Remove the ORDER BY entity_id when fetching states, and add logging

Having this ORDER BY in the query prevents it from using an index due to the range filter, so it has been removed.

We already do a `groupby` in the `states_to_json` method which accomplishes exactly what the ORDER BY in the query was trying to do anyway, so this change causes no functional difference.

Also added DEBUG-level logging to allow diagnosing a user's slow history page.

homeassistant/components/history.py

@@ -57,6 +57,7 @@ def get_significant_states(hass, start_time, end_time=None, entity_id=None,
     as well as all states from certain domains (for instance
     thermostat so that we get current temperature in our graphs).
     """
+    timer_start = time.perf_counter()
     from homeassistant.components.recorder.models import States
 
     entity_ids = (entity_id.lower(), ) if entity_id is not None else None
@@ -73,12 +74,18 @@ def get_significant_states(hass, start_time, end_time=None, entity_id=None,
         if end_time is not None:
             query = query.filter(States.last_updated < end_time)
 
+        query = query.order_by(States.last_updated)
+
         states = (
-            state for state in execute(
-                query.order_by(States.entity_id, States.last_updated))
+            state for state in execute(query)
             if (_is_significant(state) and
                 not state.attributes.get(ATTR_HIDDEN, False)))
 
+    if _LOGGER.isEnabledFor(logging.DEBUG):
+        elapsed = time.perf_counter() - timer_start
+        _LOGGER.debug(
+            'get_significant_states took %fs', elapsed)
+
     return states_to_json(hass, states, start_time, entity_id, filters)