e index don't exist.
	 */
	public function get_index_columns( string $table_name, string $index_name = '' ): array {
		global $wpdb;

		if ( empty( $index_name ) ) {
			$index_name = 'PRIMARY';
		}

		// phpcs:disable WordPress.DB.PreparedSQL
		return $wpdb->get_col(
			"
SELECT column_name FROM INFORMATION_SCHEMA.STATISTICS
WHERE table_name='$table_name'
AND table_schema='" . DB_NAME . "'
AND index_name='$index_name'"
		);
		// phpcs:enable WordPress.DB.PreparedSQL
	}
}