PostgreSQL: Get last updates by joining 2 tables - sql

Question

I have 2 tables that I need to join to get the last/latest update in the 2nd table based on valid rows in the 1st table.
Code below is en example.
Table 1: Registered users
This table contains a list of users registered in the system.
When a user gets registered it gets added into this table. A user is registered with a name, and a registration time.
A user can get de-registered from the system. When this is done, the de-registration column gets updated to the time that the user was removed. If this value is NULL, it means that the user is still registered.
CREATE TABLE users (
entry_idx SERIAL PRIMARY KEY,
name TEXT NOT NULL,
reg_time TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
dereg_time TIMESTAMP WITH TIME ZONE DEFAULT NULL
);
Table 2: User updates
This table contains updates on the users. Each time a user changes a property (example position) the change gets stored in this table. No updates must be removed since there is a requirement to keep history in the table.
CREATE TABLE user_updates (
entry_idx SERIAL PRIMARY KEY,
name TEXT NOT NULL,
position INTEGER NOT NULL,
time TIMESTAMP WITH TIME ZONE DEFAULT NOW()
);
Required output
So given the above information, I need to get a new table that contains only the last update for the current registered users.
Test Data
The following data can be used as test data for the above tables:
-- Register 3 users
INSERT INTO users(name) VALUES ('Person1');
INSERT INTO users(name) VALUES ('Person2');
INSERT INTO users(name) VALUES ('Person3');
-- Add some updates for all users
INSERT INTO user_updates(name, position) VALUES ('Person1', 0);
INSERT INTO user_updates(name, position) VALUES ('Person1', 1);
INSERT INTO user_updates(name, position) VALUES ('Person1', 2);
INSERT INTO user_updates(name, position) VALUES ('Person2', 1);
INSERT INTO user_updates(name, position) VALUES ('Person3', 1);
-- Unregister the 2nd user
UPDATE users SET dereg_time = NOW() WHERE name = 'Person2';
From the above, I want the last updates for Person 1 and Person 3.
Failed attempt
I have tried using joins and other methods but the results are not what I am looking for. The question is almost the same as one asked here. I have used the solution in answer 1 and it does give the correct answer, but it takes too long to get too the answer in my system.
Based on the above link I have created the following query that 'works':
SELECT
t1.*
, t2.*
FROM
users t1
JOIN (
SELECT
t.*,
row_number()
OVER (
PARTITION BY
t.name
ORDER BY t.entry_idx DESC
) rn
FROM user_updates t
) t2
ON
t1.name = t2.name
AND
t2.rn = 1
WHERE
t1.dereg_time IS NULL;
Problem
The problem with the above query is that it takes very long to complete. Table 1 contains a small list of users, while table 2 contains a huge amount of updates. I think that the query might be inefficient in the way that it handles the 2 tables (based on my limited understanding of the query). From pgAdmin's explain it does a lot of sorting and aggregation on the updates 1st before joining with the registered table.
Question
How can I formulate a query to efficiently and fast get the latest updates for registered users?

Answer 1

PostgreSQL have a special distinct on syntax for such type of queries:
select distinct on(t1.name)
--it's better to specify columns explicitly, * just for example
t1.*, t2.*
from users as t1
left outer join user_updates as t2 on t2.name = t1.name
where t1.dereg_time is null
order by t1.name, t2.entry_idx desc
sql fiddle demo
you can try it, but for me your query should work fine too.

Answer 2

I am using q1 to get the last update of each user. Then joining with users to remove entries that have been deregistered. Then joining with q2 to get rest of user_update fields.
select users.*,q2.* from users
join
(select name,max(time) t from user_updates group by name) q1
on users.name=q1.name
join user_updates q2 on q1.t=q2.time and q1.name=q2.name
where
users.dereg_time is null
(I haven't tested it. have edited some things)