public function begin_table_import($tablename, $schemaHash) {
        if ($this->transactionmode == 'pertable') {
            $this->transaction = $this->mdb->start_delegated_transaction();
        }
        if (!$table = $this->schema->getTable($tablename)) {
            throw new dbtransfer_exception('unknowntableexception', $tablename);
        }
        if ($schemaHash != $table->getHash()) {
            throw new dbtransfer_exception('differenttableexception', $tablename);
        }
        // this should not happen, unless someone drops tables after import started
        if (!$this->manager->table_exists($table)) {
            throw new ddl_table_missing_exception($tablename);
        }
        $this->mdb->delete_records($tablename);
    }