diff --git a/synapse/storage/background_updates.py b/synapse/storage/background_updates.py
index d4cf0fc59..12a8b8259 100644
--- a/synapse/storage/background_updates.py
+++ b/synapse/storage/background_updates.py
@@ -210,7 +210,8 @@ class BackgroundUpdateStore(SQLBaseStore):
         self._background_update_handlers[update_name] = update_handler
 
     def register_background_index_update(self, update_name, index_name,
-                                         table, columns, where_clause=None):
+                                         table, columns, where_clause=None,
+                                         unique=False):
         """Helper for store classes to do a background index addition
 
         To use:
@@ -245,9 +246,11 @@ class BackgroundUpdateStore(SQLBaseStore):
                 c.execute(sql)
 
                 sql = (
-                    "CREATE INDEX CONCURRENTLY %(name)s ON %(table)s"
+                    "CREATE %(unique)s INDEX CONCURRENTLY %(name)s"
+                    " ON %(table)s"
                     " (%(columns)s) %(where_clause)s"
                 ) % {
+                    "unique": "UNIQUE" if unique else "",
                     "name": index_name,
                     "table": table,
                     "columns": ", ".join(columns),
@@ -270,9 +273,10 @@ class BackgroundUpdateStore(SQLBaseStore):
             # down at the wrong moment - hance we use IF NOT EXISTS. (SQLite
             # has supported CREATE TABLE|INDEX IF NOT EXISTS since 3.3.0.)
             sql = (
-                "CREATE INDEX IF NOT EXISTS %(name)s ON %(table)s"
+                "CREATE %(unique)s INDEX IF NOT EXISTS %(name)s ON %(table)s"
                 " (%(columns)s)"
             ) % {
+                "unique": "UNIQUE" if unique else "",
                 "name": index_name,
                 "table": table,
                 "columns": ", ".join(columns),
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index dbd63078c..1fae1aeac 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -207,6 +207,17 @@ class EventsStore(SQLBaseStore):
             where_clause="contains_url = true AND outlier = false",
         )
 
+        # an event_id index on event_search is useful for the purge_history
+        # api. Plus it means we get to enforce some integrity with a UNIQUE
+        # clause
+        self.register_background_index_update(
+            "event_search_event_id_idx",
+            index_name="event_search_event_id_idx",
+            table="event_search",
+            columns=["event_id"],
+            unique=True,
+        )
+
         self._event_persist_queue = _EventPeristenceQueue()
 
     def persist_events(self, events_and_contexts, backfilled=False):
diff --git a/synapse/storage/schema/delta/41/event_search_event_id_idx.sql b/synapse/storage/schema/delta/41/event_search_event_id_idx.sql
new file mode 100644
index 000000000..5d9cfecf3
--- /dev/null
+++ b/synapse/storage/schema/delta/41/event_search_event_id_idx.sql
@@ -0,0 +1,17 @@
+/* Copyright 2017 Vector Creations Ltd
+ *
+ * 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.
+ */
+
+INSERT into background_updates (update_name, progress_json)
+    VALUES ('event_search_event_id_idx', '{}');