Split a mysql query into two to avoid a join -> sum ids and use where in

Question

What is faster:

Using a join to get userdetails for posts or only get the post data which includes the userid, collect the userIDs and after the posts are queried run one:

SELECT x,y,z FROM users WHERE id in (1,2,3,4,5,6,7...etc.)

Short:

What is better?:

SELECT x,y,z,userid 
FROM posts 
WHERE id > x 
ORDER BY id
LIMIT 20

SELECT x,y,z 
FROM users 
WHERE id IN (1,2,3,4,5,6,7...etc.)

or:

SELECT p.x,p.y,p.z, u.username,u.useretc,u.user.etc 
FROM posts p 
INNER JOIN users u 
ON u.id = p.userid 
AND id > n 
ORDER BY id
LIMIT 20

In some scenarios this could reduce the querying of the user table to 2 instead of 20 times. A page in a discussion where only two user posted.

Answer 1

anyway the second way is better:

1. You have only one call to database instead of two - so the channel between your DB and Application server is less loaded
2. Second way usually should be faster and less memory consuming because analyser can decide better how to manage its resources (it has all the requirements in one query)
3. In first example you force database to use not-cached queries (second query of the first example is not constant because in-list has different amount of inputs) so it parses the second query more often which leads to performance losses

Answer 2

If I'm not wrong... normally dealing with INNER JOIN is more readable and cleaner.

Answer 3

I would suggest for join query. Because of following reason :

• Cleaner and readable.
• Join will hit the DB only once. Which will be fast. Otherwise you will have to get the details in a data structure and again use the same details in another query.

Answer 4

Usual usage is JOIN over separate queries as it is more readable and easier to write.

But there frameworks like http://www.notorm.com/#performance which leverge first method using separate queries and have impressive results.