How to efficiently calculate averages from a big table? - php

Question

I have a table called ratings with the following fields:
+-----------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------+------+-----+---------+----------------+
| rating_id | bigint(20) | NO | PRI | NULL | auto_increment |
| user_id | int(11) | NO | MUL | NULL | |
| movie_id | int(11) | NO | | NULL | |
| rating | float | NO | | NULL | |
+-----------+------------+------+-----+---------+----------------+
Indexes on this table:
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| ratings | 0 | PRIMARY | 1 | rating_id | A | 100076 | NULL | NULL | | BTREE | | |
| ratings | 0 | user_id | 1 | user_id | A | 564 | NULL | NULL | | BTREE | | |
| ratings | 0 | user_id | 2 | movie_id | A | 100092 | NULL | NULL | | BTREE | | |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
I have another table called movie_average_ratings which has the following fields:
+----------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------+------+-----+---------+-------+
| movie_id | int(11) | NO | PRI | NULL | |
| average_rating | float | NO | | NULL | |
+----------------+---------+------+-----+---------+-------+
As it is obvious by this point I want to calculate the average rating of movies from ratings table and update the movie_average_ratingstable. I tried the following SQL query.
UPDATE movie_average_ratings
SET average_rating = (SELECT AVG(rating)
FROM ratings
WHERE ratings.movie_id = movie_average_ratings.movie_id);
Currently, there are around 10,000 movie records and 100,000 rating records and I get Lock wait timeout exceeded; try restarting transaction error. The number of records can grow significantly so I don't think increase timeout is a good solution.
So, how can I write 'scalable' query to acheive this? Is iterating the movie_average_ratings table records and calculate averages individually the most efficient solution to this?

Answer 1

Without an explain, it's hard to be clear on what's holding you up. It's also not clear that you will get a performance improvement by storing this aggregated data as a denormalized table - if the query to calculate the ratings executes in 0.04 seconds, it's unlikely querying your denormalized table will be much faster.
In general, I recommend only denormalizing if you know you have a performance problem.
But that's not the question.
I would do the following:
delete from movie_average_ratings;
insert into movie_average_ratings
Select movie_ID, avg(rating)
from ratings
group by movie_id;

Answer 2

I just found something in another post:
What is happening is, some other thread is holding a record lock on
some record (you're updating every record in the table!) for too long,
and your thread is being timed out.
This means that some of your records are locked you can force unlock them in the console:
1) Enter MySQL mysql -u your_user -p
2) Let's see the list of locked tables mysql> show open tables where in_use>0;
3) Let's see the list of the current processes, one of them is locking
your table(s) mysql> show processlist;
4) Kill one of these processes mysql> kill put_process_id_here;

Answer 3

You could redesign the movie_average_ratings table to
movie_id (int)
sum_of_ratings (int)
num_of_ratings (int)
Then, if a new rating is added you can add it to movie_average_ratings and calculate the average if needed