Why not B+-Tree MongoDB

Question

Do anyone know why MongoDB use B-Tree but not B+-Tree?

As I know most DBMS use B+-Tree. Are there any special reason for MongoDB to use B-Tree?

thanks.

Answer 1

The question has confused me when I learned B/B+.Now I get some answers:

1. mysql is a relational db, while mongo isn't. It means that we do more range operations in mysql(such as select * from xx where id > 23). So the advantages of B+ tree aren't obvious.
2. B tree's best search time is O(1), while B+ is always O(log n). So when search some 'hot' data. B tree has a better performance.(however if you always search the data in the leaf when using B tree, it takes more disk IO times so it may not perform well.)

In my opinion, it depends on the details how mongo implements.But I am not a Mongo developer. :D

Answer 2

MongoDB uses B+ by WiredTiger default storage engine.

1、https://docs.mongodb.com/manual/core/wiredtiger/

Starting in MongoDB 3.2, the WiredTiger storage engine is the default storage engine.

2、http://source.wiredtiger.com/3.2.1/tune_page_size_and_comp.html

WiredTiger maintains a table's data in memory using a data structure called a B-Tree ( B+ Tree to be specific), referring to the nodes of a B-Tree as pages. Internal pages carry only keys. The leaf pages store both keys and values.