mail: 'Send a message' generates excessively slow queries

[Guewen Baconnier @ Camptocamp]

Using the "Send a message" action on a model having a mail.thread, I notice a very long delay (8-10 seconds) before the action is done.

I used the debug_sql log level and could see that this slowness is due to the queries generated by the ORM.

In the `mail_message` module, in `mail_message._notify`, we can see this query:

            fol_ids = fol_obj.search(cr, SUPERUSER_ID, [
                ('res_model', '=', message.model),
                ('res_id', '=', message.res_id),
                ('subtype_ids', 'in', message.subtype_id.id)
                ], context=context)

Which results in:
2014-04-22 14:00:05 CEST LOG: duration: 1138.682 ms statement: SELECT "mail_followers_id" FROM "mail_followers_mail_message_subtype_rel" WHERE "mail_message_subtype_id" IN (1)
2014-04-22 14:00:21 CEST LOG: duration: 2544.561 ms statement: SELECT "mail_followers".id FROM "mail_followers" WHERE ((("mail_followers"."res_model" = 'crm.claim') AND ("mail_followers"."res_id" = 894)) AND ("mail_followers"."id" in (1,2,3,4,5, .....))) ORDER BY "mail_followers"."id"
where (1,2,3,4,5, .....) contains the IDs returned by the first statement (2.500.000 ids here).

Same result could be done with a query using JOINs:

SELECT mail_followers.id
FROM mail_followers
INNER JOIN mail_followers_mail_message_subtype_rel rel
ON rel.mail_followers_id = mail_followers.id
WHERE mail_followers.res_model = 'crm.claim'
AND mail_followers.res_id = 894
AND rel.mail_message_subtype_id = 1
ORDER BY mail_followers.id

Then, it takes merely a millisecond instead of several seconds.