Comment 4 for bug 1383617

Reviewed: https://review.openstack.org/134740
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=699b46799fe408bab1f23c108c2e9887638ec5b0
Submitter: Jenkins
Branch: stable/juno

commit 699b46799fe408bab1f23c108c2e9887638ec5b0
Author: Joe Gordon <email address hidden>
Date: Tue Oct 21 16:46:19 2014 -0700

    Don't make a no-op DB call

    drivers_uuid can be empty, but we are still doing a DB call. This is causing
    a sqlalchemy SAWarning.

    SAWarning: The IN-predicate on "instances.uuid" was invoked with an
    empty sequence. This results in a contradiction, which nonetheless can
    be expensive to evaluate. Consider alternative strategies for improved
    performance.

    Conflicts:
            nova/tests/unit/compute/test_compute_mgr.py

    Change-Id: Ib8c9b85e84800c9e2ddcdf204851f1b51101926c
    Closes-Bug: #1383617
    (cherry picked from commit 150428f76e6149abb21d5264142673bc4961ec75)