$bucket and $bucketAuto for distribution in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Using $bucket to group ages into fixed ranges

Given a collection people with documents containing an age field, what is the output of this aggregation pipeline?

db.people.aggregate([
  {
    $bucket: {
      groupBy: "$age",
      boundaries: [0, 20, 40, 60],
      default: "Other",
      output: { count: { $sum: 1 } }
    }
  }
])

MongoDB

db.people.aggregate([
  {
    $bucket: {
      groupBy: "$age",
      boundaries: [0, 20, 40, 60],
      default: "Other",
      output: { count: { $sum: 1 } }
    }
  }
])

A [ { _id: 0, count: 3 }, { _id: 20, count: 5 }, { _id: 60, count: 2 }, { _id: "Other", count: 1 } ]

B [ { _id: 0, count: 3 }, { _id: 20, count: 5 }, { _id: 40, count: 2 } ]

C [ { _id: 0, count: 3 }, { _id: 20, count: 5 }, { _id: 40, count: 2 }, { _id: 60, count: 1 } ]

D [ { _id: 0, count: 3 }, { _id: 20, count: 5 }, { _id: 40, count: 2 }, { _id: "Other", count: 1 } ]

💡 Hint

Remember that $bucket groups values into the specified boundaries and uses the default bucket for values outside.

The $bucket stage groups ages into ranges [0-20), [20-40), [40-60). Values outside these ranges go to the 'Other' bucket. The output counts how many documents fall into each bucket.

❓ query_result

intermediate

Using $bucketAuto to create automatic buckets

Consider a collection sales with a price field. What does this aggregation output?

db.sales.aggregate([
  {
    $bucketAuto: {
      groupBy: "$price",
      buckets: 3,
      output: { count: { $sum: 1 } }
    }
  }
])

MongoDB

db.sales.aggregate([
  {
    $bucketAuto: {
      groupBy: "$price",
      buckets: 3,
      output: { count: { $sum: 1 } }
    }
  }
])

A [ { _id: { min: 10, max: 30 }, count: 4 }, { _id: { min: 31, max: 60 }, count: 3 }, { _id: { min: 61, max: 90 }, count: 3 } ]

B [ { _id: { min: 10, max: 40 }, count: 5 }, { _id: { min: 41, max: 70 }, count: 3 }, { _id: { min: 71, max: 90 }, count: 2 } ]

C [ { _id: { min: 10, max: 33 }, count: 3 }, { _id: { min: 34, max: 66 }, count: 4 }, { _id: { min: 67, max: 90 }, count: 3 } ]

D [ { _id: { min: 10, max: 25 }, count: 2 }, { _id: { min: 26, max: 55 }, count: 5 }, { _id: { min: 56, max: 90 }, count: 3 } ]

💡 Hint

The $bucketAuto stage divides data into roughly equal groups based on the data distribution.

$bucketAuto automatically calculates boundaries to create buckets with roughly equal counts. The output shows the ranges and counts per bucket.

📝 Syntax

advanced

Identify the syntax error in $bucket usage

Which option contains a syntax error in the $bucket stage?

MongoDB

db.collection.aggregate([
  {
    $bucket: {
      groupBy: "$score",
      boundaries: [0, 50, 100],
      output: { count: { $sum: 1 } }
    }
  }
])

A { $bucket: { groupBy: "$score", boundaries: [0, 50, 100], output: { count: { $sum: 1 } } } }

B { $bucket: { groupBy: "$score", boundaries: [0, 50], output: { count: { $sum: 1 } } } }

C { $bucket: { groupBy: "$score", boundaries: [0, 50, 100], default: 0, output: { count: { $sum: 1 } } } }

D { $bucket: { groupBy: "$score", boundaries: [0, 50, 100], default: "Other", output: { count: { $sum: 1 } } } }

💡 Hint

Check if the boundaries array has enough values for buckets.

The boundaries array must have at least two values and define N+1 boundaries for N buckets. Option B has only two boundaries, which is invalid for creating buckets.

❓ optimization

advanced

Optimizing $bucketAuto for large datasets

Which option best improves performance when using $bucketAuto on a large collection with millions of documents?

A Add an index on the field used in groupBy before running $bucketAuto.

B Increase the number of buckets to the maximum allowed to reduce processing time.

C Use $bucket instead of $bucketAuto to avoid automatic boundary calculation.

D Run $bucketAuto without any indexes and limit the output to 10 documents.

💡 Hint

Indexes help queries and aggregations filter and group data faster.

Adding an index on the field used in groupBy helps MongoDB quickly access and sort data, improving $bucketAuto performance on large datasets.

🧠 Conceptual

expert

Difference between $bucket and $bucketAuto

Which statement correctly describes a key difference between $bucket and $bucketAuto in MongoDB aggregation?

A $bucket requires predefined boundaries, while $bucketAuto calculates boundaries automatically to create roughly equal-sized buckets.

B $bucketAuto requires predefined boundaries, while $bucket calculates boundaries automatically based on data distribution.

C Both $bucket and $bucketAuto require predefined boundaries but differ in output formatting.

D $bucket and $bucketAuto are identical in functionality but differ in syntax only.

💡 Hint

Think about how boundaries are set in each stage.

$bucket needs you to specify exact boundaries for buckets. $bucketAuto figures out boundaries itself to balance bucket sizes.