When you e.g. delete rows, these rows are just marked as deleted not really physically deleted from indexes and free space introduced is not returned to operating system for later reuse. Purge thread will physically delete index keys and rows, but still free space introduced is not returned to operating system and this operation can lead holes on page. If you have variable length rows, this could lead to situation where this free space can’t be used for new rows (if these rows are larger than old ones). User may use
OPTIMIZE TABLE or ALTER TABLE <table> ENGINE=InnoDB to reconstruct the table.
OPTIMIZE TABLE against an