Mysql php count number of inner join work faster with subqueries - php

Question

Two tables, first one is users and 2nd is posts, table posts structure is id,body,parent_id,user_id in this table all posts are inserted with parent_id is null, and if its a comment the parent_id is set to post id.
What I'm trying to do is join users table -to get user details- and get count of comments on each post.
I tried a couple of queries
select p.id,
users.id as 'from_id',
users.fullname as 'from_fullname',
users.role as 'from_role',
users.picture as 'from_picture',
p.body,
p.time_posted as 'time_posted',
p.attachment,
p.parent_id,
count(c.id) as counts
from
wall p
join
users on users.id = p.user_id
left join
wall c on c.parent_id = p.id
where
p.class_id = 8 and p.parent_id is null
group by
p.id
order by
`counts` ---->EXPLAIN RESULTS
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE p ref PRIMARY,parent_id,class_id,user_id parent_id 5 const 49920 Using index condition; Using where; Using temporary; Using filesort
1 SIMPLE users eq_ref PRIMARY PRIMARY 4 ischool.p.user_id 1 NULL
1 SIMPLE c ref parent_id parent_id 5 ischool.p.id 49920 Using index
This one takes average of around 2.7 sec to complete.
While my 2nd attempt
select p.id,
users.id as 'from_id',
users.fullname as 'from_fullname',
users.role as 'from_role',
users.picture as 'from_picture',
p.body,
(select count(*) from wall where parent_id= p.id ) as comments_count,
p.time_posted as 'time_posted',
p.attachment,
p.parent_id
from
wall p
left join
users on users.id = p.user_id
where
p.class_id = 8 and p.parent_id is NULL
order by
p.id DESC; --->Explain results
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY p ref parent_id,class_id parent_id 5 const 49920 Using where
1 PRIMARY users eq_ref PRIMARY PRIMARY 4 ischool.p.user_id 1 NULL
2 DEPENDENT SUBQUERY wall ref parent_id parent_id 5 ischool.p.id 49920 Using index
This query takes 1.4 sec to complete
given that im using MYSQL innodb with index on each and every id column.
so is there a better method to fetch posts and comment counts ?
why is subquery working nearly 2x faster than join ?

Answer 1

Basically Joins are more faster than sub query. Generally, new developer prefers to use sub-query.
The execution plan of query may impact on performance. Almost all cases queries joins are more faster than sub-query.
The basic difference is that sub-query execute query and load all data and based on those data execute another query.
and joins are all tables all loads with data so it performs fast execution of query.
In you query, if you want to show why you sub-query is faster than join use "explain " command to show how query is actually executed.
Following links may helpful to understand concepts:
http://www.chrislondon.co/joins-vs-subqueries/
http://www.sitepoint.com/using-explain-to-write-better-mysql-queries/