When a user is subscribed to a channel with many messages,
the discuss initial rpc /mail/client_action can become very
slow (in production, the rpc took > 20s when subscribed to
the Community mailing list). This is due to the query in
_get_message_unread, counting the number of unread messages.
This commit adds an index on the relation mail_channel_partner,
this allows postgres to perform only index lookups, and speeds
up the query by 2 order of magnitudes.
Géry Debongnie
7922370f12