mysql_High.Performance.MySQL.3rd.Edition.Mar.2012
A B-Tree index speeds up data access because the storage engine doesn’t have to scan
the whole table to find the desired data. Instead, it starts at the root node (not shown
in this figure). The slots in the root node hold pointers to child nodes, and the storage
engine follows these pointers. It finds the right pointer by looking at the values in the
node pages, which define the upper and lower bounds of the values in the child nodes.
Eventually, the storage engine either determines that the desired value doesn’t exist or
successfully reaches a leaf page.