⚡️(backend) stop using LEFT(value, LENGTH(path)) in sql queries

Comparing path with LEFT(value, LENGTH(path)) makes a sequential scan on
all the Document table, the more this table grow, the more the query
using it will be slow. We dediced instead to lookup on the path
extracting all ancestors path for a given document and then make a path
IN statement to use the index existing on the path column.

src/backend/core/models.py

@@ -1158,6 +1158,20 @@ class Document(MP_Node, BaseModel):
             Bucket=default_storage.bucket_name, Key=self.file_key, VersionId=version_id
         )
 
+    def get_self_and_ancestors_paths(self):
+        """
+        Return the paths of the document and of all its ancestors, computed from
+        the materialized path without querying the database.
+
+        Filtering on `path__in` with this list hits the unique index on `path`,
+        whereas comparing `path` with `LEFT(value, LENGTH(path))` forces a
+        sequential scan of the whole table.
+        """
+        return [
+            self.path[:pos]
+            for pos in range(self.steplen, len(self.path) + 1, self.steplen)
+        ]
+
     def get_nb_accesses_cache_key(self):
         """Generate a unique cache key for each document."""
         return f"document_{self.id!s}_nb_accesses"
@@ -1175,9 +1189,7 @@ class Document(MP_Node, BaseModel):
             nb_accesses = (
                 DocumentAccess.objects.filter(document=self).count(),
                 DocumentAccess.objects.filter(
-                    document__path=Left(
-                        models.Value(self.path), Length("document__path")
-                    ),
+                    document__path__in=self.get_self_and_ancestors_paths(),
                     document__ancestors_deleted_at__isnull=True,
                 ).count(),
             )
@@ -1217,7 +1229,7 @@ class Document(MP_Node, BaseModel):
         except AttributeError:
             roles = DocumentAccess.objects.filter(
                 models.Q(user=user) | models.Q(team__in=user.teams),
-                document__path=Left(models.Value(self.path), Length("document__path")),
+                document__path__in=self.get_self_and_ancestors_paths(),
             ).values_list("role", flat=True)
 
         return RoleChoices.max(*roles)