In mysql with InnoDB storage engine, When create a multi column index, such as (a, b, c), how many B+ tree/B tree will be create?
Explain the detail if you can.
InnoDB creates B+trees only. There is one tree for PRIMARY index. For every secondary index InnoDB creates one B+tee no matter how many columns are in the key.
See my slides with more examples https://twindb.com/efficient-use-of-indexes-in-mysql/