bw_usage_update makes an unnecessary query

[Chris Behrens]

bw_usage_update() in sqlalchemy api always does a select before doing an update or create. It needn't do this. It could just try to update and if 0 rows are updated, then create an entry. In the most common case (updating an existing row), this will result in 1 DB query vs 2.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/11267

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/11267
Committed: http://github.com/openstack/nova/commit/7a500ecc8799914e45ca8534a15f81b8823a1d3e
Submitter: Jenkins
Branch: master

commit 7a500ecc8799914e45ca8534a15f81b8823a1d3e
Author: Chris Behrens <email address hidden>
Date: Mon Aug 13 07:33:15 2012 +0000

    Improve bw_usage_update() performance

    Fixes bug 1036084

    bw_usage_update() most commonly updates rows vs creates rows. New rows
    are only created on the 1st bandwidth update for an instance or when
    the audit period rolls over. This cuts down DB queries to 1 for the
    'update' case.

    Added bonus:
    Remove unused 'import inspect' in compute manager.

    Change-Id: Ie5f6c919676046d817b842138dc9d17d1115d3c0