In this article Im going to talk about partial and sparse indexes in MongoDB® and Percona Server for MongoDB®. Ill show you how to use them, and look at cases where they can be helpful. Prior to discussing these indexes in MongoDB in detail, though, lets talk about an issue on a relational database like MySQL®. [&]

https://www.percona.com/blog/2018/12/19/using-partial-and-sparse-indexes-in-mongodb/