Slightly different greatest-n-per-group - sql

Question

I have read this comment which explains the greatest-n-per-group problem and its solution. Unfortunately, I am facing a slightly different approach, and I am failing to find a solution for it.
Let's suppose I have a table with some basic info regarding users. Due to implementation, this info may or may not repeat itself:
+----+-------------------+----------------+---------------+
| id | user_name | user_name_hash | address |
+----+-------------------+----------------+---------------+
| 1 | peter_jhones | 0xFF321345 | Some Av |
| 2 | sally_whiterspoon | 0x98AB5454 | Certain St |
| 3 | mark_jackobson | 0x0102AB32 | Some Av |
| 4 | mark_jackobson | 0x0102AB32 | Particular St |
+----+-------------------+----------------+---------------+
As you can see, mark_jackobson appears twice, although its address is different in each appearance.
Every now and then, an ETL process queries new user_names and fetches the most recent records of each. Aftewards, it stores the user_name_hash in a table to sign it has already imported that certain user_name
+----------------+
| user_name_hash |
+----------------+
| 0xFF321345 |
| 0x98AB5454 |
+----------------+
Everything begins with the following query:
SELECT DISTINCT user_name_hash
FROM my_table
EXCEPT
SELECT user_name_hash
FROM my_hash_table
This way, I am able to select the new hashes from my table. Since I need to query the most recent occurrence of a hash, I wrap it as a sub-query:
SELECT MAX(id)
FROM my_table
WHERE user_name_hash IN (
SELECT DISTINCT user_name_hash
FROM my_table
EXCEPT
SELECT user_name_hash
FROM my_hash_table)
GROUP BY user_name_hash
Perfect! With the ids of my new users, I can query the addresses as follows:
SELECT
address,
user_name_hash
FROM my_table
WHERE Id IN (
SELECT MAX(id)
FROM my_table
WHERE user_name_hash IN (
SELECT DISTINCT user_name_hash
FROM my_table
EXCEPT
SELECT user_name_hash
FROM my_hash_table)
GROUP BY user_name_hash)
From my perspective, the above query works, but it does not seem optimal. Reading this comment, I noticed I could query the same data, using joins. Since I am failing to write the desired query, could anyone help me out and point me to a direction?
This is the query I have attempted, without success.
SELECT
tb1.address,
tb1.user_name_hash
FROM my_table tb1
INNER JOIN my_table tb2
ON tb1.user_name_hash = tb2.user_name_hash
LEFT JOIN my_hash_table ht
ON tb1.user_name_hash = ht.user_name_hash AND tb1.id > tb2.id
WHERE ht.user_name_hash IS NULL;
Thanks in advance.
EDIT > I am working with PostgreSQL

Answer 1

I believe you are looking for something like this:
SELECT
address,
user_name_hash
FROM my_table t1
JOIN (
SELECT MAX(id) maxid
FROM my_table t2
WHERE NOT EXISTS (
SELECT 1
FROM my_hash_table t3
WHERE t2.user_name_hash = t3.user_name_hash
)
GROUP BY user_name_hash
) t ON t1.ID = t.maxid
I'm using NOT EXISTS instead of EXCEPT since it is more clear to the optimizer.

Answer 2

You can get a better performance using a left outer join (to get the newest records not already imported) and then compute the max id for these records (subquery in the HAVING clause).
SELECT t1.address,
t1.user_name_hash,
MAX(id) AS maxid
FROM my_table t1
LEFT JOIN my_hash_table th ON t1.user_name_hash = th.user_name_hash
WHERE th.user_name_hash IS NULL
GROUP BY t1.address,
t1.user_name_hash
HAVING MAX(id) = (SELECT MAX(id)
FROM my_table t1)