When building an index with the  { sparse : true } option, only documents that have the indexed field present will have any index entries, even if the indexed field contains a null.

If a time-series buckets control field contains information about the indexed field, then at least one measurement contains the indexed field and the bucket will be indexed. Vice-versa, if no measurements contain the indexed field, then there will be no information in the control field and the bucket will not be indexed.