diff --git a/synapse/storage/schema/main/delta/68/04partial_state_rooms.sql b/synapse/storage/schema/main/delta/68/04partial_state_rooms.sql
new file mode 100644
index 000000000000..815c0cc390b6
--- /dev/null
+++ b/synapse/storage/schema/main/delta/68/04partial_state_rooms.sql
@@ -0,0 +1,41 @@
+/* Copyright 2022 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.
+ */
+
+-- rooms which we have done a partial-state-style join to
+CREATE TABLE IF NOT EXISTS partial_state_rooms (
+    room_id TEXT PRIMARY KEY,
+    FOREIGN KEY(room_id) REFERENCES rooms(room_id)
+);
+
+-- a list of remote servers we believe are in the room
+CREATE TABLE IF NOT EXISTS partial_state_rooms_servers (
+    room_id TEXT NOT NULL REFERENCES partial_state_rooms(room_id),
+    server_name TEXT NOT NULL,
+    UNIQUE(room_id, server_name)
+);
+
+-- a list of events with partial state. We can't store this in the `events` table
+-- itself, because `events` is meant to be append-only.
+CREATE TABLE IF NOT EXISTS partial_state_events (
+    -- the room_id is denormalised for efficient indexing (the canonical source is `events`)
+    room_id TEXT NOT NULL REFERENCES partial_state_rooms(room_id),
+    event_id TEXT NOT NULL REFERENCES events(event_id),
+    UNIQUE(event_id)
+);
+
+CREATE INDEX IF NOT EXISTS partial_state_events_room_id_idx
+     ON partial_state_events (room_id);
+
+
diff --git a/synapse/storage/schema/main/delta/68/05lazy_joining_rooms_triggers.py b/synapse/storage/schema/main/delta/68/05lazy_joining_rooms_triggers.py
new file mode 100644
index 000000000000..a2ec4fc26edb
--- /dev/null
+++ b/synapse/storage/schema/main/delta/68/05lazy_joining_rooms_triggers.py
@@ -0,0 +1,72 @@
+# Copyright 2022 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.
+
+
+"""
+This migration adds triggers to the partial_state_events tables to enforce uniqueness
+
+Triggers cannot be expressed in .sql files, so we have to use a separate file.
+"""
+from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
+from synapse.storage.types import Cursor
+
+
+def run_create(cur: Cursor, database_engine: BaseDatabaseEngine, *args, **kwargs):
+    # complain if the room_id in partial_state_events doesn't match
+    # that in `events`. We already have a fk constraint which ensures that the event
+    # exists in `events`, so all we have to do is raise if there is a row with a
+    # matching stream_ordering but not a matching room_id.
+    if isinstance(database_engine, Sqlite3Engine):
+        cur.execute(
+            """
+            CREATE TRIGGER IF NOT EXISTS partial_state_events_bad_room_id
+            BEFORE INSERT ON partial_state_events
+            FOR EACH ROW
+            BEGIN
+                SELECT RAISE(ABORT, 'Incorrect room_id in partial_state_events')
+                WHERE EXISTS (
+                    SELECT 1 FROM events
+                    WHERE events.event_id = NEW.event_id
+                       AND events.room_id != NEW.room_id
+                );
+            END;
+            """
+        )
+    elif isinstance(database_engine, PostgresEngine):
+        cur.execute(
+            """
+            CREATE OR REPLACE FUNCTION check_partial_state_events() RETURNS trigger AS $BODY$
+            BEGIN
+                IF EXISTS (
+                    SELECT 1 FROM events
+                    WHERE events.event_id = NEW.event_id
+                       AND events.room_id != NEW.room_id
+                ) THEN
+                    RAISE EXCEPTION 'Incorrect room_id in partial_state_events';
+                END IF;
+                RETURN NEW;
+            END;
+            $BODY$ LANGUAGE plpgsql;
+            """
+        )
+
+        cur.execute(
+            """
+            CREATE TRIGGER check_partial_state_events BEFORE INSERT OR UPDATE ON partial_state_events
+            FOR EACH ROW
+            EXECUTE PROCEDURE check_partial_state_events()
+            """
+        )
+    else:
+        raise NotImplementedError("Unknown database engine")