[IMP] website_event_exhibitor: speed up sponsors retrieval

The filters for sponsor countries works by adding a domain term on
`partner_id.country_id`. Currently this is evaluated by the ORM by first
searching for all partners in those countries, and then substituting the
IDs in the final query (`partner_id in <ids>`).

On large databases with many partners, this makes the filtering really
slow (e.g. 3 seconds with 1M partners), even though only a few sponsors
actually match.

From a security standpoint, most of the data on the linked partner is
already available on the sponsor through related fields, and those
partners are only visible when they are already published anyway.
Besides, the calls in `_event_exhibitors_get_values` are performed
in `sudo` mode.

It should be noted that `res.partner.country_id` is not currently
indexed, and doing so helps marginally: the query goes from 3000ms to
800ms. It is still much slower than with `auto_join`, which makes it
go down to about 1-2 ms on the same dataset.

Indexing `country_id` does not help make the `auto_join` version faster,
as it only requires the primary key index to find the records.

closes odoo/odoo#66692

X-original-commit: b5246315875dfad151fcb23936499750e0f1f42a
Signed-off-by: Olivier Dony (odo) <odo@openerp.com>

addons/website_event_exhibitor/models/event_sponsor.py

@@ -40,7 +40,7 @@ class Sponsor(models.Model):
         sanitize_attributes=False, sanitize_form=True, translate=html_translate,
         readonly=False, store=True)
     # contact information
-    partner_id = fields.Many2one('res.partner', 'Partner', required=True)
+    partner_id = fields.Many2one('res.partner', 'Partner', required=True, auto_join=True)
     partner_name = fields.Char('Name', related='partner_id.name')
     partner_email = fields.Char('Email', related='partner_id.email')
     partner_phone = fields.Char('Phone', related='partner_id.phone')