All InnoDB
indexes are
B-trees where the index records
are stored in the leaf pages of the tree. The default size of an
index page is 16KB. The page size is determined by the
innodb_page_size
setting when the
MySQL instance is initialized. See
Section 14.8.1, “InnoDB Startup Configuration”.
When new records are inserted into an InnoDB
clustered index,
InnoDB
tries to leave 1/16 of the page free for
future insertions and updates of the index records. If index
records are inserted in a sequential order (ascending or
descending), the resulting index pages are about 15/16 full. If
records are inserted in a random order, the pages are from 1/2 to
15/16 full. If the fill
factor of an index page drops below 1/2,
InnoDB
tries to contract the index tree to free
the page.