Is there a simpler way to achieve this style of user messaging?

Question

I have created a messaging system for users, it allows them to send a message to another user. If it is the first time they have spoken then a new conversation is initiated,

Answer 1

If you can only have one conversation between users, I don't see a reason for a dedicated conversations table. For this query to work fast, you would need a composite index on (user, message_id) which is impossible if these fields are in different tables. Move user_id and friend_id to the userconversations. This will make your table 8 bytes per record heavier (even assuming 8-byte identifiers) which is hardly a problem for a table containing text messages.

If you have few conversations per user with many messages in each, use this:

SELECT  um.*
FROM    (
        (
        SELECT  MAX(id) AS messageId
        FROM    usermessages m1
        WHERE   user_id = :me
        GROUP BY
                friend_id
        ORDER BY
                messageId DESC
        LIMIT 10
        )
        UNION ALL
        (
        SELECT  MAX(id) AS messageId
        FROM    usermessages m1
        WHERE   frient_id = :me
        GROUP BY
                user_id
        ORDER BY
                messageId DESC
        LIMIT 10
        )
        ) q
JOIN    usermessages um
ON      um.id = q.messageId
ORDER BY
        id DESC
LIMIT 10

Create separate indexes on user_id and friend_id

If you have many conversations with few messages in each, use this query:

(
SELECT  *
FROM    usermessages um
WHERE   user_id = :me
        AND id = 
        (
        SELECT  MAX(id)
        FROM    usermessages umi
        WHERE   umi.user_id = um.user_id
                AND umi.friend_id = um.friend_id
        )
ORDER BY
        id DESC
LIMIT 10
)
UNION ALL
(
SELECT  *
FROM    usermessages um
WHERE   frient_id = :me
        AND id = 
        (
        SELECT  MAX(id)
        FROM    usermessages umi
        WHERE   umi.user_id = um.user_id
                AND umi.friend_id = um.friend_id
        )
ORDER BY
        id DESC
LIMIT 10
)
ORDER BY
        id DESC
LIMIT 10

The idea behind this query is that it just descends all messages for the given user, checking that each message is the last in its conversation. This may be much faster than sorting all last messages for all conversations (if you have many of them).

For this to work fast, create indexes on

friend_id
user_id, friend_id