$bucket and $bucketAuto for distribution in MongoDB - Step-by-Step Execution

Concept Flow - $bucket and $bucketAuto for distribution

Start with data array

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$bucket or $bucketAuto stage

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Define boundaries or bucket count

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Group data into buckets

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Count documents per bucket

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Output buckets with counts

Data flows into a bucket stage where it is grouped into ranges or automatic buckets, then counts per bucket are output.

Execution Sample

MongoDB

db.sales.aggregate([
  {$bucket: {
    groupBy: "$price",
    boundaries: [0, 50, 100, 200],
    default: "Other",
    output: {count: {$sum: 1}}
  }}
])

Groups sales documents by price ranges 0-50, 50-100, 100-200, counting how many fall in each.

Execution Table

Step	Document price	Bucket boundaries	Bucket assigned	Count update
1	23	[0,50,100,200]	0-50	count=1
2	75	[0,50,100,200]	50-100	count=1
3	150	[0,50,100,200]	100-200	count=1
4	45	[0,50,100,200]	0-50	count=2
5	210	[0,50,100,200]	Other	count=1
6	90	[0,50,100,200]	50-100	count=2
7	180	[0,50,100,200]	100-200	count=2
Exit	N/A	N/A	N/A	All documents processed

💡 All documents processed and assigned to buckets or default.

Variable Tracker

Bucket	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
0-50	0	1	1	1	2	2	2	2	2
50-100	0	0	1	1	1	1	2	2	2
100-200	0	0	0	1	1	1	1	2	2
Other	0	0	0	0	0	1	1	1	1

Key Moments - 3 Insights

Why does the document with price 210 go to 'Other' bucket?

Because 210 is outside the defined boundaries [0,50,100,200], so it is assigned to the 'default' bucket 'Other' as shown in execution_table step 5.

How does $bucket decide which bucket a document belongs to?

$bucket compares the document's value to the boundaries and assigns it to the bucket where value >= lower boundary and < upper boundary, as seen in steps 1-7.

What happens if a document's value equals a boundary?

It belongs to the bucket starting at that boundary (inclusive), for example price 50 would go to bucket 50-100.

Visual Quiz - 3 Questions

Look at the execution_table, what is the count for bucket '50-100' after step 6?

A 2

B 1

C 3

D 0

💡 Hint

Check the 'count update' column for step 6 and the variable_tracker row for '50-100' after step 6.

At which step does the 'Other' bucket first get a count?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at execution_table row where 'Bucket assigned' is 'Other' and see the count update.

If the boundaries changed to [0, 100, 200], how would the bucket for price 75 change?

A It would be in 100-200 bucket

B It would be in Other bucket

C It would be in 0-100 bucket

D It would not be assigned

💡 Hint

Check how boundaries define buckets and where 75 fits between 0 and 100.

Concept Snapshot

$bucket groups data by defined boundaries.
Values >= lower and < upper boundary go to that bucket.
Documents outside boundaries go to 'default' bucket.
$bucketAuto creates equal-sized buckets automatically.
Both count documents per bucket.

Full Transcript

This visual execution shows how MongoDB's $bucket stage groups documents by ranges. Each document's value is compared to boundaries. If it fits between two boundaries, it goes to that bucket. If not, it goes to the default bucket. Counts per bucket increase as documents are processed. This helps understand how data distribution works in aggregation.