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A very common use of an aggregation is to count elements by group:

db.items.aggregate([ {$group: {_id: "$category", count: {$sum: 1}}}])

If there is an index over category, this pipeline could be covered and no document retrieval is necessary.

Since this is a quite common pattern, you may take it into account and make use of indexes.

is related to

SERVER-57518 16% performance loss switching from Count to CountDocuments

SERVER-20066 Query planner should consider index scans on empty query predicates

related to

SERVER-4507 aggregation: optimize $group to take advantage of sorted sequences

SERVER-69257 COUNT_SCAN plan not selected when unfiltered $group + $sum on _id performed

Assignee:: Unassigned
Reporter:: Nicolas Dascanio
Participants:: Asya Kamsky, David Storch, Kelsey Schubert, Kyle Suarez, Nicolas Dascanio
Votes:: 13 Vote for this issue
Watchers:: 29 Start watching this issue