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Use subqueryload() instead of joinedload() for (system_)metadata

Currently, when we "get" a single instance from the database and we
load metadata and system_metadata, we do so using a joinedload() which
does JOINs with the respective tables. Because of the one-to-many
relationship between an instance and (system_)metadata records, doing
the database query this way can result in a large number of additional
rows being returned unnecessarily and cause a large data transfer.

This is similar to the problem addressed by change
I0610fb16ccce2ee95c318589c8abcc30613a3fe9 which added separate queries
for (system_)metadata when we "get" multiple instances. We don't,
however, reuse the same code for this change because
_instances_fill_metadata converts the instance database object to a
dict, and some callers of _instance_get_by_uuid need to be able to
access an instance database object attached to the session (example:

By using subqueryload() [1], we can perform the additional queries for
(system_)metadata to solve the problem with a similar approach.

Closes-Bug: #1799298


Change-Id: I5c071f70f669966e9807b38e99077c1cae5b4606
(cherry picked from commit e728fe668a)
(cherry picked from commit 63d2e62c3a)
(cherry picked from commit e7a45e0335)
(cherry picked from commit 4350074029)
melanie witt 1 year ago
committed by Erik Olof Gunnar Andersson
  1. 17
  2. 8


@ -48,6 +48,7 @@ from sqlalchemy.orm import aliased
from sqlalchemy.orm import contains_eager
from sqlalchemy.orm import joinedload
from sqlalchemy.orm import noload
from sqlalchemy.orm import subqueryload
from sqlalchemy.orm import undefer
from sqlalchemy.schema import Table
from sqlalchemy import sql
@ -1936,13 +1937,27 @@ def _build_instance_get(context, columns_to_join=None):
if 'extra.' in column:
query = query.options(undefer(column))
elif column in ['metadata', 'system_metadata']:
# NOTE(melwitt): We use subqueryload() instead of joinedload() for
# metadata and system_metadata because of the one-to-many
# relationship of the data. Directly joining these columns can
# result in a large number of additional rows being queried if an
# instance has a large number of (system_)metadata items, resulting
# in a large data transfer. Instead, the subqueryload() will
# perform additional queries to obtain metadata and system_metadata
# for the instance.
query = query.options(subqueryload(column))
query = query.options(joinedload(column))
# NOTE(alaski) Stop lazy loading of columns not needed.
for col in ['metadata', 'system_metadata']:
if col not in columns_to_join:
query = query.options(noload(col))
return query
# NOTE(melwitt): We need to use order_by(<unique column>) so that the
# additional queries emitted by subqueryload() include the same ordering as
# used by the parent query.
return query.order_by(
def _instances_fill_metadata(context, instances, manual_joins=None):


@ -1957,6 +1957,14 @@ class InstanceTestCase(test.TestCase, ModelsObjectComparatorMixin):
sys_meta = utils.metadata_to_dict(inst['system_metadata'])
self.assertEqual(sys_meta, self.sample_data['system_metadata'])
def test_instance_get_with_meta(self):
inst_id = self.create_instance_with_args().id
inst = db.instance_get(self.ctxt, inst_id)
meta = utils.metadata_to_dict(inst['metadata'])
self.assertEqual(meta, self.sample_data['metadata'])
sys_meta = utils.metadata_to_dict(inst['system_metadata'])
self.assertEqual(sys_meta, self.sample_data['system_metadata'])
def test_instance_update(self):
instance = self.create_instance_with_args()
metadata = {'host': 'bar', 'key2': 'wuff'}