计算帖子的平均评分

Question

I have the following entity:

post_id | post_title | post_body
-------------------------------------
1       | First post | Hello, world!

and would like to register ratings on the post, like so:

rating_id | rating_user_id | rating_post_id | rating_rating
1         | 23             | 1              | 3.5

My main concern/question is, how I should retrieve the average rating for a post. I know I can do this by using an SQL query like so:

SELECT AVG(rating_rating) FROM post_ratings WHERE rating_post_id = 2;

but I would have to call this query everytime I'd do $post->getAverageRating() - is this a big performance issue? (keeping in mind that there could be thousands of ratings per post)

An alternative would be to store the average rating in a new column in the posts table and have a cron job calculate the average or something?

What is the best way of going about this?

Answer 1

As for comparing performance- It depends on

1. How static the ratings are.If you expect new ratings for existing posts, I would go with calculating them on the fly anyway.
2. How many posts you would need the ratings for as opposed to overall posts. If you have millions of posts and need the average rating only for a few thousands, it makes sense to do them separately also.

Answer 2

This is an issue of efficiency

If and which these ratings will stack to a large amount, it would be preferable to save the calculated average I would add 2 columns to the posts table, average_rating, total_ratings

ALTER TABLE posts ADD COLUMN average_rating DECIMAL (9,2);
ALTER TABLE posts ADD COLUMN total_ratings UNSIGNED (9) DEFAULT 0;

And populate them whenever a new rating is added, so you won't need a cron job Notice the order of the queries to calculate the average

The MySQL + PHP code should be something like:

    $query = "UPDATE posts SET total_ratings  = total_ratings + " . $rating_rating . " WHERE post_id = " . $post_id;
    $db->query($query);
    $query = "UPDATE posts SET average_rating = average_rating * (total_ratings -1 / total_ratings) + " . $rating_rating ." / total_ratings WHERE post_id = " . $post_id;
$db->query($query);

This way you can update the posts table immediately without any cron job

Answer 3

Database calls are expensive especially when you have to average every time.

You have the following options that are more optimal compared to your current solution.

1. Have a column that calculates and stores the average rating of a post when the rating has been made like mentioned by the @Guy L

2. Implementing a Cache with TTL(Time To Live)

example:

public function getAverageRating() {
     $post_id = 'post' . $this->id;
     if(Cache::has($post_id))
         return Cache::get($post_id);
     $rating = //fetch rating from DB
     Cache::set($post_id, $rating, 120); //ratings will be in the cahce for 120secs
     return $rating
}

Hope it helps.