Partial indexes with filter in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a partial index in MongoDB?

A partial index is an index built only on the documents in a collection that meet a specified filter condition. It helps improve performance and reduce index size by indexing only relevant documents.

beginner

How do you create a partial index with a filter in MongoDB?

Use the createIndex() method with the option { partialFilterExpression: <filter> }, where <filter> defines the condition documents must meet to be included in the index.

intermediate

Why use a partial index instead of a full index?

Partial indexes save space and improve write performance by indexing only documents that match the filter, making queries on those documents faster without indexing the entire collection.

intermediate

Example: What does this partial index do? { age: 1 }, { partialFilterExpression: { age: { $gte: 18 } } }

This index only includes documents where the age field is 18 or older. Queries filtering on age >= 18 will be faster, but documents with age less than 18 are not indexed.

intermediate

Can a partial index speed up queries that do not match the filter condition?

No. Partial indexes only help queries that match the filter condition. Queries outside the filter will not use the partial index and may scan the collection.

What does a partial index in MongoDB do?

A Indexes only documents matching a filter condition

B Indexes all documents in a collection

C Indexes documents randomly

D Indexes documents based on size

A partial index only includes documents that match the specified filter condition.

Which option is used to specify the filter condition when creating a partial index?

A filterCondition

B indexFilter

C partialFilterExpression

D filterExpression

The option partialFilterExpression defines the filter condition for a partial index.

If a query filters documents outside the partial index filter, what happens?

A The query uses a random index

B The query fails

C The partial index is used anyway

D The query scans the whole collection

Queries outside the partial index filter do not use the partial index and scan the collection.

What is a benefit of using partial indexes?

A They reduce index size and improve write speed

B They make queries slower

C They index all fields automatically

D They delete documents automatically

Partial indexes reduce index size and improve write performance by indexing fewer documents.

Which of these is a valid partial index filter expression?

A { name: 'John' }

B All of the above

C { status: { $exists: true } }

D { age: { $lt: 30 } }

Any valid MongoDB query filter can be used as a partialFilterExpression.

Explain what a partial index is and why you might use one in MongoDB.

Think about indexing only some documents instead of all.

Describe how to create a partial index with a filter and give an example.

Remember the option name partialFilterExpression.

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'