Single field index in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a single field index in MongoDB?

A single field index is an index created on one field of a MongoDB collection to speed up queries that filter or sort by that field.

beginner

How do you create a single field index on the field 'age' in MongoDB?

Use the command:
db.collection.createIndex({ age: 1 })
Here, 1 means ascending order index.

beginner

What does the value 1 or -1 mean when creating a single field index?

1 means the index is created in ascending order, and -1 means descending order. Both speed up queries but may affect sort order.

beginner

Why use a single field index?

It helps MongoDB find documents faster when searching or sorting by that field, improving query performance.

intermediate

Can a single field index speed up queries that filter on multiple fields?

No, single field indexes only help queries filtering or sorting on that one field. For multiple fields, use compound indexes.

Which MongoDB command creates a single field index on 'name' in ascending order?

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

B db.collection.createIndex({ name: 'asc' })

C db.collection.createIndex({ name: true })

D db.collection.createIndex({ name: 'ascending' })

The correct syntax uses 1 for ascending order: db.collection.createIndex({ name: 1 })

What does the number -1 mean when creating a single field index?

A Index in ascending order

B Index in descending order

C Index disabled

D Index on multiple fields

-1 means the index is created in descending order.

Which of these is true about single field indexes?

A They speed up queries filtering on one field

B They speed up queries filtering on multiple fields

C They slow down queries

D They are only for sorting, not filtering

Single field indexes speed up queries filtering or sorting on that one field.

If you want to speed up queries filtering on two fields, what should you use?

A Text index

B Single field index

C No index

D Compound index

Compound indexes cover multiple fields and speed up queries filtering on those fields.

What happens if you create a single field index on a field that is rarely used in queries?

A It will slow down all queries

B It will always speed up queries

C It may not improve performance much

D It deletes documents

Indexes help only if queries use that field; otherwise, the index adds overhead without benefit.

Explain what a single field index is and how it helps MongoDB queries.

Think about how an index is like a shortcut to find data faster.

Describe the difference between using 1 and -1 when creating a single field index.

Consider how sorting can be from smallest to largest or largest to smallest.

Practice

1. What is the main purpose of creating a single field index in MongoDB?

easy

A. To backup the database automatically

B. To speed up queries that filter or sort by one specific field

C. To store multiple fields in a single document

D. To create relationships between collections

Solution

1. Step 1: Understand what single field index does

   A single field index helps MongoDB find documents faster when searching or sorting by that one field.

2. Step 2: Compare options with this purpose

   Only To speed up queries that filter or sort by one specific field describes speeding up queries on one field, which matches the purpose of a single field index.

3. Final Answer:

   To speed up queries that filter or sort by one specific field -> Option B

4. Quick Check:

   Single field index = speed up queries on one field [OK]

Hint: Indexes speed up queries on the indexed field [OK]

Common Mistakes:

• Thinking indexes store multiple fields
• Confusing indexes with backups
• Believing indexes create relationships

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

easy

A. db.collection.createIndex({ 'age' => 1 })

B. db.collection.createIndex(age: 1)

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

D. db.collection.createIndex({ age: 'asc' })

Solution

1. Step 1: Recall correct syntax for createIndex

   The correct syntax uses an object with the field name as key and 1 for ascending order: createIndex({ field: 1 }).

2. Step 2: Check each option

   db.collection.createIndex({ age: 1 }) matches the correct syntax exactly. The other options use invalid syntax or wrong symbols.

3. Final Answer:

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

4. Quick Check:

   Correct syntax = createIndex({ field: 1 }) [OK]

Hint: Use curly braces and 1 for ascending index [OK]

Common Mistakes:

• Missing curly braces around the field
• Using wrong symbols like =>
• Using string 'asc' instead of 1

3. Given the following commands run on a MongoDB collection:

db.users.createIndex({ username: 1 })
db.users.find({ username: 'alice' })

What is the effect of the index on the find query?

medium

A. The query will use the index to find documents faster

B. The query will cause an error due to index usage

C. The query will ignore the index and scan all documents

D. The query will run slower because of the index

Solution

1. Step 1: Understand index effect on find queries

   Creating an index on 'username' helps MongoDB quickly locate documents matching that field.

2. Step 2: Analyze the find query

   The find query filters by username 'alice', so it will use the index to speed up the search.

3. Final Answer:

   The query will use the index to find documents faster -> Option A

4. Quick Check:

   Index on field = faster find queries on that field [OK]

Hint: Find queries on indexed field run faster [OK]

Common Mistakes:

• Assuming indexes slow down queries
• Thinking queries ignore indexes
• Believing index causes errors

4. You tried to create a single field index on email using this command:

db.users.createIndex(email: 1)

What is the problem with this command?

medium

A. Indexes cannot be created on the email field

B. Using 1 instead of 'asc' for ascending order

C. The field name should be a string in quotes

D. Missing curly braces around the field and order

Solution

1. Step 1: Check syntax for createIndex

   The argument to createIndex must be an object with curly braces: { field: 1 }.

2. Step 2: Identify error in given command

   The command lacks curly braces around email: 1, so it is invalid syntax.

3. Final Answer:

   Missing curly braces around the field and order -> Option D

4. Quick Check:

   createIndex needs { field: 1 } syntax [OK]

Hint: Always use curly braces for index fields [OK]

Common Mistakes:

• Omitting curly braces
• Using strings instead of numbers for order
• Thinking any field cannot be indexed

5. You want to speed up queries that sort users by their score field in descending order. Which command correctly creates a single field index to optimize this?

hard

A. db.users.createIndex({ score: -1 })

B. db.users.createIndex({ score: 1 })

C. db.users.createIndex({ score: 'desc' })

D. db.users.createIndex({ score: 0 })

Solution

1. Step 1: Understand index order for sorting

   To optimize descending sort, the index must be created with -1 for descending order.

2. Step 2: Evaluate each option

   db.users.createIndex({ score: -1 }) uses { score: -1 } which is correct. The option with { score: 1 } is ascending order, the option with { score: 'desc' } uses invalid string, the option with { score: 0 } uses 0 which is invalid.

3. Final Answer:

   db.users.createIndex({ score: -1 }) -> Option A

4. Quick Check:

   Descending index = { field: -1 } [OK]

Hint: Use -1 for descending index order [OK]

Common Mistakes:

• Using 1 for descending sort
• Using strings like 'desc' instead of numbers
• Using 0 which is invalid