Partial indexes with filter in MongoDB

Introduction

Partial indexes help speed up searches by only indexing some documents that match a condition. This saves space and makes queries faster.

When you only want to index active users in a large user collection.

When you want to index orders that are not yet shipped.

When you want to index products that are in stock.

When you want to speed up queries on documents with a specific status.

When you want to reduce index size by excluding irrelevant documents.

Syntax

MongoDB

db.collection.createIndex(
  { field: 1 },
  { partialFilterExpression: { <filter_condition> } }
)

The is a query that decides which documents get indexed.

Use 1 for ascending or -1 for descending index order.

Examples

Creates an index on the email field only for users where isActive is true.

MongoDB

db.users.createIndex(
  { email: 1 },
  { partialFilterExpression: { isActive: true } }
)

Indexes orderDate descending only for orders not shipped yet.

MongoDB

db.orders.createIndex(
  { orderDate: -1 },
  { partialFilterExpression: { status: { $ne: "shipped" } } }
)

Sample Program

This example inserts three products, creates a partial index on the name field only for products that are in stock, then queries those products sorted by name.

MongoDB

db.products.insertMany([
  { name: "Pen", inStock: true },
  { name: "Notebook", inStock: false },
  { name: "Eraser", inStock: true }
])

// Create partial index on name only for products in stock
 db.products.createIndex(
  { name: 1 },
  { partialFilterExpression: { inStock: true } }
)

// Query using the index
 db.products.find({ inStock: true }).sort({ name: 1 })

Important Notes

Partial indexes only include documents matching the filter, so queries must use the filter condition to benefit from the index.

Partial indexes reduce index size and improve write performance compared to full indexes.

Summary

Partial indexes index only some documents based on a filter condition.

They save space and speed up queries on filtered data.

Use partialFilterExpression option when creating the index.

Practice

1. What is the main purpose of a partial index in MongoDB?

easy

A. To index only documents that match a specific filter condition

B. To index all documents in a collection

C. To create a backup of the index

D. To speed up writes by disabling indexing

Solution

1. Step 1: Understand what partial indexes do

   Partial indexes only include documents that meet a filter condition, unlike normal indexes that include all documents.

2. Step 2: Compare options

   To index only documents that match a specific filter condition correctly describes this behavior. Options A, B, and D describe other unrelated or incorrect behaviors.

3. Final Answer:

   To index only documents that match a specific filter condition -> Option A

4. Quick Check:

   Partial index = filtered indexing [OK]

Hint: Partial indexes filter documents before indexing [OK]

Common Mistakes:

• Thinking partial indexes index all documents
• Confusing partial indexes with backups
• Assuming partial indexes disable indexing

2. Which of the following is the correct syntax to create a partial index on the status field where status equals 'active'?

easy

A. db.collection.createIndex({status: 1}, {filter: {status: 'active'}})

B. db.collection.createIndex({status: 1}, {partialFilterExpression: {status: 'active'}})

C. db.collection.createPartialIndex({status: 1}, {status: 'active'})

D. db.collection.createIndex({status: 1}, {partialIndex: {status: 'active'}})

Solution

1. Step 1: Recall the correct option name for partial indexes

   The option to specify a filter for partial indexes is partialFilterExpression.

2. Step 2: Match syntax with options

   db.collection.createIndex({status: 1}, {partialFilterExpression: {status: 'active'}}) uses partialFilterExpression correctly. Options B, C, and D use incorrect option names or methods.

3. Final Answer:

   db.collection.createIndex({status: 1}, {partialFilterExpression: {status: 'active'}}) -> Option B

4. Quick Check:

   Use partialFilterExpression for partial indexes [OK]

Hint: Use partialFilterExpression option to filter index documents [OK]

Common Mistakes:

• Using 'filter' instead of partialFilterExpression
• Trying to use createPartialIndex method (doesn't exist)
• Using partialIndex option instead of partialFilterExpression

3. Given the following partial index creation:

db.users.createIndex({age: 1}, {partialFilterExpression: {age: {$gte: 18}}})

Which documents will be included in this index?

medium

A. All documents regardless of age

B. Only documents where age is less than 18

C. Only documents where age is 18 or older

D. Documents where age is exactly 18

Solution

1. Step 1: Understand the filter condition

   The filter {age: {$gte: 18}} means age greater than or equal to 18.

2. Step 2: Determine which documents match

   Only documents with age 18 or older satisfy this condition and will be indexed.

3. Final Answer:

   Only documents where age is 18 or older -> Option C

4. Quick Check:

   Filter {$gte: 18} means age ≥ 18 [OK]

Hint: Partial index includes only documents matching filter [OK]

Common Mistakes:

• Thinking all documents are indexed
• Confusing $gte with $lt
• Assuming only exact matches are indexed

4. You tried to create a partial index with this command:

db.orders.createIndex({price: 1}, {partialFilterExpression: {price: {$gt: 100}}})

But you get an error. What is the most likely cause?

medium

A. The filter expression uses an unsupported operator

B. The field name in the index is missing

C. Partial indexes cannot use comparison operators

D. The partialFilterExpression option is misspelled or misplaced

Solution

1. Step 1: Check syntax correctness

   The syntax shown is correct if partialFilterExpression is spelled and placed properly.

2. Step 2: Identify common mistakes

   Errors often happen if partialFilterExpression is misspelled or placed outside the options object.

3. Final Answer:

   The partialFilterExpression option is misspelled or misplaced -> Option D

4. Quick Check:

   Check spelling and placement of partialFilterExpression [OK]

Hint: Check spelling and position of partialFilterExpression [OK]

Common Mistakes:

• Misspelling partialFilterExpression
• Placing filter outside options object
• Assuming comparison operators are unsupported

5. You want to speed up queries on a collection products where only documents with stock greater than zero are frequently queried. Which partial index creation command is best?

hard

A. db.products.createIndex({stock: 1}, {partialFilterExpression: {stock: {$gt: 0}}})

B. db.products.createIndex({stock: 1})

C. db.products.createIndex({stock: 1}, {partialFilterExpression: {stock: 0}})

D. db.products.createIndex({stock: 1}, {filter: {stock: {$gt: 0}}})

Solution

1. Step 1: Identify the filter condition for partial index

   The goal is to index only documents where stock is greater than zero.

2. Step 2: Match the correct syntax and filter

   db.products.createIndex({stock: 1}, {partialFilterExpression: {stock: {$gt: 0}}}) uses partialFilterExpression with {stock: {$gt: 0}}, which matches the requirement.

3. Step 3: Evaluate other options

   db.products.createIndex({stock: 1}) indexes all documents (not partial). db.products.createIndex({stock: 1}, {partialFilterExpression: {stock: 0}}) filters only stock equal to zero, which is wrong. db.products.createIndex({stock: 1}, {filter: {stock: {$gt: 0}}}) uses incorrect option name filter.

4. Final Answer:

   db.products.createIndex({stock: 1}, {partialFilterExpression: {stock: {$gt: 0}}}) -> Option A

5. Quick Check:

   Use partialFilterExpression with correct filter [OK]

Hint: Use partialFilterExpression with correct filter condition [OK]

Common Mistakes:

• Indexing all documents instead of partial
• Using wrong filter values
• Using incorrect option names like 'filter'