diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 2f3a70b4e5..55ea567793 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -14,43 +14,71 @@
 # limitations under the License.
 
 from ._base import SQLBaseStore
-from twisted.internet import defer
 
 
 class StateStore(SQLBaseStore):
+    """ Keeps track of the state at a given event.
+
+    This is done by the concept of `state groups`. Every event is a assigned
+    a state group (identified by an arbitrary string), which references a
+    collection of state events. The current state of an event is then the
+    collection of state events referenced by the event's state group.
+
+    Hence, every change in the current state causes a new state group to be
+    generated. However, if no change happens (e.g., if we get a message event
+    with only one parent it inherits the state group from its parent.)
+
+    There are three tables:
+      * `state_groups`: Stores group name, first event with in the group and
+        room id.
+      * `event_to_state_groups`: Maps events to state groups.
+      * `state_groups_state`: Maps state group to state events.
+    """
 
-    @defer.inlineCallbacks
     def get_state_groups(self, event_ids):
-        groups = set()
-        for event_id in event_ids:
-            group = yield self._simple_select_one_onecol(
-                table="event_to_state_groups",
-                keyvalues={"event_id": event_id},
-                retcol="state_group",
-                allow_none=True,
-            )
-            if group:
-                groups.add(group)
+        """ Get the state groups for the given list of event_ids
 
-        res = {}
-        for group in groups:
-            state_ids = yield self._simple_select_onecol(
-                table="state_groups_state",
-                keyvalues={"state_group": group},
-                retcol="event_id",
-            )
-            state = []
-            for state_id in state_ids:
-                s = yield self.get_event(
-                    state_id,
+        The return value is a dict mapping group names to lists of events.
+        """
+
+        def f(txn):
+            groups = set()
+            for event_id in event_ids:
+                group = self._simple_select_one_onecol_txn(
+                    txn,
+                    table="event_to_state_groups",
+                    keyvalues={"event_id": event_id},
+                    retcol="state_group",
                     allow_none=True,
                 )
-                if s:
-                    state.append(s)
+                if group:
+                    groups.add(group)
 
-            res[group] = state
+            res = {}
+            for group in groups:
+                state_ids = self._simple_select_onecol_txn(
+                    txn,
+                    table="state_groups_state",
+                    keyvalues={"state_group": group},
+                    retcol="event_id",
+                )
+                state = []
+                for state_id in state_ids:
+                    s = self._get_events_txn(
+                        txn,
+                        [state_id],
+                    )
+                    if s:
+                        state.extend(s)
 
-        defer.returnValue(res)
+                res[group] = state
+
+            return res
+
+        return self.runInteraction(
+            "get_state_groups",
+            f,
+        )
 
     def store_state_groups(self, event):
         return self.runInteraction(