diff --git a/addons/survey/models/survey_user_input.py b/addons/survey/models/survey_user_input.py
index 50f13936c6b..0971bc3186b 100644
--- a/addons/survey/models/survey_user_input.py
+++ b/addons/survey/models/survey_user_input.py
@@ -614,7 +614,7 @@ class SurveyUserInputLine(models.Model):
     _order = 'question_sequence, id'
 
     # survey data
-    user_input_id = fields.Many2one('survey.user_input', string='User Input', ondelete='cascade', required=True)
+    user_input_id = fields.Many2one('survey.user_input', string='User Input', ondelete='cascade', required=True, index=True)
     survey_id = fields.Many2one(related='user_input_id.survey_id', string='Survey', store=True, readonly=False)
     question_id = fields.Many2one('survey.question', string='Question', ondelete='cascade', required=True)
     page_id = fields.Many2one(related='question_id.page_id', string="Section", readonly=False)
diff --git a/addons/website_slides_survey/models/survey_user.py b/addons/website_slides_survey/models/survey_user.py
index 5d23035dae0..6561f04739b 100644
--- a/addons/website_slides_survey/models/survey_user.py
+++ b/addons/website_slides_survey/models/survey_user.py
@@ -11,7 +11,8 @@ class SurveyUserInput(models.Model):
     slide_id = fields.Many2one('slide.slide', 'Related course slide',
         help="The related course slide when there is no membership information")
     slide_partner_id = fields.Many2one('slide.slide.partner', 'Subscriber information',
-        help="Slide membership information for the logged in user")
+        help="Slide membership information for the logged in user",
+        index='btree_not_null') # index useful for deletions in comodel
 
     @api.model_create_multi
     def create(self, vals_list):