How ORDER BY works in MySQL? - mysql

Question

assume there is a table with 100 million records in it with this schema:
+---------+---------+------+
| user_id | post_id | date |
+---------+---------+------+
We want to have latest 30 records which user with #1 is posted.
SELECT * FROM users_posts WHERE user_id = 1 ORDER BY date DESC LIMIT 30
How MySQL search in table ? Does it search all rows ( full scan ) finding all matches THEN ordering them by date?
if Yes( Full Scan ), how can we edit this query to limit it to search latest records till finds 30 matches. ( to increase reading speed and have better performance )
thanks in advanced.

Answer 1

If you want the best performance for this query:
SELECT *
FROM users_posts
WHERE user_id = 1
ORDER BY date DESC;
Then you want an index on users_posts(user_id, date). This will allow the SQL engine to only use the index to define the set of rows being used. First, it will use user_id to satisfy the where clause. Then it will use the date part for the order by. It will still have to look up the records in the original data pages, in order to get all the columns needed for the select.
If you want to limit this to 30 rows, then add limit 30:
SELECT *
FROM users_posts
WHERE user_id = 1
ORDER BY date DESC
LIMIT 30;
With the above index, this will improve performance. With no index, the engine will have to scan all the data, sort the data that matches the where clause, and then apply the limit -- in other words, it won't be much of an improvement in performance.