DynamoDB GSI Backfill: Why Queries Return Zero Items
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.
Network Topology
Refer to the exhibit. A developer runs a DynamoDB query against a global secondary index. The index's partition key is 'status' and sort key is 'created_at'. There are many items with status 'PENDING' in the table. Why does the query return zero items?
Network Topology
A
The query is incorrectly targeting the base table instead of the index.
Why wrong: The --index-name parameter correctly specifies the index.
B
The query must include a condition on the sort key to return results.
Why wrong: Query on partition key alone is valid and returns all items with that partition key value.
C
The key-condition-expression uses the wrong attribute name placeholder.
Why wrong: The expression is correct; #s resolves to status.
D
The global secondary index is not yet backfilled with data from the base table.
If the index was recently created, it may still be in the process of backfilling, so items are not yet available.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The global secondary index is not yet backfilled with data from the base table.
Option D is correct because when a global secondary index (GSI) is created on an existing DynamoDB table, the index is populated asynchronously with data from the base table. If the query returns zero items, it likely means the backfill process has not yet completed, so the index does not yet contain the items that match the query. This is a common scenario when a GSI is newly added and the table contains a large number of items.
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.
✗
The query is incorrectly targeting the base table instead of the index.
Why it's wrong here
The --index-name parameter correctly specifies the index.
✗
The query must include a condition on the sort key to return results.
Why it's wrong here
Query on partition key alone is valid and returns all items with that partition key value.
✗
The key-condition-expression uses the wrong attribute name placeholder.
Why it's wrong here
The expression is correct; #s resolves to status.
✓
The global secondary index is not yet backfilled with data from the base table.
Why this is correct
If the index was recently created, it may still be in the process of backfilling, so items are not yet available.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a GSI is immediately available with all data upon creation, but DynamoDB requires asynchronous backfill, and queries during this period can return zero items even when matching data exists in the base table.
Detailed technical explanation
How to think about this question
DynamoDB GSIs are eventually consistent and are populated asynchronously via a backfill process that reads from the base table's write-ahead log. During backfill, the index may not reflect all base table items, especially if the table is large or has high write throughput. The backfill time depends on the table size and provisioned capacity; until it completes, queries against the index may return incomplete or zero results.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
Visual reference
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.
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: The global secondary index is not yet backfilled with data from the base table. — Option D is correct because when a global secondary index (GSI) is created on an existing DynamoDB table, the index is populated asynchronously with data from the base table. If the query returns zero items, it likely means the backfill process has not yet completed, so the index does not yet contain the items that match the query. This is a common scenario when a GSI is newly added and the table contains a large number of items.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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