In the User Records part of a page, you'll find all the records that the user inserted.
There are two ways to navigate through the user records, depending whether you want to think of their organization as an unordered or an ordered list.
An unordered list is often called a "heap". If you make a pile
of stones by saying "whichever one I happen to pick up next
will go on top" — rather than organizing them according
to size and colour — then you end up with a heap.
InnoDB does not want to insert
new rows according to the B-tree's key order (that would
involve expensive shifting of large amounts of data), so it
inserts new rows right after the end of the existing rows (at
the top of the Free Space part) or wherever there's space left
by a deleted row.
But by definition the records of a B-tree must be accessible
in order by key value, so there is a record pointer in each
record (the "next" field in the Extra Bytes) which points to
the next record in key order. In other words, the records are
a one-way linked list. So
InnoDB can access
rows in key order when searching.
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