diff --git a/src/github.com/matrix-org/dendrite/federationsender/storage/room_table.go b/src/github.com/matrix-org/dendrite/federationsender/storage/room_table.go
index 66b8fa299..e11ed4213 100644
--- a/src/github.com/matrix-org/dendrite/federationsender/storage/room_table.go
+++ b/src/github.com/matrix-org/dendrite/federationsender/storage/room_table.go
@@ -21,7 +21,7 @@ import (
 const roomSchema = `
 CREATE TABLE IF NOT EXISTS rooms (
     -- The string ID of the room
-    room_id TEXT NOT NULL CONSTRAINT room_id_unique UNIQUE,
+    room_id TEXT PRIMARY KEY,
     -- The most recent event state by the room server.
     -- We can use this to tell if our view of the room state has become
     -- desynchronised.
@@ -29,10 +29,8 @@ CREATE TABLE IF NOT EXISTS rooms (
 );`
 
 const insertRoomSQL = "" +
-	"INSERT INTO rooms (room_id, last_event_id)" +
-	" VALUES ($1, '')" +
-	" ON CONFLICT ON CONSTRAINT room_id_unique" +
-	" DO NOTHING"
+	"INSERT INTO rooms (room_id, last_event_id) VALUES ($1, '')" +
+	" ON CONFLICT DO NOTHING"
 
 const selectRoomForUpdateSQL = "" +
 	"SELECT last_event_id FROM rooms WHERE room_id = $1 FOR UPDATE"
@@ -64,11 +62,16 @@ func (s *roomStatements) prepare(db *sql.DB) (err error) {
 	return
 }
 
+// insertRoom inserts the room if it didn't already exist.
+// If the room didn't exist then last_event_id is set to the empty string.
 func (s *roomStatements) insertRoom(txn *sql.Tx, roomID string) error {
 	_, err := txn.Stmt(s.insertRoomStmt).Exec(roomID)
 	return err
 }
 
+// selectRoomForUpdate locks the row for the room and returns the last_event_id.
+// The row must already exist in the table. Callers can ensure that the row
+// exists by calling insertRoom first.
 func (s *roomStatements) selectRoomForUpdate(txn *sql.Tx, roomID string) (string, error) {
 	var lastEventID string
 	err := txn.Stmt(s.selectRoomForUpdateStmt).QueryRow(roomID).Scan(&lastEventID)
@@ -78,6 +81,8 @@ func (s *roomStatements) selectRoomForUpdate(txn *sql.Tx, roomID string) (string
 	return lastEventID, nil
 }
 
+// updateRoom updates the last_event_id for the room. selectRoomForUpdate should
+// have already been called earlier within the transaction.
 func (s *roomStatements) updateRoom(txn *sql.Tx, roomID, lastEventID string) error {
 	_, err := txn.Stmt(s.updateRoomStmt).Exec(roomID, lastEventID)
 	return err