DBS-C01 Workload-Specific Database Design Practice Question
Network Topology
Refer to the exhibit. A company uses this DynamoDB table to store user session data. The application frequently queries by user_id alone to get all sessions for a user. However, the query is slow. What is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates assume any attribute can be queried efficiently, failing to recognize that DynamoDB requires a primary key or index for efficient lookups, and that a scan is the fallback for non-key attributes.
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
✓
The table's partition key is session_id, not user_id, so querying by user_id requires a scan.
The table's primary key is session_id, not user_id. Querying by user_id without a secondary index forces DynamoDB to perform a full table scan, which reads every item and is significantly slower than a query operation. This is the most likely cause of the slow performance.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The table's partition key is session_id, not user_id, so querying by user_id requires a scan.
Why this is correct
Without a GSI on user_id, queries on user_id are scans.
- ✗
The table has no sort key on user_id.
Why it's wrong here
user_id is a sort key, but it is only efficient when combined with session_id.
- ✗
The table has too many items, causing slow scans.
Why it's wrong here
10,000 items is not large; the issue is key design.
- ✗
The provisioned read capacity is too low.
Why it's wrong here
5 RCUs for 10,000 items is sufficient for small queries.
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