Count retweets per user by linking two tables
===

I have the following tables:

    tweets               retweets
    -----------------    ----------------
    user_id  retweets    user_id (etc...)
    -----------------    ----------------
    1        0           1
    2        0           1
                         1
                         2
                         2

I want to count the number of retweets per user and update tweets.retweets accordingly:

    UPDATE users 
    SET retweets = (
      SELECT COUNT(*) FROM retweets WHERE retweets.user_id = users.user_id
    )

I have been running this query two times, but it times out (on tables that are not that large). Is my query wring?

Also see the SQL Fiddle (although it apparently doesn't allow `UPDATE` statements): http://www.sqlfiddle.com/#!2/f591e/1