DBS-C01 Workload-Specific Database Design Practice Question
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?
⚠ Common exam trap
It's easy for candidates to 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.
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 global secondary index is not yet backfilled with data from the base table.
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.
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.
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