This is somewhat related to MySQL Bug #68546. In non-leaf InnoDB index pages, the "node pointer" records are currently constucted from child page records. This makes the keys unnecessarily large.

A non-leaf page pointer could have a one-character key that would be sufficient to distinguish longer VARCHAR columns in the child or leaf pages. For example, if the maximum and minimum keys on two sibling pages are "Jones" and "Smith", any single-character key between "K" and "S" would distinguish those child pages in the parent page.

In case of a multi-column key, we could store the last columns as empty strings or NULL when the columns are not declared NOT NULL.