Partial

A partial index on a collection is an index that applies only to the documents that satisfy the partialFilterExpression query.

We'll use our familiar books collection:

> db.books.createIndex(
  { price: 1, name: 1 },
  { partialFilterExpression: { price: { $gt: 30 } } }
)

Using this, we can have an index just for the books that have a price greater than 30. The advantage of partial indexes is that they are more lightweight in creation and maintenance and use less storage.

The partialFilterExpression filter, supports the following operators:

Equality expressions (that is, field: value or using the $eq operator)
The $exists: true expression
The $gt, $gte, $lt, and $lte expressions
$type expressions
The $and operator at the top-level only

Partial indexes will only be used if the query can be satisfied as a whole by the partial index.

If our query matches, or is more restrictive than, the partialFilterExpression filter then the partial index will be used. If the results may not be contained in the partial index then the index will be totally ignored.

The partialFilterExpression does not need to be part of the sparse index fields. The following index is a valid sparse index:
> db.books.createIndex({ name: 1 },{ partialFilterExpression: { price: { $gt: 30 } } })

To use this partial index, however, we need to query for both name and price greater than 30 or more.

Prefer partial to sparse indexes. Sparse indexes offer a subset of the functionality offered by partial indexes. Partial indexes were introduced in MongoDB 3.2 so if you have sparse indexes from earlier versions it may be a good idea to upgrade them. The _id field cannot be part of a partial index. A shard key index cannot be a partial index. partialFilterExpression cannot be combined with the sparse option.

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