Identifying access patterns first in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Query Result for Single Partition Key Access

You have a DynamoDB table with PartitionKey as the primary key. You run this query to get all items with PartitionKey = 'User#123'. What will be the output?

DynamoDB

SELECT * FROM Table WHERE PartitionKey = 'User#123';

A Only one item with PartitionKey 'User#123' will be returned, even if multiple exist.

B All items with PartitionKey 'User#123' will be returned.

C Query will fail because PartitionKey must be unique.

D No items will be returned unless SortKey is specified.

💡 Hint

Think about how DynamoDB handles queries on partition keys.

In DynamoDB, querying by PartitionKey returns all items with that key, regardless of SortKey values.

🧠 Conceptual

intermediate

Choosing Partition Key for Efficient Access

You want to design a DynamoDB table to store orders. You expect frequent queries by CustomerID and occasional queries by OrderDate. Which is the best choice for the partition key to optimize access patterns?

A Use <code>OrderDate</code> as the partition key.

B Use a composite key with <code>OrderDate</code> as partition key and <code>CustomerID</code> as sort key.

C Use a random UUID as the partition key.

D Use <code>CustomerID</code> as the partition key.

💡 Hint

Think about which attribute is used most often in queries.

Choosing CustomerID as partition key optimizes frequent queries by customer. OrderDate can be a sort key or accessed via secondary indexes.

📝 Syntax

advanced

Identify the Correct DynamoDB Query Syntax

Which of the following DynamoDB query expressions correctly retrieves items with PartitionKey = 'User#456' and SortKey starting with 'Order#'?

DynamoDB

Assume table has PartitionKey and SortKey attributes.

A QueryInput={ TableName: 'Table', KeyConditionExpression: 'PartitionKey = :pk and begins_with(SortKey, :sk)', ExpressionAttributeValues: { ':pk': 'User#456', ':sk': 'Order#' } }

B QueryInput={ TableName: 'Table', KeyConditionExpression: 'PartitionKey = :pk and SortKey = :sk', ExpressionAttributeValues: { ':pk': 'User#456', ':sk': 'Order#' } }

C QueryInput={ TableName: 'Table', FilterExpression: 'begins_with(SortKey, :sk)', ExpressionAttributeValues: { ':sk': 'Order#' } }

D QueryInput={ TableName: 'Table', KeyConditionExpression: 'PartitionKey = :pk or begins_with(SortKey, :sk)', ExpressionAttributeValues: { ':pk': 'User#456', ':sk': 'Order#' } }

💡 Hint

Remember that KeyConditionExpression must use AND and can use begins_with for sort keys.

Option A correctly uses KeyConditionExpression with AND and begins_with for SortKey. Option A uses equality for SortKey which won't match prefixes. Option A uses FilterExpression which is less efficient. Option A uses OR which is invalid in KeyConditionExpression.

❓ optimization

advanced

Optimizing Access Patterns with Secondary Indexes

You have a DynamoDB table with primary key CustomerID and OrderID. You want to efficiently query orders by OrderStatus. What is the best way to optimize this access pattern?

A Use a Scan operation filtering by <code>OrderStatus</code>.

B Add <code>OrderStatus</code> as a sort key to the primary key.

C Create a Global Secondary Index (GSI) with <code>OrderStatus</code> as partition key.

D Store <code>OrderStatus</code> in a separate table.

💡 Hint

Think about how to query efficiently by an attribute not in the primary key.

A GSI allows querying efficiently by attributes other than the primary key. Adding OrderStatus as sort key won't help if you want to query by OrderStatus alone. Scan is inefficient. Separate table adds complexity.

🔧 Debug

expert

Debugging a Query Returning No Results

You run this DynamoDB query to get all orders for CustomerID = 'Cust#789' with OrderDate after '2023-01-01'. The query returns no results, but you know matching items exist. What is the likely cause?

QueryInput={
  TableName: 'Orders',
  KeyConditionExpression: 'CustomerID = :cid and OrderDate > :date',
  ExpressionAttributeValues: { ':cid': 'Cust#789', ':date': '2023-01-01' }
}

A OrderDate is not part of the primary key or sort key, so it cannot be used in KeyConditionExpression.

B The date format '2023-01-01' is invalid for DynamoDB queries.

C CustomerID value is case-sensitive and 'Cust#789' does not match stored keys.

D ExpressionAttributeValues keys must be prefixed with a hash (#) symbol.

💡 Hint

Check which attributes can be used in KeyConditionExpression.

KeyConditionExpression can only use partition key and sort key attributes. If OrderDate is not part of the key schema, it cannot be used there, causing no results.