According to the search the indexing is used to speed up the search database. Indexing makes database searches more efficient, which means you can search for all the rows or columns to access the data you want, instead of working your way through the database. In other words, it can be accessed from anywhere in the database. Meanwhile, in Mysql, the index is stored in b-trees. B-tree is not the same as the binary tree. Because B-tree nodes can have many children, a B-tree is not the same as a binary tree, which is limited to 2 children per node. When I search on the web, I find the index has two different types: the clustered and non-clustered. “A non-clustered index is different in that you can have many of them and they then point at the data in the clustered index. You could have e.g. a non-clustered index at the back of a phone book which is keyed on (town, address); you can open the book at 'Hilditch, David' and find all the information for all of the 'Hilditch's right next to each other. Here the keys for the clustered index are (Lastname, first name).” This is someone tell on the website and that’s I learn from the web.