src/Pure/Tools/build_process.scala

     catch { case exn: Throwable => close(); throw exn }
 
   private val _build_database: Option[SQL.Database] =
     try {
       for (db <- store.maybe_open_build_database(server = server)) yield {
+        if (!db.is_postgresql) {
+          error("Required PostgreSQL for cluster build (option database_server)")
+        }
         val store_tables = db.is_postgresql
         Build_Process.private_data.transaction_lock(db,
           create = true,
           label = "Build_Process.build_database"
         ) {