[AIRFLOW-3392] Add index on dag_id in sla_miss table (apache#4235)

The select queries on sla_miss table produce a great % of DB traffic and
thus made the DB CPU usage unnecessarily high. It would be a low hanging
fruit to add an index and reduce the load.

airflow/migrations/versions/03bc53e68815_add_sm_dag_index.py

@@ -0,0 +1,40 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you 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.
+
+"""merge_heads_2
+
+Revision ID: 03bc53e68815
+Revises: 0a2a5b66e19d, bf00311e1990
+Create Date: 2018-11-24 20:21:46.605414
+
+"""
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = '03bc53e68815'
+down_revision = ('0a2a5b66e19d', 'bf00311e1990')
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_index('sm_dag', 'sla_miss', ['dag_id'], unique=False)
+
+
+def downgrade():
+    op.drop_index('sm_dag', table_name='sla_miss')

airflow/models.py

@@ -5481,6 +5481,10 @@ class SlaMiss(Base):
     description = Column(Text)
     notification_sent = Column(Boolean, default=False)
 
+    __table_args__ = (
+        Index('sm_dag', dag_id, unique=False),
+    )
+
     def __repr__(self):
         return str((
             self.dag_id, self.task_id, self.execution_date.isoformat()))