How MongoDB indexes work (B-tree mental model) - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an index on the 'name' field in a MongoDB collection.

MongoDB

db.collection.createIndex({ name: [1] })

Drag options to blanks, or click blank then click option'

A1

B-1

C0

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 2 as index direction, which are invalid.

Confusing ascending (1) with descending (-1).

In MongoDB, creating an index with 1 means ascending order, which is the standard way to index a field.

2fill in blank

medium

Complete the code to find documents where the 'age' field is greater than 25 using an index.

MongoDB

db.collection.find({ age: { $[1]: 25 } })

Drag options to blanks, or click blank then click option'

Alt

Bne

Ceq

Dgt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gt, which finds smaller values.

Using $eq which looks for exact matches.

The $gt operator means 'greater than', which finds documents with 'age' greater than 25.

3fill in blank

hard

Fix the error in the index creation code to ensure it uses a valid B-tree index on 'score'.

MongoDB

db.collection.createIndex({ score: [1] })

Drag options to blanks, or click blank then click option'

A"text"

B2

C1

D"hashed"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 which is invalid for index direction.

Using 'text' or 'hashed' when a B-tree index is needed.

For a B-tree index, use 1 (ascending) or -1 (descending). Here, 1 is correct. 'text' and 'hashed' create different index types.

4fill in blank

hard

Fill both blanks to create a compound index on 'lastName' ascending and 'age' descending.

MongoDB

db.collection.createIndex({ lastName: [1], age: [2] })

Drag options to blanks, or click blank then click option'

A1

B-1

C0

D"text"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 which is invalid for index direction.

Using 'text' which creates a text index, not a B-tree index.

Compound indexes can mix ascending (1) and descending (-1) directions. Here, lastName is ascending, age is descending.

5fill in blank

hard

Fill all three blanks to create a filtered index on 'status' equals 'active' and sort by 'createdAt' descending.

MongoDB

db.collection.createIndex({ status: [1], createdAt: [2] }, { partialFilterExpression: { status: [3] } })

Drag options to blanks, or click blank then click option'

A1

B-1

C"active"

D"inactive"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong sort directions.

Using 'inactive' instead of 'active' in the filter.

The index sorts 'status' ascending (1) and 'createdAt' descending (-1). The filter only includes documents where status is 'active'.

Practice

1. What is the main purpose of an index in MongoDB using a B-tree structure?

easy

A. To encrypt data for security

B. To store data in a compressed format

C. To speed up search and sort operations on data

D. To backup the database automatically

Solution

1. Step 1: Understand what a B-tree index does

   A B-tree index organizes data so that searching and sorting can happen quickly.

2. Step 2: Identify the main benefit of indexing

   Indexes help queries find data faster by avoiding full collection scans.

3. Final Answer:

   To speed up search and sort operations on data -> Option C

4. Quick Check:

   Index = Faster search and sort [OK]

Hint: Indexes speed up queries by organizing data [OK]

Common Mistakes:

• Thinking indexes compress data
• Confusing indexes with backups
• Assuming indexes encrypt data

2. Which of the following is the correct syntax to create an ascending index on the field age in MongoDB?

easy

A. db.collection.createIndex({age: 1})

B. db.collection.createIndex(age: ascending)

C. db.collection.index({age: 'asc'})

D. db.collection.addIndex({age: 1})

Solution

1. Step 1: Recall MongoDB index creation syntax

   The correct method is createIndex with a document specifying field and order.

2. Step 2: Check the correct format for ascending order

   Ascending order is represented by 1, so {age: 1} is correct.

3. Final Answer:

   db.collection.createIndex({age: 1}) -> Option A

4. Quick Check:

   createIndex + {field: 1} = correct syntax [OK]

Hint: Use createIndex({field: 1}) for ascending index [OK]

Common Mistakes:

• Using addIndex instead of createIndex
• Passing string 'asc' instead of 1
• Omitting curly braces around field

3. Given a collection with an index on score, what will be the order of documents returned by this query?
db.collection.find().sort({score: 1})

medium

A. Documents sorted by score in descending order

B. Documents sorted by insertion order

C. Documents in random order

D. Documents sorted by score in ascending order

Solution

1. Step 1: Understand the sort parameter

   The sort({score: 1}) means sort by score ascending (lowest to highest).

2. Step 2: Recognize index usage

   Since there is an index on score, MongoDB uses it to return documents in ascending order efficiently.

3. Final Answer:

   Documents sorted by score in ascending order -> Option D

4. Quick Check:

   sort({score: 1}) = ascending order [OK]

Hint: sort({field: 1}) means ascending order [OK]

Common Mistakes:

• Confusing 1 with descending order
• Assuming no order without explicit index
• Thinking insertion order is default

4. You created a unique index on the field email but still see duplicate emails in your collection. What is the likely cause?

medium

A. The index was created with ascending order instead of unique

B. The unique index was created after duplicates existed

C. MongoDB does not enforce uniqueness on indexes

D. The field email is missing in some documents

Solution

1. Step 1: Understand unique index behavior

   Unique indexes prevent duplicates only for new inserts after index creation.

2. Step 2: Consider existing duplicates

   If duplicates existed before creating the unique index, MongoDB allows them unless you drop or clean data first.

3. Final Answer:

   The unique index was created after duplicates existed -> Option B

4. Quick Check:

   Unique index blocks new duplicates, not old ones [OK]

Hint: Unique index blocks new duplicates, not existing ones [OK]

Common Mistakes:

• Thinking ascending order affects uniqueness
• Believing MongoDB ignores unique constraints
• Assuming missing fields cause duplicates

5. You want to optimize queries filtering by category and sorting by price. Which index is best to improve performance?

hard

A. A compound index on {category: 1, price: 1}

B. Two separate indexes: one on category, one on price

C. A unique index on price

D. No index needed if collection is small

Solution

1. Step 1: Understand compound index benefits

   A compound index on {category: 1, price: 1} supports queries filtering by category and sorting by price efficiently.

2. Step 2: Compare with separate indexes

   Separate indexes cannot optimize combined filter and sort as well as a compound index.

3. Step 3: Consider uniqueness and collection size

   Unique index on price is irrelevant here; small collections may not need indexes but that is not optimization.

4. Final Answer:

   A compound index on {category: 1, price: 1} -> Option A

5. Quick Check:

   Compound index = best for filter + sort [OK]

Hint: Use compound index for filter and sort fields together [OK]

Common Mistakes:

• Using separate indexes instead of compound
• Confusing unique index with performance index
• Ignoring index for small collections