Optimize mistral queries for 'get_task_executions'

During workflow executions there are a lot of calls of function
'find_task_executions_by_name' from lookup_utils. This function
internally calls 'get_task_executions' with two filters: name and
workflow_execution_id.

To optimize this call it's reasonable to add a composite db index
by these two fields.

Another optimization is to explicitely disable sorting by 'created_at',
because it's unnecessary in this particulat case.

Change-Id: Ife150a384575cc4ad2fe45dc093816b48268df6f

mistral/db/sqlalchemy/migration/alembic_migrations/versions/024_add_composite_index_workflow_execution_id_name.py

@@ -0,0 +1,34 @@
+# Copyright 2017 OpenStack Foundation.
+#
+# 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 database indices
+
+Revision ID: 024
+Revises: 023
+Create Date: 2017-10-11 15:23:04.904251
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '024'
+down_revision = '023'
+
+from alembic import op
+
+
+def upgrade():
+    op.create_index('task_executions_v2_workflow_execution_id_name',
+                    'task_executions_v2',
+                    ['workflow_execution_id', 'name'])

mistral/db/v2/sqlalchemy/models.py

@@ -345,6 +345,11 @@ sa.Index(
     TaskExecution.workflow_execution_id
 )
 
+sa.Index(
+    '%s_workflow_execution_id_name' % TaskExecution.__tablename__,
+    TaskExecution.workflow_execution_id, TaskExecution.name
+)
+
 
 # Other objects.
 

mistral/workflow/lookup_utils.py

@@ -67,7 +67,8 @@ def find_task_executions_by_name(wf_ex_id, task_name):
 
     t_execs = db_api.get_task_executions(
         workflow_execution_id=wf_ex_id,
-        name=task_name
+        name=task_name,
+        sort_keys=[]  # disable sorting
     )
 
     # We can cache only finished tasks because they won't change.