Compound index and field order in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a compound index in MongoDB?

A compound index is an index on multiple fields in a MongoDB document. It helps speed up queries that filter or sort by those fields together.

intermediate

Why does the order of fields matter in a compound index?

The order matters because MongoDB uses the index from left to right. Queries must match the prefix of the index fields to use it efficiently.

intermediate

Given a compound index on { age: 1, name: 1 }, which queries can use this index?

Queries filtering by age alone or by both age and name can use this index. Queries filtering only by name cannot use it efficiently.

intermediate

How does MongoDB use a compound index for sorting?

MongoDB can use a compound index to sort results if the sort order matches the index field order and direction.

advanced

What happens if you reverse the field order in a compound index?

Reversing the field order creates a different index. Queries optimized for the original order may not benefit from the reversed index.

Which of these is a valid compound index in MongoDB?

A { age }

B { age: 'ascending', name: 'descending' }

C { name: 1 }

D { age: 1, name: 1 }

Compound indexes specify multiple fields with sort order (1 for ascending, -1 for descending). Option D is correct.

If you have a compound index on { age: 1, name: 1 }, which query uses the index efficiently?

A { age: 25 }

B { name: 'Alice' }

C { name: 'Alice', age: 25 }

D { city: 'NY' }

Queries filtering on the first field (age) or both fields can use the index. Filtering only by name cannot.

What is the effect of field order in a compound index?

A It does not matter

B MongoDB uses fields from left to right for queries

C Only the last field is used

D Fields are used randomly

MongoDB uses the compound index fields in the order they are defined, from left to right.

Can a compound index be used to sort query results?

A Yes, if the sort matches the index field order

B No, indexes are only for filtering

C Only single-field indexes can sort

D Sorting is done after fetching all results

Compound indexes can support sorting if the sort order matches the index field order.

What happens if you create a compound index with reversed field order?

A MongoDB merges both indexes

B It creates the same index

C It creates a different index

D It causes an error

Reversing the field order creates a different compound index with different query optimization.

Explain what a compound index is and why the order of fields matters in MongoDB.

Think about how MongoDB uses indexes to speed up queries.

Describe how MongoDB uses a compound index to optimize queries and sorting.

Consider both filtering and sorting when using compound indexes.

Practice

1. What does a compound index in MongoDB do?

easy

A. Speeds up queries that filter or sort on multiple fields together

B. Stores data in multiple collections

C. Automatically backs up the database

D. Encrypts data for security

Solution

1. Step 1: Understand what an index is

   An index helps MongoDB find data faster by organizing it.

2. Step 2: Understand compound index purpose

   A compound index uses more than one field to speed up queries that use those fields together.

3. Final Answer:

   Speeds up queries that filter or sort on multiple fields together -> Option A

4. Quick Check:

   Compound index = Speeds up multi-field queries [OK]

Hint: Compound index = multiple fields speed up queries [OK]

Common Mistakes:

• Thinking it stores data in multiple collections
• Confusing with backup or encryption features
• Believing it only indexes one field

2. Which is the correct way to create a compound index on fields age ascending and score descending in MongoDB?

easy

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

B. db.collection.createIndex({ age: -1, score: 1 })

C. db.collection.createIndex(["age", "score"])

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

Solution

1. Step 1: Recall syntax for createIndex

   MongoDB uses an object with field names and 1 or -1 for ascending/descending.

2. Step 2: Match field order and directions

   Age ascending is 1, score descending is -1, so { age: 1, score: -1 } is correct.

3. Final Answer:

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

4. Quick Check:

   Use 1/-1 for asc/desc in object [OK]

Hint: Use {field: 1/-1} object for compound index [OK]

Common Mistakes:

• Using array instead of object for fields
• Using strings like 'asc' or 'desc' instead of 1/-1
• Mixing ascending and descending incorrectly

3. Given a compound index on { city: 1, rating: -1 }, which query will use the index efficiently?

medium

A. db.collection.find({ rating: { $gt: 4 } })

B. db.collection.find({ city: 'NYC' }).sort({ rating: 1 })

C. db.collection.find({ city: 'NYC' }).sort({ rating: -1 })

D. db.collection.find({ rating: -1, city: 'NYC' })

Solution

1. Step 1: Understand index field order and directions

   The index is on city ascending, then rating descending.

2. Step 2: Check query filter and sort usage

   Query B filters on city (first field) and sorts rating descending (matching index direction).

3. Final Answer:

   db.collection.find({ city: 'NYC' }).sort({ rating: -1 }) -> Option C

4. Quick Check:

   Filter first field, sort second field matching index [OK]

Hint: Filter first field, sort second field matching index order [OK]

Common Mistakes:

• Sorting rating ascending when index is descending
• Filtering only on rating without city
• Ignoring field order in index

4. You created a compound index { a: 1, b: 1 } but your query { b: 5 } is slow. What is the likely issue?

medium

A. MongoDB does not support compound indexes

B. The query does not filter on the first indexed field a

C. The index fields are in wrong order; should be { b: 1, a: 1 }

D. The index must use -1 for descending order

Solution

1. Step 1: Recall how compound indexes work

   Compound indexes work best when queries filter on the first field in the index.

2. Step 2: Analyze query filter

   The query filters only on b, the second field, so index on a then b is not used efficiently.

3. Final Answer:

   The query does not filter on the first indexed field a -> Option B

4. Quick Check:

   Filter must include first index field [OK]

Hint: Query must filter first indexed field to use compound index [OK]

Common Mistakes:

• Thinking MongoDB doesn't support compound indexes
• Believing field order doesn't matter
• Assuming descending order is always required

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

hard

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

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

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

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

Solution

1. Step 1: Identify query pattern

   Queries filter by category and sort by price ascending.

2. Step 2: Choose index field order and direction

   Index should have category first (filter), then price ascending (sort) to match query.

3. Final Answer:

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

4. Quick Check:

   Filter field first, sort field second with matching order [OK]

Hint: Filter field first, sort field second with matching order [OK]

Common Mistakes:

• Putting sort field before filter field in index
• Using descending order when query sorts ascending
• Ignoring field order importance