The answer is to change the clustering order to descending on timestamp. This optimization directly improves Cosmos DB Cassandra API time-series queries with clustering order by ensuring the most recent telemetry data is stored first within each partition, allowing queries for a specific device and time range to stop scanning as soon as the requested rows are found. On the DP-900 exam, this tests your understanding of how composite primary keys and clustering order affect RU consumption—a common trap is confusing partition keys with clustering columns, or thinking that simply increasing RUs fixes inefficient data access. Remember the memory tip: "Newest first, RU thirst reversed"—descending order on timestamp means the latest data is retrieved immediately, slashing read costs for time-series workloads.
DP-900 Practice Question: Describe considerations for working with non-relational data on Azure
This DP-900 practice question tests your understanding of describe considerations for working with non-relational data on azure. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company uses Azure Cosmos DB with the Cassandra API for a time-series telemetry application. The data model uses a composite primary key (device_id, timestamp). The application queries telemetry for a specific device within a time range. Recently, query performance has degraded as data volume grows. The RU consumption per query is high. Which action should they take to improve performance and reduce RU cost?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change the clustering order to descending on timestamp.
Changing the clustering order to descending on timestamp (option C) allows the most recent data to be retrieved first without scanning all rows. Option A (increasing RU) does not improve efficiency. Option B (adding a secondary index) may increase RU. Option D (changing partition key to timestamp) would scatter device data across partitions, hurting queries by device.
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.
✗
Change the partition key to timestamp.
Why it's wrong here
Partitioning by timestamp scatters device data, making queries by device slow.
✗
Increase the provisioned RU/s on the container.
Why it's wrong here
Higher RU does not fix inefficient queries; it just allows more throughput.
✗
Add a secondary index on the timestamp column.
Why it's wrong here
Secondary indexes increase RU consumption and are not efficient for range queries.
✓
Change the clustering order to descending on timestamp.
Why this is correct
Descending order makes time-range queries efficient by reading only relevant rows.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this DP-900 question in full detail.
Identify which DP-900 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Describe considerations for working with non-relational data on Azure — This question tests Describe considerations for working with non-relational data on Azure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Change the clustering order to descending on timestamp. — Changing the clustering order to descending on timestamp (option C) allows the most recent data to be retrieved first without scanning all rows. Option A (increasing RU) does not improve efficiency. Option B (adding a secondary index) may increase RU. Option D (changing partition key to timestamp) would scatter device data across partitions, hurting queries by device.
What should I do if I get this DP-900 question wrong?
Identify which DP-900 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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