Single field index in MongoDB - Step-by-Step Execution

Concept Flow - Single field index

Create collection

↓

Define index on one field

↓

Insert documents

↓

Query uses index

↓

Faster search on that field

↓

Return matching documents

This flow shows how creating an index on a single field helps MongoDB find documents faster when querying that field.

Execution Sample

MongoDB

db.users.createIndex({ age: 1 })
db.users.insertMany([
  { name: "Alice", age: 25 },
  { name: "Bob", age: 30 },
  { name: "Carol", age: 25 }
])
db.users.find({ age: 25 })

Create an index on the 'age' field, insert documents, then query documents where age is 25.

Execution Table

Step	Action	Details	Result
1	Create index	Index on field 'age' ascending	Index created on 'age'
2	Insert documents	Insert Alice (25), Bob (30), Carol (25)	3 documents added
3	Query documents	Find documents where age = 25	Uses index to find Alice and Carol
4	Return results	Documents matching age=25	[{name: 'Alice', age: 25}, {name: 'Carol', age: 25}]
5	End	No more steps	Query complete

💡 Query completes after returning all documents with age 25 using the index

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Index on age	None	Created	Created	Used in query	Used
Documents in collection	Empty	Empty	3 documents	3 documents	3 documents
Query result	None	None	None	2 documents found	2 documents returned

Key Moments - 2 Insights

Why does the query run faster after creating the index?

Because the index on 'age' lets MongoDB quickly locate documents with age 25 without scanning all documents, as shown in execution_table step 3.

Does the index store the whole document?

No, the index stores the values of the 'age' field and pointers to documents, so it is smaller and faster to search, as implied in step 1 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 3?

A Index is created

B Documents are inserted

C Documents with age 25 are found

D Query ends

💡 Hint

Check the 'Result' column in row with Step 3 in execution_table

At which step are documents added to the collection?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for document insertion

If we did not create the index, how would the query step change?

A Query would scan all documents (slower)

B Query would use the index anyway

C Query would fail

D Query would return no results

💡 Hint

Indexes speed up queries by avoiding full scans, see key_moments explanation

Concept Snapshot

Single field index in MongoDB:
- Created with createIndex({field: 1})
- Speeds up queries filtering on that field
- Index stores field values + pointers
- Query uses index to find matches fast
- Improves read performance without changing data

Full Transcript

This visual execution shows how creating a single field index in MongoDB works. First, an index is created on the 'age' field. Then, documents are inserted into the collection. When a query searches for documents where age equals 25, MongoDB uses the index to quickly find matching documents instead of scanning all documents. The result is faster query performance. The index stores only the 'age' values and pointers to documents, not the whole documents. This example helps beginners see how indexes improve search speed step-by-step.