[IMP] mass_mailing: add missing indexes

These two columns are used to compute the mailing statistics, to be
displayed in the kanban dashboards. Adding them makes the dashboards
a lot faster on large databases (e.g. a few milliseconds instead of
several seconds)

addons/mass_mailing/models/mass_mailing_stats.py

@@ -27,11 +27,11 @@ class MailMailStats(models.Model):
     model = fields.Char(string='Document model')
     res_id = fields.Integer(string='Document ID')
     # campaign / wave data
-    mass_mailing_id = fields.Many2one('mail.mass_mailing', string='Mass Mailing')
+    mass_mailing_id = fields.Many2one('mail.mass_mailing', string='Mass Mailing', index=True)
     mass_mailing_campaign_id = fields.Many2one(
         related='mass_mailing_id.mass_mailing_campaign_id',
         string='Mass Mailing Campaign',
-        store=True, readonly=True)
+        store=True, readonly=True, index=True)
     # Bounce and tracking
     scheduled = fields.Datetime(help='Date when the email has been created', default=fields.Datetime.now)
     sent = fields.Datetime(help='Date when the email has been sent')