.suffix,
person.lastname,
thriller.title,
year(thriller.orig_pubdate),
thriller.id,
person.id
order by
person.lastname
However, if we make the subquery a little more complex by selecting the book with the average rating it takes a full 20 seconds to generate a resultset.
The query would then be as follows:
select
person.id as pers_id,
person.firstname,
person.suffix,
person.lastname,
thriller.title,
year(thriller.orig_pubdate) as year,
thriller.id as thrill_id,
count(user_rating.id) as nr,
AVG(user_rating.rating) as avgrating
from
thriller
inner join
thriller_form
on thriller_form.thriller_id = thriller.id
inner join
thriller_person
on thriller_person.thriller_id = thriller.id
and thriller_person.person_type_id = 1
inner join
person
on person.id = thriller_person.person_id
left outer join
user_rating
on user_rating.thriller_id = thriller.id
and user_rating.rating_type_id = 1
where thriller.id in
(select top 1 B.id from thriller as B
inner join thriller_person as C on B.id=C.thriller_id
and person.id=C.person_id
inner join user_rating as D on B.id=D.thriller_id
group by B.id
order by AVG(D.rating))
group by
person.firstname,
person.suffix,
person.lastname,
thriller.title,
year(thriller.orig_pubdate),
thriller.id,
person.id
order by
person.lastname
Anyone got a good suggestion to speed up this query?

Answer 1

Calculating an average requires a table scan since you've got to sum the values and then divide by the number of (relevant) rows. This in turn means that you're doing a lot of rescanning; that's slow. Can you calculate the averages once and store them? That would let your query use those pre-computed values. (Yes, it denormalizes the data, but denormalizing for performance is often necessary; there's a trade-off between performance and minimal data.)
It might be appropriate to use a temporary table as the store of the averages.