[MM-36387] Address db performance audit items (#18078)

store/sqlstore/thread_store.go

@@ -61,7 +61,7 @@ func (s *SqlThreadStore) createIndexesIfNotExists() {
 	s.CreateIndexIfNotExists("idx_thread_memberships_last_update_at", "ThreadMemberships", "LastUpdated")
 	s.CreateIndexIfNotExists("idx_thread_memberships_last_view_at", "ThreadMemberships", "LastViewed")
 	s.CreateIndexIfNotExists("idx_thread_memberships_user_id", "ThreadMemberships", "UserId")
-	s.CreateIndexIfNotExists("idx_threads_channel_id", "Threads", "ChannelId")
+	s.CreateCompositeIndexIfNotExists("idx_threads_channel_id_last_reply_at", "Threads", []string{"ChannelId", "LastReplyAt"})
 }
 
 func (s *SqlThreadStore) SaveMultiple(threads []*model.Thread) ([]*model.Thread, int, error) {