when I learned B+ tree knowledge , I saw this article, but I don't quite understand how this 100 is calculated. can anyone help explain?
There are conflicting definitions of what "order" means in a B-tree.
On Wikipedia we find:
The literature on B-trees is not uniform in its terminology.
Bayer and McCreight (1972), Comer (1979), and others define the order of B-tree as the minimum number of keys in a non-root node.
With this definition, nodes of a B tree with order 2 can have up to 4 keys and up to 5 children (like depicted in the image).
When nodes of a 3-layer tree (i.e. depth 2) are filled to the maximum, then the root node has 5 children, and each of those children has 5 children of their own, i.e. the root has 5x5 grandchildren. Each of those grandchildren has 4 keys. So the grandchildren together have 100 keys.