Change to use selectin for DB load strategy
During a mailing list discussion on some OOM issues neutron
has been seeing [0], Mike Bayer recommended we should change
from using subquery to selectin DB load strategy.
A full description of this strategy can be found here [1],
but in short:
- “subquery” loading incurs additional performance / complexity
issues when used on a many-levels-deep eager load, as
subqueries will be nested repeatedly.
- "The subqueryload() eager loader is mostly legacy at this
point, superseded by selectinload()
- "The only scenario in which selectin eager loading is not
feasible is when the model is using composite primary keys,
and the backend database does not support tuples with IN,
which currently includes SQL Server." So that does not
apply to us.
The plan agreed to at the neutron drivers meeting [2] was to
make this change early in the cycle so we would be able to
see if there were any issues through the D cycle.
Added hacking checks so new code using subquery loads is
not added back.
[0] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/EHLQQXNG3NLLZYPDGG2ES3DINIJ7YT3N/
[1] https://docs.sqlalchemy.org/en/20/orm/queryguide/relationships.html#selectin-eager-loading
[2] https://meetings.opendev.org/meetings/neutron_drivers/2024/neutron_drivers.2024-05-31-14.00.log.html#l-67
Closes-bug: #2067770
Depends-on: https://review.opendev.org/c/openstack/neutron-lib/+/920936
Change-Id: I6e40a15284da392a3d48d45205a7a5770c14c297
05fcfef6ce