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: instance_update_and_get_original). By using subqueryload() [1], we can perform the additional queries for (system_)metadata to solve the problem with a similar approach. Closes-Bug: #1799298 [1] https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html#subquery-eager-loading Change-Id: I5c071f70f669966e9807b38e99077c1cae5b4606 (cherry picked from commite728fe668a
) (cherry picked from commit63d2e62c3a
) (cherry picked from commite7a45e0335
)
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@ -48,6 +48,7 @@ from sqlalchemy.orm import aliased
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from sqlalchemy.orm import contains_eager
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from sqlalchemy.orm import joinedload
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from sqlalchemy.orm import noload
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from sqlalchemy.orm import subqueryload
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from sqlalchemy.orm import undefer
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from sqlalchemy.schema import Table
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from sqlalchemy import sql
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@ -1930,13 +1931,27 @@ def _build_instance_get(context, columns_to_join=None):
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continue
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if 'extra.' in column:
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query = query.options(undefer(column))
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elif column in ['metadata', 'system_metadata']:
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# NOTE(melwitt): We use subqueryload() instead of joinedload() for
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# metadata and system_metadata because of the one-to-many
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# relationship of the data. Directly joining these columns can
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# result in a large number of additional rows being queried if an
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# instance has a large number of (system_)metadata items, resulting
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# in a large data transfer. Instead, the subqueryload() will
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# perform additional queries to obtain metadata and system_metadata
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# for the instance.
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query = query.options(subqueryload(column))
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else:
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query = query.options(joinedload(column))
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# NOTE(alaski) Stop lazy loading of columns not needed.
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for col in ['metadata', 'system_metadata']:
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if col not in columns_to_join:
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query = query.options(noload(col))
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return query
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# NOTE(melwitt): We need to use order_by(<unique column>) so that the
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# additional queries emitted by subqueryload() include the same ordering as
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# used by the parent query.
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# https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html#the-importance-of-ordering
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return query.order_by(models.Instance.id)
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def _instances_fill_metadata(context, instances, manual_joins=None):
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@ -2086,6 +2086,14 @@ class InstanceTestCase(test.TestCase, ModelsObjectComparatorMixin):
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sys_meta = utils.metadata_to_dict(inst['system_metadata'])
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self.assertEqual(sys_meta, self.sample_data['system_metadata'])
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def test_instance_get_with_meta(self):
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inst_id = self.create_instance_with_args().id
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inst = db.instance_get(self.ctxt, inst_id)
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meta = utils.metadata_to_dict(inst['metadata'])
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self.assertEqual(meta, self.sample_data['metadata'])
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sys_meta = utils.metadata_to_dict(inst['system_metadata'])
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self.assertEqual(sys_meta, self.sample_data['system_metadata'])
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def test_instance_update(self):
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instance = self.create_instance_with_args()
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metadata = {'host': 'bar', 'key2': 'wuff'}
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