Question

I am using PHP , MySQL for the following
I have two tables

1. users
2. users_messages

The table “users” stores records of all users .

id  name  password
1   XYZ   XYZ

[Here id is actually the user_id]

The table “messages” contains messages posted by users

id  user_id messages
1   1       XYZ

The “users” table contains thousands of records (>5000)
The “users_messages” contains about 100 records (will keep on varying)

I am trying to get the no of messages of all the users and show them in a table .
Currently i am trying to paginate them so i need only 20 users at one page load.

I have my query as follows

SELECT u.username as username,u.id as id, COUNT( f.user_id ) AS no_of_messages 
FROM users AS u
LEFT JOIN users_messages AS f ON f.user_id = u.id
GROUP BY u.id LIMIT 20

This works , but it takes a long time … may be it scans the complete table just to get the no of messages of only 20 users .

The id column of table “users” does not start from 1 , the start is random , but it auto increments from the next record .

Answer 1

The id column of table “users” does not start from 1 , the start is
random

Try this instead:

SELECT 
  u.username as username, 
  u.id as id, 
  f.no_of_messages 
FROM users AS u
LEFT JOIN
(
   SELECT user_id, COUNT(*) no_of_messages 
   FROM users_messages
   GROUP BY user_id
) AS f ON f.user_id = u.id
ORDER BY u.id
LIMIT 20;