Add index on trunk_id in the subports model

An index on trunk_id will make queries that return subports from a given
trunk_id efficient.  When I first approved the trunk and subports model,
I thought that having trunk_id in the primary key would be sufficient.
However, thinking about it some more, I realize that isn't the case
because trunk_id is listed as the second column in the primary key.

Think about it like searching a string index for something that matches
in the last half of a string.  The index doesn't make that efficient.
The DB must still search all entries for matches.  It is the same with
the index on (port_id, trunk_id) that the primary key provides.  You
need to look through every row to find matching trunk_ids.  Hence the
addition of an index.

Change-Id: I566863fd44aa402230d2eb83e99bc66e7fed8d80
Partially-Implements: blueprint vlan-aware-vms

neutron/db/migration/alembic_migrations/versions/EXPAND_HEAD

@@ -1 +1 @@
-30107ab6a3ee
+0e877ec3cba3

neutron/db/migration/alembic_migrations/versions/newton/expand/0e877ec3cba3_add_index_to_trunk_model_and_adjust_.py

@@ -0,0 +1,25 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+"""Add index on trunk_id to subports model """
+
+revision = '0e877ec3cba3'
+down_revision = '30107ab6a3ee'
+
+from alembic import op
+
+
+def upgrade():
+    op.create_index('ix_subports_trunk_id',
+                    'subports',
+                    ['trunk_id'],
+                    unique=False)

neutron/services/trunk/models.py

@@ -56,6 +56,7 @@ class SubPort(model_base.BASEV2):
     segmentation_id = sa.Column(sa.Integer, nullable=False)
 
     __table_args__ = (
+        sa.Index('ix_subports_trunk_id', 'trunk_id'),
         sa.UniqueConstraint(
             'trunk_id',
             'segmentation_type',