Latest entry for append-only log in postgresql - sql

Question

I want to store users' settings in a postgresql database.
I would like to keep full history of their settings, and also be able to query the latest settings for a given user.
I have tried storing settings in a table like this:
CREATE TABLE customer (
customer_id INTEGER PRIMARY KEY,
name VARCHAR NOT NULL
);
CREATE TABLE customer_settings (
customer_id INTEGER REFERENCES customer NOT NULL,
sequence INTEGER NOT NULL, -- start at 1 and increase, set by the application
settings JSONB NOT NULL,
PRIMARY KEY(customer_id, sequence)
);
So customer_settings is an append-only log, per customer.
Then to query latest settings I use a long query that will do a subquery to SELECT the max sequence for the given customer_id, then will select the settings for that id.
This is awkward! I wonder if there is a better way? May I use a view or a trigger to make a second table latest_customer_settings??

Answer 1

You can make a view. To get the settings for multiple customers in Postgres, I would recommend:
select distinct on (customer_id)
from customer_settings cs
order by customer_id, sequence desc;
And for this query, I would recommend an index on customer_settings(customer_id, sequence desc).
In addition, you can have Postgres generate the sequence -- if you can deal with one overall sequence number for all customers.
CREATE TABLE customer_settings (
customer_settings_id bigserial primary key,
customer_id INTEGER REFERENCES customer NOT NULL,
settings JSONB NOT NULL
);
Then, the application does not need to set a sequence number. You can just insert customer_id and settings into the table.
Having the application maintain this information has some short-comings. First, the application has to read from the database before it can insert anything into the table. Second, you can have race conditions if multiple threads are updating the table at the same time (in this case for the same customer).

Answer 2

you can use row_number() window function , It will help to you to get each customers latest settings
with cte as (select cs.*,
row_number() over(partition by c.customer_id order by sequence desc) rn
from customer c join customer_settings cs on c.customerid=cs.customerid
) select * from cte where rn=1

Answer 3

Assuming you just want the single latest log for a given user, and also assuming that the sequence is always increasing and unique, then actually you only need a simple query:
SELECT *
FROM customer_settings
WHERE customer_id = 123
ORDER BY sequence DESC
LIMIT 1;
If you want to invest some time into creating a better logging framework, then try looking into things like MDC (Mapped Diagnostic Context, see here). With MDC, each log statement is written out with a completely unique identifier, which also gets sent in the response header or body. Then, it becomes easy and foolproof to correlate an exception between backend and frontend or consumer.