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(mysql.info)InnoDB physical structure

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Physical structure of an index
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All indexes in InnoDB are B-trees where the index records are stored in
the leaf pages of the tree. The default size of an index page is 16 kB.
When new records are inserted, 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 (ascending or descending)
order, the resulting index pages will be about 15/16 full.  If records
are inserted in a random order, then the pages will be 1/2 - 15/16
full. If the fillfactor of an index page drops below 1/2, InnoDB will
try to contract the index tree to free the page.