Nested query performance - mysql

Question

I have two queries below. The first one has a nested select. The second one makes use of a group by clause.
select
posts.*,
(select count(*) from comments where comments.post_id = posts.id and comments.is_approved = 1) as comments_count
from
posts
select
posts.*,
count(comments.id) comments_count
from
posts
left join comments on
comments.post_id = posts.id
group by
posts.*
From my understanding the first query is worse because it has to do a select for each record in posts where as the second query does not.
Is this true or false?

Answer 1

As with all performance questions, you should test the performance on your system with your data.
However, I would expect the first to perform better, with the right indexes. The right index for:
select p.*,
(select count(*)
from comments c
where c.post_id = p.id and c.is_approved = 1
) as comments_count
from posts p
is comments(post_id, is_approved).
MySQL implements a group by by doing a file sort. This version saves a file sort on all the data. My guess is that will be faster than the second method.
As a note: group by posts.* is not valid syntax. I assume this was intended for illustration purposes only.

Answer 2

This is the standard way I would do it (the use of LEFT JOIN, and SUM lets you also know which posts have no comments.)
SELECT posts.*
, SUM(IF(comments.id IS NULL, 0, 1)) AS comments_count
FROM posts
LEFT JOIN comments USING (post_id)
GROUP BY posts.post_id
;
But if I were trying for faster, this might be better.
SELECT posts.*, IFNULL(subQ.comments_count, 0) AS comments_count
FROM posts
LEFT JOIN (
SELECT post_id, COUNT(1) AS comments_count
FROM comments
GROUP BY post_id
) As subQ
USING (post_id)
;

Answer 3

After a bit more research I found no time difference between the two queries
Benchmark.bm do |b|
b.report('joined') do
1000.times do
ActiveRecord::Base.connection.execute('
select
p.id,
(select count(c.id) from comments c where c.post_id = p.id) comment_count
from
posts l;')
end
end
b.report('nested') do
1000.times do
ActiveRecord::Base.connection.execute('
select
p.id,
count(c.id) comment_count
from
posts File.join(File.dirname(__FILE__), *%w[rel path here])
left join comments c on
c.post_id = p.id
group by
p.id;')
end
end
end
user system total real
nested 2.120000 0.900000 3.020000 ( 3.349015)
joined 2.110000 0.990000 3.100000 ( 3.402986)
However I did notice that when running an explain for both queries, more indexes are possible in the first query. Which makes me think it is a better option if the attributes needed in the select changed.