$avg accumulator in MongoDB - Step-by-Step Execution

Concept Flow - $avg accumulator

Start with documents

↓

Group documents by key

↓

Extract values of field to average

↓

Sum values and count documents

↓

Calculate average = sum / count

↓

Output average per group

The $avg accumulator groups documents, sums values, counts them, then divides sum by count to get average.

Execution Sample

MongoDB

db.sales.aggregate([
  { $group: { _id: "$item", avgQuantity: { $avg: "$quantity" } } }
])

Calculate average quantity sold per item from sales documents.

Execution Table

Step	Document	Group _id	Quantity	Sum	Count	Average
1	{item: 'apple', quantity: 5}	apple	5	5	1	5
2	{item: 'banana', quantity: 3}	banana	3	3	1	3
3	{item: 'apple', quantity: 7}	apple	7	12	2	6
4	{item: 'banana', quantity: 2}	banana	2	5	2	2.5
5	{item: 'apple', quantity: 4}	apple	4	16	3	5.33
6	No more documents	-	-	-	-	-

💡 All documents processed; averages computed as sum/count per group.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
sum_apple	0	5	5	12	12	16	16
count_apple	0	1	1	2	2	3	3
avg_apple	-	5	5	6	6	5.33	5.33
sum_banana	0	0	3	3	5	5	5
count_banana	0	0	1	1	2	2	2
avg_banana	-	-	3	3	2.5	2.5	2.5

Key Moments - 2 Insights

Why does the average for 'apple' change after each document?

Because each new 'apple' document updates sum and count, changing average as sum/count (see execution_table rows 1,3,5).

What happens if a group has only one document?

The average equals that document's value since sum and count are both 1 (see banana at step 2).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the average quantity for 'banana'?

A 3

B 2.5

C 5

D 1

💡 Hint

Check the 'Average' column for 'banana' at step 4 in execution_table.

At which step does the average for 'apple' first become less than 6?

A Step 5

B Step 3

C Step 1

D Step 2

💡 Hint

Look at the 'Average' values for 'apple' in execution_table rows 3 and 5.

If a new document {item: 'banana', quantity: 5} is added after step 5, what happens to banana's average?

A It stays the same

B It decreases

C It increases

D It becomes zero

💡 Hint

Adding quantity 5 increases sum and count for banana, so average = new sum / new count.

Concept Snapshot

$avg accumulator in MongoDB groups documents by a key.
It sums the values of a field and counts documents.
Then it divides sum by count to get average.
Syntax: { $avg: "$fieldName" } inside $group stage.
Useful to find average values per group in collections.

Full Transcript

The $avg accumulator in MongoDB is used inside an aggregation pipeline's $group stage. It groups documents by a specified key, then calculates the average of a numeric field within each group. This happens by summing the field values and counting the documents, then dividing sum by count. For example, grouping sales by item and averaging quantity sold per item. The execution table shows step-by-step how each document updates sums, counts, and averages. Beginners often wonder why averages change after each document; it's because sums and counts update incrementally. Also, if a group has only one document, average equals that document's value. Adding more documents changes sums and counts, thus changing averages accordingly.