Count retweets per user by linking two tables - mysql

Question

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

Answer 1

This solution should be much faster than using subqueries for getting the count of tweets of each user (your correlated subquery will execute for each user):
UPDATE users a
LEFT JOIN
(
SELECT user_id, COUNT(1) AS retweet_count
FROM retweets
GROUP BY user_id
) b ON a.user_id = b.user_id
SET a.retweets = COALESCE(b.retweet_count, 0)

Answer 2

If your retweets table is not changing dynamically why not to gather data at first and then update destination table like this:
create table retweets_hist AS SELECT COUNT(*) AS retweets,user_id FROM retweets group by user_id;
then
UPDATE users
SET retweets = NVL(
SELECT retweets FROM retweets_hist WHERE retweets_hist.user_id = users.user_id
),0)
If it is dynamic, then I think using triggers is better.
The main issue here is when there is a user which has never retweeted ever counting it's retweets is time-consuming.
In answer to your question, Yes counting takes a fraction but counting something which never existed take time! this is the problem!
May this one would have better timing:
UPDATE users
SET retweets = NVL(
SELECT retweets
FROM retweets
WHERE retweets.user_id = users.user_id),0)
WHERE EXISTS(select *
FROM retweets
WHERE retweets.user_id = users.user_id)
But then again you have to update never retweets to Zero.
**Keyword EXISTS is in Oracle I don't know if mysql supports it