Index direction (ascending vs descending) in MongoDB - Quick Revision & Key Differences

Recall & Review

beginner

What does an ascending index mean in MongoDB?

An ascending index sorts the values from smallest to largest (like A to Z or 1 to 10). It helps MongoDB find data quickly in that order.

beginner

What does a descending index mean in MongoDB?

A descending index sorts the values from largest to smallest (like Z to A or 10 to 1). It helps MongoDB find data quickly in reverse order.

beginner

How do you specify an ascending index on the field 'age' in MongoDB?

You create it by using: { age: 1 } where 1 means ascending order.

beginner

How do you specify a descending index on the field 'score' in MongoDB?

You create it by using: { score: -1 } where -1 means descending order.

intermediate

Does the direction of an index (ascending or descending) affect query speed in MongoDB?

No, MongoDB uses indexes efficiently in both ascending and descending directions. The direction mainly affects sorting order, not speed.

In MongoDB, what does the value 1 mean when creating an index on a field?

A No order

B Descending order

C Ascending order

D Random order

1 means ascending order when defining an index in MongoDB.

What value do you use to create a descending index on a field in MongoDB?

A -1

B 1

C 0

D 2

-1 means descending order for an index in MongoDB.

If you want to sort query results from highest to lowest using an index, which index direction should you use?

A Ascending

B Descending

C Either ascending or descending

D No index needed

Descending index sorts values from highest to lowest.

Does MongoDB use ascending indexes to speed up descending sorts?

A Yes, but only for single-field indexes

B No, never

C Yes, always

D Only if the index is compound

MongoDB can use ascending indexes for descending sorts on single fields, but descending indexes are better for descending sorts.

Which of these is a valid MongoDB index definition for ascending order on 'name'?

A { name: -1 }

B { name: 2 }

C { name: 0 }

D { name: 1 }

1 means ascending order in MongoDB index definitions.

Explain the difference between ascending and descending index directions in MongoDB and when you might use each.

Think about sorting order and how indexes help find data faster.

How do you define an ascending and a descending index on a field in MongoDB? Give the syntax.

Remember the numbers 1 and -1 represent direction.

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