MongoDB provides a compact operation for removing (or re-allocating) padding for all cases in a collection. We have a use-case where it would be nice to compact a collection, but not to re-allocate padding for a select subset of the documents. This subset is easily described by a query.

This feature would be a simple additional field to the compact operation. For example, the docs might include:

field: spec
type: object
description: A MongoDB query indicating which records to be compacted.

And the docs might state, "If a spec is provided, docs that don't match the spec may be moved but will retain their current padding (allocation size on disk)."