Unique Indexes
- 確保沒有其他documents也有同樣的值（field value）
- 在同一個documents內保持其唯一性（uniqueness）, 但不包含分片(not across shards)
- single value field 也適用在 array
A unique index ensures that the indexed fields do not store duplicate values; i.e. enforces uniqueness for the indexed fields. By default, MongoDB creates a unique index on the _id field during the creation of a collection.
注意：null也算是重複！
Sparse + Unique
- Unique enforced when value exists
- Null values allowed in multiple docs
- Missing field allowed in multiple docs