From what MySQL index would this query benefit? - mysql

Question

UPDATE: Added the query that runs second most:
(maybe needed when taking an index in consideration???)
SELECT m.time, m.message, m.receiver_uid AS receiver, m.sender_uid AS sender
FROM messages AS m, users AS u
WHERE u.uid = '$coID'
AND ( (m.receiver_uid = '$meID' AND m.sender_uid = '$coID') OR
(m.receiver_uid = '$coID' AND m.sender_uid = '$meID') )
ORDER BY m.time DESC
$meID is the iD of the user who runs the wuery,
$coID is the ID of the contact.
I've got a somewhat big query and it runs everytime an user visits my page.
SELECT m2.message, m2.time, m2.sender_uid AS sender, m2.receiver_uid AS receiver,
m.contact, u.ufirstname
FROM ( SELECT CASE
WHEN sender_uid = '$me' THEN receiver_uid
ELSE sender_uid
END AS contact,
MAX(time) AS maxtime
FROM messages
WHERE sender_uid = '$me' OR receiver_uid = '$me'
GROUP BY CASE
WHEN sender_uid = '$me' THEN receiver_uid
ELSE sender_uid
END ) AS m
INNER JOIN messages m2 ON m.maxtime = m2.time
AND ((m2.sender_uid = '$me' AND m2.receiver_uid = m.Contact)
OR (m2.receiver_uid = '$me' AND m2.sender_uid = m.Contact))
INNER JOIN users AS u ON m.contact = u.uid
ORDER BY time DESC
$me is the ID of the user who runs the query
This query will (successfully) retrieve:
LAST MESSAGE from EVERY 'CONVERSATION' ordered by TIME.
So it will get the last message (whether the message is send or received) in every PM session
And than sort those by time, and retrieves the contacts information.
Please tell me if I didn't explain it correctly.
My MySQL table looks like this:
receiver_id | sender_id | message | time
From what index(es) would this query benefit?
(The user table already has an primary key on the ID so the part where the join retrieves the contacts name should be efficient)
EXPLAIN OUTPUTs:
The BIG query:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 4 Using temporary; Using filesort
1 PRIMARY m2 ALL NULL NULL NULL NULL 42 Using where
1 PRIMARY u eq_ref PRIMARY PRIMARY 4 m.contact 1 Using where
2 DERIVED messages ALL NULL NULL NULL NULL 42 Using where; Using temporary; Using filesort
The query in the update part:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE u const PRIMARY PRIMARY 4 const 1 Using index; Using filesort
1 SIMPLE m ALL NULL NULL NULL NULL 42 Using where

Answer 1

As your messages table grows, this query will start becoming slower and slower. Depending on the number of conversations that the user is a part of, you will start seeing exponentially decaying performance. While an individual index on messages.time, messages.sender_uid and messages.receiver_uid will help for now, no index will help you in your long run, unless you trim your messages table. Especially when you have more than a few hundred thousand messages.
I would suggest maintaining an association type type that links a user to a conversation and their last message id. Something looking like:
user_id | conversation_id | message_id
You then look up this table, instead of performing a complicated and expensive query. This greatly reduces the number of scans that you need to do on your messages table. While, it does slightly increase the complexity, the performance will not degrade as much as your query above.

Answer 2

I found by trail and error that indexing the time decreases the load time.
So that will probably be the answer to my question.