mysql composite index Why follow the principles of the leftmost

1,> mysql: multi-column indexes

　　 https://dev.mysql.com/doc/refman/5.7/en/multiple-column-indexes.html

1>, B + Tree;

      https://blog.csdn.net/Fmuma/article/details/80287924;

　 Summarized : B + tree will present data on the leaf nodes; and is continuous; so the composite index (a, b, c); at a, ab, abc three cases will be used a composite index;

　  Case situation (ac), resulting leaf node can not find the data in the order, so the composite index fail. If not entered a start index tree, or b Example C, you can not use the composite index.

 

    Composite index fail if the wrong conclusion, please correct me friends.