Previous versions of InnoDB used an unnamed file format (now called “Antelope”) for database files. With that format, tables were defined with ROW_FORMAT=COMPACT (or ROW_FORMAT=REDUNDANT) and InnoDB stored up to the first 768 bytes of variable-length columns (such as BLOB and VARCHAR) in the index record within the B-tree node, with the remainder stored on the overflow page(s).

To preserve compatibility with those prior versions, tables created with the InnoDB Plugin use the prefix format, unless one of ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED is specified (or implied) on the CREATE TABLE command.

With the “Antelope” file format, if the value of a column is not longer than 768 bytes, no overflow page is needed, and some savings in I/O may result, since the value is in the B-tree node. This works well for relatively short BLOBs, but may cause B-tree nodes to fill with data rather than key values, thereby reducing their efficiency. Tables with many BLOB columns could cause B-tree nodes to become too full of data, and contain too few rows, making the entire index less efficient than if the rows were shorter or if the column values were stored off-page.