Handling long texts/blobs in InnoDB - 1 to 1 relationship, covering index

I have seen that 1 to 1 relationship is sometimes used for MySQL(InnoDB) to avoid significant performance slowdown. To understand the performance difference, it is necessary to understand how InnoDB stores column values to data blocks. Suppose you have the following "diary" table (for storing blog entries).CREATE TABLE diary ( diary_id INT UNSIGNED AUTO_INCREMENT, user_id INT UNSIGNED NOT NULL, po