Question

I have a fairly complicated operation that I’m trying to perform with just one SQL query but I’m not sure if this would be more or less optimal than breaking it up into n queries. Basically, I have a table called “Users” full of user ids and their associated fb_ids (id is the pk and fb_id can be null).

+-----------------+
| id | .. | fb_id |
|====|====|=======|
| 0  | .. | 12345 |
| 1  | .. | 31415 |
| .. | .. |  ..   |
+-----------------+

I also have another table called “Friends” that represents a friend relationship between two users. This uses their ids (not their fb_ids) and should be a two-way relationship.

+----------------+
| id | friend_id |
|====|===========|
| 0  |     1     |
| 1  |     0     |
| .. |    ..     |
+----------------+
// user 0 and user 1 are friends

So here’s the problem:
We are given a particular user’s id (“my_id”) and an array of that user’s Facebook friends (an array of fb_ids called fb_array). We want to update the Friends table so that it honors a Facebook friendship as a valid friendship among our users. It’s important to note that not all of their Facebook friends will have an account in our database, so those friends should be ignored. This query will be called every time the user logs in so it can update our data if they’ve added any new friends on Facebook. Here’s the query I wrote:

INSERT INTO Friends (id, friend_id)
SELECT "my_id", id FROM Users WHERE id IN
  (SELECT id FROM Users WHERE fb_id IN fb_array)
AND id NOT IN
  (SELECT friend_id FROM Friends WHERE id = "my_id")

The point of the first IN clause is to get the subset of all Users who are also your Facebook friends, and this is the main part I’m worried about. Because the fb_ids are given as an array, I have to parse all of the ids into one giant string separated by commas which makes up “fb_array.” I’m worried about the efficiency of having such a huge string for that IN clause (a user may have hundreds or thousands of friends on Facebook). Can you think of any better way to write a query like this?

It’s also worth noting that this query doesn’t maintain the dual nature of a friend relationship, but that’s not what I’m worried about (extending it for this would be trivial).

Answer 1

If I am not mistaken, your query can be simplified, if you have a UNIQUE constraint on the combination (id, friend_id), to:

INSERT IGNORE INTO Friends 
  (id, friend_id)
SELECT "my_id", id 
FROM Users 
WHERE fb_id IN fb_array ;

You should have index on User (fb_id, id) and test for efficiency. if the number of the itmes in the array is too big (more than a few thousands), you may have to split the array and run the query more than once. Profile with your data and settings.