⚡️(backend) improve DocumentViewset.get_queryset

The filtering made in the DocumentViewset.get_queryset method is not
optimal and lead to a full scan of the Document table. The heavy part is
on the filtering on what the user can access between the accesses and
the link traces. To have better performance we make an union operation
of both document_id list and the filter the id on this list. Postgresql
will use the index on the id column.

src/backend/core/api/viewsets.py

@@ -581,22 +581,27 @@ class DocumentViewSet(
         queryset = queryset.filter(ancestors_deleted_at__isnull=True)
 
         # Filter documents to which the current user has access...
-        access_documents_ids = models.DocumentAccess.objects.filter(
-            db.Q(user=user) | db.Q(team__in=user.teams)
-        ).values_list("document_id", flat=True)
+        access_documents_ids = (
+            models.DocumentAccess.objects.filter(
+                db.Q(user=user) | db.Q(team__in=user.teams)
+            )
+            .order_by()
+            .values_list("document_id", flat=True)
+        )
 
         # ...or that were previously accessed and are not restricted
-        traced_documents_ids = models.LinkTrace.objects.filter(user=user).values_list(
-            "document_id", flat=True
+        traced_documents_ids = (
+            models.LinkTrace.objects.filter(user=user)
+            .exclude(document__link_reach=models.LinkReachChoices.RESTRICTED)
+            .order_by()
+            .values_list("document_id", flat=True)
         )
 
-        return queryset.filter(
-            db.Q(id__in=access_documents_ids)
-            | (
-                db.Q(id__in=traced_documents_ids)
-                & ~db.Q(link_reach=models.LinkReachChoices.RESTRICTED)
-            )
-        )
+        # A single `IN (... UNION ...)` lets PostgreSQL drive the query from the
+        # (small) set of document ids and probe the primary key index. The
+        # equivalent `id IN (...) OR (id IN (...) AND ...)` results in a sequential
+        # scan of the whole document table.
+        return queryset.filter(id__in=access_documents_ids.union(traced_documents_ids))
 
     def filter_queryset(self, queryset):
         """Override to apply annotations to generic views."""