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 commit e728fe668a)
(cherry picked from commit 63d2e62c3a)
(cherry picked from commit e7a45e0335)
(cherry picked from commit 4350074029)
(cherry picked from commit ad7e4fb8f4)
68f80d2013