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Workload-Specific Database Design →mediumMultiple ChoiceObjective-mapped

DBS-C01 Workload-Specific Database Design Practice Question

This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A social media application uses Amazon DynamoDB as its primary data store. The application stores user posts and allows users to retrieve the most recent 10 posts of users they follow. The access pattern is a followee-based query that needs to be highly scalable and low-latency. Which DynamoDB table design should the database specialist recommend?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.

A
Use a partition key of post ID and a local secondary index on the followee ID
Why wrong: Partition key as post ID does not support querying by follower or followee efficiently.
B
Use a single table with a scan operation and filter on the followee attribute
Why wrong: Scan operations are expensive and not recommended for low-latency access patterns.
C
Use a composite primary key with a partition key of follower ID and a sort key of timestamp, and store the followee ID as an attribute
This design allows efficient Query on the follower ID to retrieve recent posts in reverse order by timestamp.
D
Design the table with a partition key of user ID and a sort key of timestamp, and create a global secondary index (GSI) on followee ID
Why wrong: A GSI with followee ID as partition key might still lead to hot partitions if a popular user has many followers.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Use a composite primary key with a partition key of follower ID and a sort key of timestamp, and store the followee ID as an attribute

Option C is correct because it models the access pattern directly: the follower ID as the partition key ensures all posts from followed users are co-located, and the sort key of timestamp allows efficient retrieval of the most recent 10 posts via a Query with a limit of 10 and descending order. This design avoids expensive scans or secondary index lookups, meeting the low-latency and scalability requirements.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Use a partition key of post ID and a local secondary index on the followee ID
Why it's wrong here
Partition key as post ID does not support querying by follower or followee efficiently.
✗
Use a single table with a scan operation and filter on the followee attribute
Why it's wrong here
Scan operations are expensive and not recommended for low-latency access patterns.
✓
Use a composite primary key with a partition key of follower ID and a sort key of timestamp, and store the followee ID as an attribute
Why this is correct
This design allows efficient Query on the follower ID to retrieve recent posts in reverse order by timestamp.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Design the table with a partition key of user ID and a sort key of timestamp, and create a global secondary index (GSI) on followee ID
Why it's wrong here
A GSI with followee ID as partition key might still lead to hot partitions if a popular user has many followers.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often choose Option D because they think a GSI on followee ID solves the query pattern, but they overlook that the base table's partition key (user ID) does not match the follower-based access pattern, requiring multiple queries or a Scan, and the GSI still incurs additional latency and cost for index maintenance.

Detailed technical explanation

How to think about this question

In DynamoDB, a composite primary key with a partition key of follower ID and a sort key of timestamp enables a single Query operation to retrieve the 10 most recent posts from all followed users if the followee ID is stored as an attribute and the application issues separate queries for each followee. However, a more optimized design would use a single-table design with a composite key of (follower_id, timestamp) and store the followee_id as an attribute, then use a Query with a filter expression on followee_id to limit results to specific followees, though this still requires filtering after retrieval. The key insight is that DynamoDB's Query operation on a partition key is the most efficient access pattern, and the sort key enables ordering by timestamp without additional indexing.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Use a composite primary key with a partition key of follower ID and a sort key of timestamp, and store the followee ID as an attribute — Option C is correct because it models the access pattern directly: the follower ID as the partition key ensures all posts from followed users are co-located, and the sort key of timestamp allows efficient retrieval of the most recent 10 posts via a Query with a limit of 10 and descending order. This design avoids expensive scans or secondary index lookups, meeting the low-latency and scalability requirements.

What should I do if I get this DBS-C01 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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