Querying GSI in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying a GSI with a partition key

Given a DynamoDB table with a Global Secondary Index (GSI) named GSI1 having category as the partition key, what will be the output of the following query?

const params = {
  TableName: 'Products',
  IndexName: 'GSI1',
  KeyConditionExpression: 'category = :cat',
  ExpressionAttributeValues: { ':cat': { S: 'Books' } }
};

// Assume the table has these items:
// { id: '1', category: 'Books', title: 'Learn SQL' }
// { id: '2', category: 'Electronics', title: 'Smartphone' }
// { id: '3', category: 'Books', title: 'DynamoDB Guide' }

// Query is executed with these params.

What items will be returned?

DynamoDB

const params = {
  TableName: 'Products',
  IndexName: 'GSI1',
  KeyConditionExpression: 'category = :cat',
  ExpressionAttributeValues: { ':cat': { S: 'Books' } }
};

A [{ id: '1', category: 'Books', title: 'Learn SQL' }]

B [{ id: '2', category: 'Electronics', title: 'Smartphone' }]

C []

D [{ id: '1', category: 'Books', title: 'Learn SQL' }, { id: '3', category: 'Books', title: 'DynamoDB Guide' }]

💡 Hint

Remember, querying a GSI with a partition key returns all items matching that key in the index.

The query filters items where category equals 'Books' in the GSI. Two items match this condition.

🧠 Conceptual

intermediate

Understanding GSI consistency

Which statement about querying a Global Secondary Index (GSI) in DynamoDB is true?

A Queries on a GSI return eventually consistent results by default.

B Queries on a GSI always return strongly consistent results.

C Queries on a GSI cannot use filter expressions.

D Queries on a GSI require the base table's primary key in the query condition.

💡 Hint

Think about how DynamoDB replicates data to GSIs.

GSIs provide eventually consistent reads by default because data replication to GSIs is asynchronous.

📝 Syntax

advanced

Correct syntax for querying a GSI with sort key condition

Which of the following DynamoDB query parameter objects correctly queries a GSI named GSI2 with partition key category equal to 'Books' and sort key price less than 20?

A { TableName: 'Products', IndexName: 'GSI2', KeyConditionExpression: 'category = :cat AND price < :p', ExpressionAttributeValues: { ':cat': 'Books', ':p': 20 } }

B { TableName: 'Products', IndexName: 'GSI2', KeyConditionExpression: 'category = :cat AND price < :p', ExpressionAttributeValues: { ':cat': { S: 'Books' }, ':p': { N: '20' } } }

C { TableName: 'Products', IndexName: 'GSI2', KeyConditionExpression: 'category = :cat AND price <= :p', ExpressionAttributeValues: { ':cat': { S: 'Books' }, ':p': { N: '20' } } }

D { TableName: 'Products', IndexName: 'GSI2', KeyConditionExpression: 'category = :cat AND price < :p', ExpressionAttributeValues: { ':cat': { S: 'Books' }, ':p': 20 } }

💡 Hint

Check the format of ExpressionAttributeValues for DynamoDB SDK.

ExpressionAttributeValues must specify the data type (S for string, N for number) as objects. Option B correctly uses this format.

❓ optimization

advanced

Optimizing GSI queries with projections

You want to reduce the amount of data returned by a GSI query to only the id and title attributes. Which parameter should you add to your query?

A Add <code>KeyConditionExpression: 'id, title'</code> to the query parameters.

B Add <code>FilterExpression: 'id, title'</code> to the query parameters.

C Add <code>ProjectionExpression: 'id, title'</code> to the query parameters.

D Add <code>ExpressionAttributeNames: { '#id': 'id', '#title': 'title' }</code> only.

💡 Hint

Think about how to specify which attributes to return in DynamoDB queries.

ProjectionExpression specifies which attributes to return, reducing data size and improving performance.

🔧 Debug

expert

Diagnosing a GSI query returning no results

A developer runs this query on a DynamoDB table with a GSI named GSI1:

const params = {
  TableName: 'Orders',
  IndexName: 'GSI1',
  KeyConditionExpression: 'orderDate = :date',
  ExpressionAttributeValues: { ':date': { S: '2023-01-01' } }
};

But the query returns no items, even though there are orders on that date. What is the most likely cause?

A The GSI's partition key is not <code>orderDate</code>, so the query condition is invalid.

B The <code>ExpressionAttributeValues</code> is missing a data type for <code>:date</code>.

C The query must include the base table's primary key in the condition.

D The <code>IndexName</code> parameter should be omitted when querying a GSI.

💡 Hint

Check the GSI's key schema and what the query condition uses.

The query uses orderDate as the partition key condition, but if the GSI's partition key is different, the query will return no results.