Multikey indexes for arrays in MongoDB - Step-by-Step Execution

Concept Flow - Multikey indexes for arrays

Start: Document with array field

↓

Create index on array field

↓

MongoDB extracts each array element

↓

Index stores each element as separate key

↓

Query uses index to find matching elements

↓

Return documents matching any array element

MongoDB creates a multikey index by indexing each element of an array field separately, allowing efficient queries on any array element.

Execution Sample

MongoDB

db.products.createIndex({ tags: 1 })
db.products.insertMany([
  { _id: 1, tags: ["red", "blue"] },
  { _id: 2, tags: ["blue", "green"] }
])
db.products.find({ tags: "blue" })

Create a multikey index on 'tags' array, insert documents with arrays, then query documents containing 'blue' in tags.

Execution Table

Step	Action	Index Keys Created	Query Condition	Documents Matched
1	Create index on 'tags' array field	empty
2	Insert document {_id:1, tags:["red","blue"]}	Index keys updated with 'red', 'blue'
3	Insert document {_id:2, tags:["blue","green"]}	Index keys updated with 'blue', 'green'
4	Query for tags: 'blue'		tags = 'blue'	Documents with _id 1 and 2
5	Query for tags: 'red'		tags = 'red'	Document with _id 1
6	Query for tags: 'green'		tags = 'green'	Document with _id 2
7	Query for tags: 'yellow'		tags = 'yellow'	No documents matched

💡 Queries stop after matching all documents with the requested array element or none if no match.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Index keys	empty	['red', 'blue']	['red', 'blue', 'blue', 'green']	['red', 'blue', 'blue', 'green']
Documents	empty	[{_id:1, tags:['red','blue']}]	[{_id:1, tags:['red','blue']}, {_id:2, tags:['blue','green']}]	[{_id:1, tags:['red','blue']}, {_id:2, tags:['blue','green']}]

Key Moments - 3 Insights

Why does the index have duplicate keys like 'blue' twice?

Because each array element is indexed separately, and multiple documents or the same document can have the same element, so 'blue' appears multiple times in the index as shown in execution_table rows 2-3.

Does the multikey index store the whole array or each element separately?

It stores each element separately as individual keys, not the whole array as one key. This is why queries can match any element efficiently, as shown in execution_table step 4.

What happens if we query for an element not in any array?

No documents are matched because the index has no keys for that element, as shown in execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are the index keys first created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Index Keys Created' column in execution_table row for Step 2.

According to variable_tracker, what is the state of 'Index keys' after Step 3?

A ['red', 'blue']

B ['blue', 'green']

C ['red', 'blue', 'blue', 'green']

D empty

💡 Hint

Look at the 'Index keys' row under 'After Step 3' in variable_tracker.

From execution_table, which query returns no documents?

A tags = 'yellow'

B tags = 'red'

C tags = 'green'

D tags = 'blue'

💡 Hint

Check the 'Documents Matched' column for queries in execution_table.

Concept Snapshot

Multikey indexes in MongoDB index each element of an array field separately.
This allows queries to efficiently find documents matching any array element.
Duplicates appear in the index if elements repeat across documents.
Queries use these keys to quickly locate matching documents.
If no element matches, no documents are returned.
Create with createIndex({ arrayField: 1 }).

Full Transcript

This visual execution trace shows how MongoDB creates and uses multikey indexes for array fields. First, an index is created on the array field. MongoDB extracts each element from arrays in documents and stores them as separate keys in the index. When documents are inserted, their array elements are added to the index keys. Queries on the array field use these keys to find documents containing the queried element. If the element exists in any document's array, those documents are returned. If not, no documents match. The index can have duplicate keys if multiple documents or arrays contain the same element. This process allows efficient querying of array contents in MongoDB.