Why does index size not decrease when deleting documents from a large mongodb collection?

Question

Environment: MongoDB 4.2.7, Centos7

I have a collection with about 500 million document and an index that is about 8GB. If I delete half the documents, I would expect the indexSize to decrease by about 50%. But I don't. Why does it not go down? Is there a way to compact the index?

Just to verify that the index should be smaller, I copied 50% of the documents to a brand new mongodb instance and created the index there. The index is indeed about 50% less.

Answer 1

Well, I just found the answer to my own question. MongoDB actually has a compact command:

https://docs.mongodb.com/manual/reference/command/compact/

This should rebuild the indexes let me reclaim the free space.