Fetch multi-participant conversations with last message for each - sql

Question

I am trying to create a simple chat application database schema, and query the conversations. My current table setup is the following:
CREATE TABLE chat_user (
id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
display_name VARCHAR(140),
... other user stuff ...
);
CREATE TABLE conversation (
id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
title VARCHAR(140),
created timestamp with time zone NOT NULL
);
CREATE TABLE conversation_message (
id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
conversation_id bigint NOT NULL,
sender_id bigint NOT NULL,
body TEXT NOT NULL,
created timestamp with time zone NOT NULL
);
CREATE TABLE conversation_participant (
id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
conversation_id bigint NOT NULL,
user_id bigint NOT NULL
);
So basically each conversation has its own title, and multiple participants. What I would like to fetch my conversations sorted by the date of the latest message in the conversation (so the conversations with the newest messages are shown first). The result set should contain the id, title of the conversation and list of participants + the id, sender_id and body of the latest message.
It would also be required to fetch the conversations paginated based on the creation date of the conversation (20 per page)
Is my table setup efficient enough to satisfy the above constraint? Seems to me that this could result in a rather large query with multiple subqueries?.

Answer 1

This answers the original version of the question.
You seem to want a join and agregation:
select cm.conversation_id, max(created)
from conversation_message cm join
conversation_participant cp
on cm.conversation_id = cp.conversation_id
where cp.user_id = ?
group by cm.conversation_id
order by max(created) desc;

Answer 2

You can try using lateral join.
So your query would look something like this.
You can fetch all of your needed data, apply limits and offsets, and retrieve the last message for each conversation. Hope it would help.
select * from conversation c
left join lateral (
select * from conversation_message cm
where cm.conversation_id=c.id
order by created desc
limit 1
) cm on true
left join conversation_participant cp on cp.id = cm.sender_id;
Left joins here are for the chat rooms without any messages.

Answer 3

In short: I think you have a reasonable design for a normalized (3NF) OLTP database. That's what you should aim for, not the number of JOINs for a specific use case. The design you have will satisfy the use case you defined and many other use cases, which I'm sure involved in this application of yours.
In details:
You're designing for an OLTP system, where the data is kept normalized to ensure the data consistency and improve the efficiency of OLTP transactions.
This however means you will have to do much more JOINs than a de-normalized database( which is suitable more for OLAP, reporting, analytics systems). This is just the nature of OLTP relational databases.
Trying to reduce the number of JOINs in a normalized database (i.e 3NF - Third normal form) means you will be combining data from different granularity into the same table and cause duplication, thus making updates harder and slower, and eventually data inconsistency.
So, you really shouldn't design aiming to reduce the number of JOINs. Instead make sure, you have a normalized design and avoid over-normalizing. In cases, where you may want to avoid writing long queries, you can add VIEWS and use view to write queries to simplify your queries (but that can cause sub-optimal query performance sometimes by bringing unnecessary joins).

Answer 4

To get latest message for your conversations there are ways to achieve it like self joins or window functions (row_number(), rank() etc). Using window function you can write your query as
with cm as (
select *,
rank() over (partition by conversation_id order by created desc) as r
from conversation_message
)
select c.id,
c.title,
cm.body,
cm.created,
cm.r,
cu.display_name
from conversation as c
left join cm on c.id = cm.conversation_id and cm.r <= 1
left join chat_user cu on cu.id = cm.sender_id
DEMO
In above query I have used left joins to include conversions with no messages, If you need only conversations which has messages then use inner joins. If you need more than 1 latest messages for each conversation change cm.r <= #no
To get participants list for each conversation you can add new CTE like
with cm as (
select *,
rank() over (partition by conversation_id order by created desc) as r
from conversation_message
),
message_participants as (
select
m.conversation_id,
array_agg(u.display_name order by m.created desc) as participants
from chat_user as u
join conversation_message as m on u.id = m.sender_id
group by m.conversation_id
)
select c.id,
c.title,
cm.body,
cm.created,
cm.r,
cu.display_name,
cmp.participants
from conversation c
left join cm on c.id = cm.conversation_id and cm.r <= 1
left join chat_user cu on cu.id = cm.sender_id
left join message_participants cmp on c.id = cmp.conversation_id
DEMO
Improvements
Add user_id in conversation table to identify who has created
this conversation.
Table conversation_participant is redundant while you can extract
list of participants from conversation_message