Cardinality aggregation in Elasticsearch - Step-by-Step Execution

Concept Flow - Cardinality aggregation

Start Query

↓

Search Documents

↓

Extract Field Values

↓

Count Unique Values

↓

Return Cardinality Result

↓

End

The query searches documents, extracts values of a field, counts unique values, and returns the count.

Execution Sample

Elasticsearch

{
  "aggs": {
    "unique_users": {
      "cardinality": { "field": "user_id" }
    }
  }
}

This aggregation counts the number of unique user_id values in the documents.

Execution Table

Step	Action	Field Values Extracted	Unique Values Counted	Result
1	Start query execution	-	-	-
2	Search documents matching criteria	[user_id: 1, 2, 1, 3, 2]	-	-
3	Extract user_id field values	[1, 2, 1, 3, 2]	-	-
4	Count unique values	-	3 (values: 1, 2, 3)	-
5	Return cardinality result	-	-	3

💡 All documents processed, unique user_id count computed as 3

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
field_values	-	[1, 2, 1, 3, 2]	[1, 2, 1, 3, 2]	[1, 2, 3]	[1, 2, 3]
unique_count	-	-	-	3	3

Key Moments - 2 Insights

Why does the count of unique values ignore repeated user_id values?

Because cardinality aggregation counts only distinct values, duplicates like user_id 1 and 2 are counted once, as shown in step 4 of the execution_table.

What happens if the field has missing values in some documents?

Documents without the field are ignored in counting unique values, so only existing field values contribute to the cardinality, as implied in step 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, how many unique user_id values are counted?

A 3

B 5

C 2

D 4

💡 Hint

Check the 'Unique Values Counted' column at step 4 in the execution_table.

At which step are the field values extracted from documents?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column describing extraction in the execution_table.

If a document has no user_id field, how does it affect the unique count?

A It increases the unique count by 1

B It decreases the unique count by 1

C It does not affect the unique count

D It causes an error

💡 Hint

Refer to the key_moments explanation about missing field values.

Concept Snapshot

Cardinality aggregation counts unique values of a field.
Syntax: { "aggs": { "name": { "cardinality": { "field": "fieldname" } } } }
It returns the count of distinct values.
Duplicates are counted once.
Missing fields are ignored.

Full Transcript

Cardinality aggregation in Elasticsearch counts the number of unique values for a specified field. The process starts by executing a query to find documents. Then, it extracts the values of the chosen field from these documents. Next, it counts how many distinct values appear, ignoring duplicates. Finally, it returns this count as the result. For example, if user_id values are [1, 2, 1, 3, 2], the unique count is 3 because only values 1, 2, and 3 are distinct. Documents missing the field do not affect the count. This aggregation helps find how many different values exist in a dataset.