Mongodb huge date index size

Question

I have a mongodb database storing documents with a publication date and an end date. For my request i need to create an index with both on those dates.

My collection size is about 17GB for a total of 6 Millions documents.

When i create my index, the index size is about 600MB ... but when i use it intensively, the size get up to 70GB ... 6 times more than the documents themselves oO

Am i doing something wrong ? Is there some special considerations with date fields ? (I only have problem with date indexes).

Note: I suspected my dates to be "too precises" to be indexed, so i rounded them to the nearest hour ... without any index size decrease.

Answer 1

Okay i've found the reason of this really strange bug. I am putting it here since it can happens to others.

1. I'm using moment.js for managing dates on node.js
2. I'm using mongodb driver directly (not mongoose)
3. (I forgot to precise it in my question), i make an update on my dates in my intensive scripts.
4. I update those dates with a moment object instead of a pure javascript date making mongodb try to index every bit of subpath i think which is responsible of this huge index size.