Early versions of InnoDB used an unnamed file format (now called
Antelope) for database files. With that file format, tables are
defined with ROW_FORMAT=REDUNDANT. InnoDB stores up to the first
768 bytes of variable-length columns (such as VARCHAR) in the
index record within the B-tree node, with the remainder stored
on the overflow pages. To preserve compatibility with those
prior versions, tables created with the newest InnoDB default to
the COMPACT row format. See Section 220.127.116.11, “ COMPRESSED
Row Formats” for information about COMPRESSED row formats. With
the Antelope file format, if the value of a column is 768 bytes
or less, 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, reducing their efficiency.
Tables with many could cause B-tree nodes to become too full of
data, and contain too few rows, making the entire index less
efficient