3 files changed, 75 insertions, 2 deletions
diff --git a/synapse/storage/data_stores/main/events.py b/synapse/storage/data_stores/main/events.py
index aafc2007d3..301f8ea128 100644
--- a/synapse/storage/data_stores/main/events.py
+++ b/synapse/storage/data_stores/main/events.py
@@ -29,7 +29,7 @@ from prometheus_client import Counter
 from twisted.internet import defer
 
 import synapse.metrics
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventContentFields, EventTypes
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase  # noqa: F401
 from synapse.events.snapshot import EventContext  # noqa: F401
@@ -932,6 +932,13 @@ class EventsStore(
 
             self._handle_event_relations(txn, event)
 
+            # Store the labels for this event.
+            labels = event.content.get(EventContentFields.LABELS)
+            if labels:
+                self.insert_labels_for_event_txn(
+                    txn, event.event_id, labels, event.room_id, event.depth
+                )
+
         # Insert into the room_memberships table.
         self._store_room_members_txn(
             txn,
@@ -1917,6 +1924,33 @@ class EventsStore(
             get_all_updated_current_state_deltas_txn,
         )
 
+    def insert_labels_for_event_txn(
+        self, txn, event_id, labels, room_id, topological_ordering
+    ):
+        """Store the mapping between an event's ID and its labels, with one row per
+        (event_id, label) tuple.
+
+        Args:
+            txn (LoggingTransaction): The transaction to execute.
+            event_id (str): The event's ID.
+            labels (list[str]): A list of text labels.
+            room_id (str): The ID of the room the event was sent to.
+            topological_ordering (int): The position of the event in the room's topology.
+        """
+        return self._simple_insert_many_txn(
+            txn=txn,
+            table="event_labels",
+            values=[
+                {
+                    "event_id": event_id,
+                    "label": label,
+                    "room_id": room_id,
+                    "topological_ordering": topological_ordering,
+                }
+                for label in labels
+            ],
+        )
+
 
 AllNewEventsResult = namedtuple(
     "AllNewEventsResult",
diff --git a/synapse/storage/data_stores/main/schema/delta/56/event_labels.sql b/synapse/storage/data_stores/main/schema/delta/56/event_labels.sql
new file mode 100644
index 0000000000..5e29c1da19
--- /dev/null
+++ b/synapse/storage/data_stores/main/schema/delta/56/event_labels.sql
@@ -0,0 +1,30 @@
+/* Copyright 2019 The Matrix.org Foundation C.I.C.
+ *
+ * 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.
+ */
+
+-- room_id and topoligical_ordering are denormalised from the events table in order to
+-- make the index work.
+CREATE TABLE IF NOT EXISTS event_labels (
+    event_id TEXT,
+    label TEXT,
+    room_id TEXT NOT NULL,
+    topological_ordering BIGINT NOT NULL,
+    PRIMARY KEY(event_id, label)
+);
+
+
+-- This index enables an event pagination looking for a particular label to index the
+-- event_labels table first, which is much quicker than scanning the events table and then
+-- filtering by label, if the label is rarely used relative to the size of the room.
+CREATE INDEX event_labels_room_id_label_idx ON event_labels(room_id, label, topological_ordering);
diff --git a/synapse/storage/data_stores/main/stream.py b/synapse/storage/data_stores/main/stream.py
index 263999dfca..616ef91d4e 100644
--- a/synapse/storage/data_stores/main/stream.py
+++ b/synapse/storage/data_stores/main/stream.py
@@ -229,6 +229,14 @@ def filter_to_clause(event_filter):
         clauses.append("contains_url = ?")
         args.append(event_filter.contains_url)
 
+    # We're only applying the "labels" filter on the database query, because applying the
+    # "not_labels" filter via a SQL query is non-trivial. Instead, we let
+    # event_filter.check_fields apply it, which is not as efficient but makes the
+    # implementation simpler.
+    if event_filter.labels:
+        clauses.append("(%s)" % " OR ".join("label = ?" for _ in event_filter.labels))
+        args.extend(event_filter.labels)
+
     return " AND ".join(clauses), args
 
 
@@ -864,8 +872,9 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         args.append(int(limit))
 
         sql = (
-            "SELECT event_id, topological_ordering, stream_ordering"
+            "SELECT DISTINCT event_id, topological_ordering, stream_ordering"
             " FROM events"
+            " LEFT JOIN event_labels USING (event_id, room_id, topological_ordering)"
             " WHERE outlier = ? AND room_id = ? AND %(bounds)s"
             " ORDER BY topological_ordering %(order)s,"
             " stream_ordering %(order)s LIMIT ?"