sort against documents should be done on server

[Tong Li]

in impl_mongodb.py, sort was done after documents returned from the db calls. sort should be done on the server side. Here is where the problem was found.

    def get_users(self, source=None):
        """Return an iterable of user id strings.

        :param source: Optional source filter.
        """
        q = {}
        if source is not None:
            q['source'] = source
        return sorted(self.db.user.find(q).distinct('_id'))

Notice the sorted method call at the end.

same problem in get_projects method.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/38537
Committed: http://github.com/openstack/ceilometer/commit/4514eb87675ed9cf46d9fd4397727059b7881a37
Submitter: Jenkins
Branch: master

commit 4514eb87675ed9cf46d9fd4397727059b7881a37
Author: Tong Li <email address hidden>
Date: Thu Jul 25 10:33:55 2013 -0400

    calling distinct on _id field against a collection is slow

    To optimize the code, distinct method should be avoided especially
    against _id field. Sort should also be done on the server side to
    take advantage of generator to avoid using up memory for large
    data set.

    Change-Id: Ib417f73e2238a5d9865a2a1a9990e891b0f7a24c
    fixes: bug #1204607
    fixes: bug #1204609