Is this possible in one fast mysql query? - mysql

Question

I have three tables and i need different data from all of them. Sadly i also need to be able to extract the latest row.
Here are my tables:
messages: I am just storing the content of the messages inside a table because one text could be sent to multiple users
+------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------+------+-----+---------+----------------+
| message_id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| content | varchar(255) | NO | | 0 | |
+------------+------------------+------+-----+---------+----------------+
conversations: This table just reflects a single conversation between two users.
+-----------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+------------------+------+-----+---------+----------------+
| partner_id | int(10) unsigned | NO | MUL | NULL | |
| conversation_id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| expedition_id | int(11) | NO | | NULL | |
| active | tinyint(4) | NO | | 1 | |
+-----------------+------------------+------+-----+---------+----------------+
conversation_messages: This table stores the information about the actual messages exchanged.
+-----------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+------------------+------+-----+---------+-------+
| message_id | int(11) unsigned | NO | PRI | NULL | |
| receiver_id | int(11) unsigned | NO | PRI | NULL | |
| conversation_id | int(11) unsigned | NO | MUL | NULL | |
| status | tinyint(4) | NO | | NULL | |
| timestamp | datetime | YES | | NULL | |
+-----------------+------------------+------+-----+---------+-------+
What i want to do now to select the latest message inside each conversation and get the content from this message aswell. (It sounds simple, but it did not find a simple solution). What i tried is the following:
SELECT max(c_m.message_id), m.content, c_m.`status`
FROM expedition_conversations e_c, conversation_messages c_m
INNER JOIN messages m ON m.message_id = c_m.message_id
WHERE e_c.expedition_id = 1 AND (c_m.conversation_id = e_c.conversation_id)
GROUP BY c_m.conversation_id;
Sadly since GROUP BY internally seems to selecting the first inserted row most of the time, the content i select from the messages table is wrong, while the message_id selected from conversation_messages is correct.
Any idea how to perform this in one query? If you have any suggestions to alter the table structure, i would also appreciate those.
Thanks in advance.

Answer 1

You may want to try this version:
SELECT c_m.message_id, m.content, c_m.`status`
FROM expedition_conversations e_c join
conversation_messages c_m
ON c_m.conversation_id = e_c.conversation_id INNER JOIN
messages m
ON m.message_id = c_m.message_id
WHERE e_c.expedition_id = 1 AND
NOT EXISTS (SELECT 1
FROM conversation_messages cm2
WHERE cm2.conversation_id = c_m.conversation_id AND
cm2.timestamp > c_m.timestamp
)
For performance, you want an index on conversation_messages(conversation_id, timestamp).

Answer 2

This is possible, because your usage of AUTO_INCREMENT means, that the highest id belongs to the latest message:
SELECT
messages.*,
FROM
conversations
INNER JOIN (
SELECT conversation_id, MAX(message_id) AS maxmsgid
FROM conversation_messages
GROUP BY conversation_id
) AS latest ON latest.conversation_id=conversations.id
INNER JOIN messages
ON messages.message_id=latest.maxmsgid
WHERE
1=1 -- whatever you want or need!
Since this query is bound to be quite slow, you might want to consider a few options:
Throw hardware at it: Use enough RAM and configure MySQL to go to disk for the interims table as late as possibel
Use denormalization and have a ON AFTER INSERT trigger on messages update a field on conversation_messages, that holds the latest message ID

Answer 3

Try this little trick:
SELECT c_m.message_id, m.content, c_m.status
FROM expedition_conversations e_c
JOIN (select * from (
select * from conversation_messages
order by message_id desc) x
group by conversation_id) c_m ON c_m.conversation_id = e_c.conversation_id
INNER JOIN messages m ON m.message_id = c_m.message_id
WHERE e_c.expedition_id = 1
This will work on your version if mysql - 5.6.19 - and should out-perform other approaches.