protected function makeIndexedColumnsDDL()
    {
        $columnDDLs = [];

        foreach ($this->indexedColumns as $indexedColumn) {
            $columnDDLs[] = '`' . $indexedColumn->getColumnName() . '`'
                . ($indexedColumn->hasLength() ? '(' . $indexedColumn->getLength() . ')' : null)
                . ($indexedColumn->hasCollation() ? ' ' . strtoupper($indexedColumn->getCollation()) : null);
        }

        return '(' . implode(',', $columnDDLs) . ')';
    }