Adjacency list pattern in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve all direct children of a parent node

Given a DynamoDB table using the adjacency list pattern where each item has a NodeId and a ParentId, which query will return all direct children of the node with NodeId = 'A1'?

DynamoDB

Table: Nodes
Attributes: NodeId (PK), ParentId
Sample data:
{NodeId: 'A1', ParentId: null}
{NodeId: 'B1', ParentId: 'A1'}
{NodeId: 'B2', ParentId: 'A1'}
{NodeId: 'C1', ParentId: 'B1'}

A Query with KeyConditionExpression: ParentId = 'A1' using ParentId as partition key

B Query with KeyConditionExpression: NodeId = 'A1'

C Scan with FilterExpression: ParentId = 'A1'

D Scan with FilterExpression: NodeId = 'A1'

💡 Hint

Remember that in DynamoDB, Query works only on the partition key. If ParentId is not the partition key, you cannot query by it directly.

Since ParentId is not the partition key, you cannot use Query to find children by ParentId. You must use Scan with a FilterExpression to find all items where ParentId equals 'A1'.

🧠 Conceptual

intermediate

Understanding adjacency list pattern in DynamoDB

In the adjacency list pattern, how is the parent-child relationship typically represented in a DynamoDB table?

A Each item stores its own <code>NodeId</code> and the <code>ParentId</code> of its parent node

B Each item stores a list of all its children <code>NodeIds</code> in an attribute

C Each item stores only its <code>NodeId</code> without any reference to parent or children

D Each item stores the entire path from root to itself as a string

💡 Hint

Think about how you can find a node's parent or children by looking at attributes.

The adjacency list pattern stores each node with its own unique NodeId and a ParentId attribute pointing to its parent node. This allows traversal up and down the tree.

📝 Syntax

advanced

Identify the syntax error in this DynamoDB query expression

Which option contains a syntax error in the DynamoDB query expression to find children nodes?

DynamoDB

Query parameters example:
{
  TableName: 'Nodes',
  KeyConditionExpression: 'ParentId = :pid',
  ExpressionAttributeValues: { ':pid': 'A1' }
}

A KeyConditionExpression: 'ParentId = :pid', ExpressionAttributeValues: { ':pid': 'A1' }

B } '1A' :'dip:' { :seulaVetubirttAnoisserpxE ,'dip: = dItneraP' :noisserpxEnoitidnoCyeK

C KeyConditionExpression: 'ParentId = :pid', ExpressionAttributeValues: { ':pid': { S: 'A1' } }

D KeyConditionExpression: 'ParentId == :pid', ExpressionAttributeValues: { ':pid': 'A1' }

💡 Hint

Check the operator used in the KeyConditionExpression.

DynamoDB expressions use a single equals sign '=' for comparison, not '=='. Using '==' causes a syntax error.

❓ optimization

advanced

Optimize adjacency list queries for children nodes

You want to efficiently query all children of a node by ParentId in DynamoDB. Which table design improves query performance?

A Use <code>ParentId</code> as the partition key and <code>NodeId</code> as the sort key

B Use <code>NodeId</code> as the partition key and <code>ParentId</code> as a regular attribute

C Use a composite key with <code>NodeId</code> and a timestamp

D Use a global secondary index with <code>NodeId</code> as partition key

💡 Hint

Think about which key allows you to query all children by parent efficiently.

Using ParentId as the partition key allows you to Query all items with that ParentId quickly. NodeId as sort key uniquely identifies children.

🔧 Debug

expert

Why does this recursive adjacency list traversal fail in DynamoDB?

A developer tries to recursively fetch all descendants of a node by repeatedly querying children using ParentId. The code runs but returns incomplete results. What is the most likely cause?

A The developer used Scan instead of Query, causing inconsistent results

B DynamoDB Query only returns up to 1MB of data per request, so pagination is needed to get all children

C The table does not have a <code>ParentId</code> attribute, so queries fail silently

D Recursive queries are not supported in DynamoDB and cause runtime errors

💡 Hint

Consider DynamoDB limits on query results and how to handle large datasets.

DynamoDB Query returns a maximum of 1MB of data per call. If there are more children, the developer must use pagination with LastEvaluatedKey to fetch all results.