Compound index and field order in MongoDB

Introduction

A compound index helps you find data faster when you search using more than one field. The order of fields in the index matters because it changes how quickly you get results.

You want to speed up searches that use two or more fields together.

You have queries that filter by one field and then sort by another.

You want to improve performance for reports that group by multiple fields.

You often search by a main category and then by a subcategory.

You want to avoid creating many single-field indexes and save space.

Syntax

MongoDB

db.collection.createIndex({ field1: 1, field2: -1 })

Use 1 for ascending order and -1 for descending order.

The order of fields in the index affects which queries can use it efficiently.

Examples

This index helps queries that filter by category and then by price in ascending order.

MongoDB

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

This index speeds up queries filtering by customerId and sorting orders by newest date first.

MongoDB

db.orders.createIndex({ customerId: 1, orderDate: -1 })

This index supports searches by last name and then first name in alphabetical order.

MongoDB

db.users.createIndex({ lastName: 1, firstName: 1 })

Sample Program

First, we create a compound index on region (ascending) and salesAmount (descending). Then, we find sales in the 'West' region and sort them by salesAmount from highest to lowest. The index helps this query run faster.

MongoDB

db.sales.createIndex({ region: 1, salesAmount: -1 })
db.sales.find({ region: 'West' }).sort({ salesAmount: -1 })

Important Notes

If you search only by the first field in the compound index, MongoDB can still use the index efficiently.

But if you search only by the second field, the index is not helpful.

Think of the index like a phone book sorted first by last name, then by first name.

Summary

Compound indexes speed up queries using multiple fields together.

The order of fields in the index matters for query performance.

Use ascending (1) or descending (-1) to control sorting in the index.

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