public function initExplainCols() {
        $this->header_row = array(
            'id' => 'The SELECT identifier. This is the sequential number of the SELECT within the query. The value can be NULL if the row refers to the union result of other rows. In this case, the table column shows a value like <unionM,N> to indicate that the row refers to the union of the rows with id values of M and N.',
            'select_type' => 'The type of SELECT',
            'table' => 'The name of the table to which the row of output refers.',