1

I want to create simple conversation module on my site. So I need a list of users who had conversation with current user. I need to show amount of new messages in each conversation. I think about next tables

User
id
...

Conversation
id
user_id1
user_id2
cnt_new_msg
last_msg_id

Message
id
conversation_id
sender_id
type
is_read
created_at

For selecting data for list of conversation I can use next query:

SELECT *
FROM conversation c
LEFT JOIN message m ON m.id = c.last_msg_id l.last_ms
WHERE user_id1=[current_user_id] OR user_id2=[current_user_id]

When user send new message I will increment Conversation.cnt_new_msg and when user open conversation I will set Message.is_read to 1 and recalculate Conversation.cnt_new_msg again. Also to sync this value I can create cronjob.

But this approach has problems. I don't like user_id1=[current_user_id] OR user_id2=[current_user_id] for each query. My tables will be very denormalized but I think it will be faster.

Please advise best practices to design conversations tables.

2

Don't store functionally dependent data. If you should need that for performance, you can always add a materialize view later (see below). But it's often better (and sometimes even faster) to calculate derived values in queries on the fly.

In particular, you would create many dead rows, table bloat, more work for VACUUM and nasty concurrency issues in conversation if you UPDATE that row with every new message and with every status change.

I suggest a relational model like this:

CREATE TABLE users (user_id serial PRIMARY KEY, ...)

CREATE TABLE conversation (
  conversation_ id serial PRIMARY KEY
, user_id1         int NOT NULL REFERENCES users
, user_id2         int NOT NULL REFERENCES users
-- cnt_new_msg   -- redundant
-- last_msg_id
, created_at       timestamptz NOT NULL DEFAULT now() 
);

CREATE TABLE message (
message_id      serial PRIMARY KEY
conversation_id int  NOT NULL REFERENCES conversation
sender          "char" NOT NULL CHECK (sender IN ('1', '2')
message         text NOT NULL
type            ???
unread          boolean NOT NULL DEFAULT TRUE
created_at      timestamptz NOT NULL DEFAULT now() 
);

Details for data types in the manual.
You can create a VIEW or MATERIALIZED VIEW to present conversation data like you had in mind:

CREATE VIEW conversation_plus_ct AS
SELECT c.*
     , count(m.unread OR NULL) AS cnt_new_msg
     , max(m.created_at) AS last_msg_id
FROM   conversation c
JOIN   message m USING (conversation_id);

And your query could look like this (not using the VIEW, but it could ..)

SELECT *
     , count(m.unread OR NULL) AS cnt_new_msg
     , max(m.created_at) AS last_msg_id
FROM   conversation c
LEFT   JOIN message m USING (conversation_id)
WHERE  [current_user_id] IN (user_id1, user_id2);
  • Thank you for reply. Why message.sender is "char"? Is it bad to set reference to user? – Inokentiy Jul 30 '15 at 15:24
  • @Inokentiy: I chose "char" (with double-quotes!) because it only requires 1 byte internally (like a boolean column). It's enough to store the information whether user_id1 or user_id2 is the author. You could store the complete user_id (int, 4 bytes) instead, but how do you enforce that no unrelated user_id (not member of this conversation) can be entered? – Erwin Brandstetter Jul 30 '15 at 16:46

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