[AIRFLOW-291] Add index for state in TI table

Closes apache#1635 from aoen/ddavydov/add_index_to_task_instance_state

airflow/migrations/versions/211e584da130_add_ti_state_index.py

@@ -0,0 +1,23 @@
+"""add TI state index
+
+Revision ID: 211e584da130
+Revises: 2e82aab8ef20
+Create Date: 2016-06-30 10:54:24.323588
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '211e584da130'
+down_revision = '2e82aab8ef20'
+branch_labels = None
+depends_on = None
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.create_index('ti_state', 'task_instance', ['state'], unique=False)
+
+def downgrade():
+    op.drop_index('ti_state', table_name='task_instance')

airflow/models.py

@@ -715,6 +715,7 @@ class TaskInstance(Base):
 
     __table_args__ = (
         Index('ti_dag_state', dag_id, state),
+        Index('ti_state', state),
         Index('ti_state_lkp', dag_id, task_id, execution_date, state),
         Index('ti_pool', pool, state, priority_weight),
     )