Index direction (ascending vs descending) in MongoDB - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an ascending index on the field 'age'.

MongoDB

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

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

A0

B-1

Ctrue

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1 instead of 1 for ascending index.

Using 0 or true which are invalid index directions.

In MongoDB, an ascending index is created by specifying 1 as the index direction.

2fill in blank

medium

Complete the code to create a descending index on the field 'score'.

MongoDB

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

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

A-1

B0

C1

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of -1 for descending index.

Using 0 or boolean values which are invalid.

Descending indexes in MongoDB use -1 as the direction.

3fill in blank

hard

Fix the error in the index creation code to specify descending order on 'date'.

MongoDB

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

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

A1

B-1

Ctrue

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of -1.

Using boolean or zero values.

Descending order requires -1, not 1 or boolean values.

4fill in blank

hard

Fill both blanks to create a compound index with 'name' ascending and 'score' descending.

MongoDB

db.collection.createIndex({ name: [1], score: [2] })

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

A1

B-1

C0

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or true as index directions.

Mixing up ascending and descending values.

Compound indexes can mix ascending (1) and descending (-1) directions.

5fill in blank

hard

Fill all three blanks to create a compound index with 'category' descending, 'price' ascending, and 'rating' descending.

MongoDB

db.collection.createIndex({ category: [1], price: [2], rating: [3] })

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

A1

B-1

C0

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or true as index directions.

Confusing ascending and descending values.

Compound indexes can combine ascending (1) and descending (-1) directions as needed.

Practice

1. What does the index direction 1 mean in MongoDB indexes?

easy

A. Descending order

B. No order

C. Ascending order

D. Random order

Solution

1. Step 1: Understand index direction values

   In MongoDB, index direction 1 means ascending order, and -1 means descending order.

2. Step 2: Match the value to the meaning

   Since the question asks about 1, it corresponds to ascending order.

3. Final Answer:

   Ascending order -> Option C

4. Quick Check:

   Index direction 1 = Ascending order [OK]

Hint: 1 means ascending, -1 means descending [OK]

Common Mistakes:

• Confusing 1 with descending order
• Thinking 1 means no order
• Mixing up ascending and descending

2. Which of the following is the correct syntax to create a descending 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: 'desc' })

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

Solution

1. Step 1: Recall MongoDB index syntax for direction

   MongoDB uses 1 for ascending and -1 for descending in index creation.

2. Step 2: Identify the correct descending syntax

   The correct way to create a descending index on age is { age: -1 }.

3. Final Answer:

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

4. Quick Check:

   Descending index uses -1 [OK]

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

Common Mistakes:

• Using string values like 'desc' instead of -1
• Using 1 which means ascending
• Incorrect object syntax

3. Given the index { name: 1, score: -1 }, which query will use this index efficiently?

medium

A. db.collection.find().sort({ name: 1, score: -1 })

B. db.collection.find().sort({ name: -1, score: 1 })

C. db.collection.find().sort({ score: -1, name: 1 })

D. db.collection.find().sort({ name: 1 })

Solution

1. Step 1: Understand compound index direction

   The index sorts name ascending and score descending.

2. Step 2: Match query sort order with index

   Only the query sorting { name: 1, score: -1 } matches the index directions exactly, so it uses the index efficiently.

3. Final Answer:

   db.collection.find().sort({ name: 1, score: -1 }) -> Option A

4. Quick Check:

   Sort matches index directions exactly [OK]

Hint: Sort order must match index directions exactly [OK]

Common Mistakes:

• Reversing sort directions
• Changing field order in sort
• Using partial sort that doesn't match index

4. You created an index with { date: 1, price: -1 } but your query sorts by { price: -1, date: 1 }. What is the problem?

medium

A. The index fields order does not match the sort order

B. The index direction values are invalid

C. MongoDB does not support mixed ascending and descending indexes

D. The query syntax is incorrect

Solution

1. Step 1: Check index field order

   The index is created with fields date then price.

2. Step 2: Compare query sort order

   The query sorts by price first, then date, which does not match the index field order.

3. Final Answer:

   The index fields order does not match the sort order -> Option A

4. Quick Check:

   Field order in sort must match index order [OK]

Hint: Index and sort field order must match exactly [OK]

Common Mistakes:

• Assuming direction values are invalid
• Thinking MongoDB disallows mixed directions
• Blaming query syntax instead of order

5. You want to optimize queries that sort by category ascending and then by rating descending. Which index should you create?

hard

A. db.collection.createIndex({ rating: -1, category: 1 })

B. db.collection.createIndex({ category: -1, rating: 1 })

C. db.collection.createIndex({ rating: 1, category: -1 })

D. db.collection.createIndex({ category: 1, rating: -1 })

Solution

1. Step 1: Identify query sort order

   The queries sort by category ascending and rating descending.

2. Step 2: Create matching compound index

   The index must have category with direction 1 and rating with direction -1 in that order.

3. Final Answer:

   db.collection.createIndex({ category: 1, rating: -1 }) -> Option D

4. Quick Check:

   Index matches query sort directions and order [OK]

Hint: Match index fields and directions to query sort exactly [OK]

Common Mistakes:

• Reversing field order in index
• Swapping ascending and descending directions
• Creating index on wrong fields