Firestore index limitation for document

Question

According to Firestore documentation about index limitations:

Maximum sum of the following for a document: 20,000

• The number of single-field index entries

• The number of composite index entries

I can't understand what is that means. Is it:

• Single document will not be indexed more than 20,000 time for different indexes.
• OR, collection to have index should not be more than 20,000 document

Answer 1

Update: The limit is now 40,000 index entries per document.

This limit affects the maximum number of fields you can index for each document. For example, Cloud Firestore automatically creates an ascending single-field index entry and a descending single-field index entry for each string field. A document with only string fields and no composite indexes, could have a maximum of 20,000 fields.

We updated the docs to clarify: https://firebase.google.com/docs/firestore/query-data/index-overview#index_limitations