Planned lessonMID-042-L04

Aggregation pipelines

On This Lesson 4 topics

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

compute summaries in the database.

A pipeline is a list of stages; each passes documents to the next: $match, $group, $sort, $project, $lookup.
Put $match first so it can use an index and shrink the data.
$group with $avg and $sum computes counts and averages.
Do the work in the database instead of loading documents into Java.
$lookup is a join; use sparingly, it is not a relational join engine.

Draft outline details

db.reviews.aggregate([
  { $match: { productId: 42 } },
  { $group: { _id: "$rating", count: { $sum: 1 } } },
  { $sort: { _id: -1 } }
])

Why put $match first?

Show answer

It uses an index and reduces the documents later stages process.

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