We've seen several cases in the field of $group operations which use $addToSet or $push to create very large arrays. This can lead to the memory footprint being large. In related ticket SERVER-44174, we added a check that will fail a query if a particular $addToSet or $push accumulator's memory consumption exceeds 100MB. However, understanding the memory consumption for a query can still be difficult since there are limited diagnostics in this area.

In order to help users understand which operators in the query plan are consuming memory, we should extend "executionStats" and "allPlansExecution" verbosity explain output for aggregate commands to include the maximum memory footprint attained at query runtime on a per-accumulator basis.