fix: improve question count query performance with vote

The query was really slow, as we needed a seqscan to know the exact
count of insights following the criteria, including the vote criterion.

The count could be really high
(ex: on https://hunger.openfoodfacts.org/questions we have 1.5M
insights).
By limiting to 100, we dramatically reduce query performance.

robotoff/app/api.py

@@ -1429,6 +1429,11 @@ def get_questions_resource_on_get(
         # Limit the number of brands to prevent slow SQL queries
         brands = brands[:10]
 
+    avoid_voted_on = _get_skip_voted_on(auth, device_id)
+    # Counting the number of insights that match the vote
+    # criteria can be very costly, so we limit the count to 100
+    # if avoid_voted_on is not None
+    max_count = 100 if avoid_voted_on is not None else None
     get_insights_ = functools.partial(
         get_insights,
         server_type=server_type,
@@ -1438,7 +1443,8 @@ def get_questions_resource_on_get(
         brands=brands,
         order_by=order_by,
         reserved_barcode=reserved_barcode,
-        avoid_voted_on=_get_skip_voted_on(auth, device_id),
+        avoid_voted_on=avoid_voted_on,
+        max_count=max_count,
         automatically_processable=False,
         campaigns=campaigns,
         predictor=predictor,

robotoff/app/core.py

@@ -79,6 +79,7 @@ def get_insights(
     limit: Optional[int] = 25,
     offset: Optional[int] = None,
     count: bool = False,
+    max_count: Optional[int] = None,
     avoid_voted_on: Optional[SkipVotedOn] = None,
     group_by_value_tag: Optional[bool] = False,
     automatically_processable: Optional[bool] = None,
@@ -116,6 +117,10 @@ def get_insights(
     :param offset: query offset (used for pagination), defaults to None
     :param count: if True, return the number of results instead of the
         results, defaults to False
+    :param count_max: an upper bound on the number of insights to count,
+        defaults to None. If provided, the count will be limited to this
+        value. It allows to dramatically speed up the count query.
+        If not provided, an exact count will be returned.
     :param avoid_voted_on: a SkipVotedOn used to remove results insights the
         user previously ignored, defaults to None
     :param group_by_value_tag: if True, group results by value_tag, defaults
@@ -181,6 +186,8 @@ def get_insights(
         query = query.where(*where_clauses)
 
     if count:
+        if max_count is not None:
+            query = query.limit(max_count)
         return query.count()
 
     if limit is not None: