What do lookup, unwind and facet do?

Definition:

$lookup adds related data from another collection, $unwind expands an array into individual output records, and $facet runs multiple sub-pipelines on the same input. These operations change the shape or number of documents flowing through a pipeline. They are useful for joins and reports but can increase memory use and intermediate result size.

Key points:

• $lookup adds matching data from another collection.

• $unwind turns array items into separate output documents.

• $facet produces multiple grouped outputs, such as a page and a count.

• Unwinding an array of ten items can turn one document into ten; choose whether empty arrays should be preserved.

Interview point:

• Explain row multiplication and what it does to later counts or sums, not just the operator names.