https://www.mongodb.com/docs/manual/core/aggregation-pipeline/
    https://www.mongodb.com/docs/manual/core/aggregation-pipeline-optimization/

https://www.mongodb.com/docs/manual/reference/aggregation/

https://www.mongodb.com/docs/manual/meta/aggregation-quick-reference/
https://www.mongodb.com/docs/manual/reference/operator/aggregation/interface/
    commands: aggregate, count, distinct, mapReduce

https://www.mongodb.com/docs/manual/reference/operator/aggregation-pipeline/
    stages
    https://www.mongodb.com/docs/manual/core/map-reduce/
    ! deprecated from 5.0
https://www.mongodb.com/docs/manual/reference/operator/aggregation/
https://www.mongodb.com/docs/manual/reference/aggregation-variables/
    $$<var-name>
https://www.mongodb.com/docs/manual/reference/sql-aggregation-comparison/

https://www.mongodb.com/docs/manual/tutorial/update-documents-with-aggregation-pipeline/

misc
    https://www.mongodb.com/docs/manual/reference/operator/aggregation/accumulator/

from-java-driver
    https://www.mongodb.com/docs/drivers/java/sync/current/fundamentals/aggregation/
    https://www.mongodb.com/docs/drivers/java/sync/current/fundamentals/builders/aggregates/

2015
https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-2-of-3-worked-examples
https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-1-of-3-introduction
    $lookup (left-outer equi-join)
    $match, $project, $lookup, $group, ..., geoNear, unwind, sample, sort, skip(limit number of docs), redact, out

books
https://www.practical-mongodb-aggregations.com/front-cover.html
mongosh
https://www.mongodb.com/docs/manual/reference/method/db.collection.aggregate