- A
Partition key: deviceId, Indexing: automatic on all properties
Correct. deviceId as partition key targets a single partition; automatic indexing ensures the timestamp filter uses an index, minimizing RUs.
- B
Partition key: timestamp, Indexing: automatic on all properties
Why wrong: timestamp is a poor partition key because it can cause hot partitions (e.g., all recent data on one partition) and queries for a specific deviceId would be cross-partition.
- C
Partition key: deviceId, Indexing: none
Why wrong: Without indexing, the query cannot use an index on timestamp, causing a full scan of the partition, which increases RU consumption.
- D
Partition key: temperature, Indexing: automatic on all properties
Why wrong: temperature is not ideal as a partition key because it has low cardinality and uneven distribution; queries for a specific deviceId would be cross-partition.
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. A key principle to apply: a partition key determines the logical partition for a document.. 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 manufacturing company stores IoT sensor data as JSON documents in Azure Cosmos DB. Each document has fields: deviceId (high cardinality, many unique values), timestamp, temperature, and humidity. The most frequent query is: 'Retrieve all readings for a specific deviceId from the last hour.' To minimize Request Unit (RU) consumption, which combination of partition key and indexing policy should be chosen?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"minimum / minimize"Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
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
Partition key: deviceId, Indexing: automatic on all properties
Option A is correct because deviceId is the most frequently filtered attribute (in the WHERE clause), making it an ideal partition key that ensures queries are scoped to a single physical partition, minimizing cross-partition fan-out. Automatic indexing on all properties allows efficient filtering on timestamp within the partition, while the index on deviceId is not strictly needed since the partition key itself routes the query, but it does not harm RU consumption significantly. This combination balances query performance and RU cost for the described workload.
Key principle: A partition key determines the logical partition for a document.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Partition key: deviceId, Indexing: automatic on all properties
Why this is correct
Correct. deviceId as partition key targets a single partition; automatic indexing ensures the timestamp filter uses an index, minimizing RUs.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
A partition key determines the logical partition for a document.
- ✗
Partition key: timestamp, Indexing: automatic on all properties
Why it's wrong here
timestamp is a poor partition key because it can cause hot partitions (e.g., all recent data on one partition) and queries for a specific deviceId would be cross-partition.
- ✗
Partition key: deviceId, Indexing: none
Why it's wrong here
Without indexing, the query cannot use an index on timestamp, causing a full scan of the partition, which increases RU consumption.
- ✗
Partition key: temperature, Indexing: automatic on all properties
Why it's wrong here
temperature is not ideal as a partition key because it has low cardinality and uneven distribution; queries for a specific deviceId would be cross-partition.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often pick timestamp as the partition key because it seems logical for time-range queries, but they overlook that the most frequent query filters on deviceId, making deviceId the correct partition key to avoid cross-partition queries.
Detailed technical explanation
How to think about this question
Under the hood, Azure Cosmos DB uses the partition key to distribute data across physical partitions; queries with a partition key filter are routed directly to the relevant partition, avoiding the RU overhead of fan-out. The indexing policy on timestamp (within the partition) leverages a range index to efficiently locate documents matching the last-hour filter without scanning all documents. In real-world scenarios, choosing a high-cardinality partition key like deviceId also prevents storage skew, which can throttle throughput if one partition exceeds 20 GB or consumes disproportionate RU.
KKey Concepts to Remember
- A partition key determines the logical partition for a document.
- Queries targeting a single partition key value are 'single-partition queries' and are highly efficient.
- Indexing allows queries to quickly locate documents without full scans.
- Automatic indexing in Cosmos DB indexes all properties by default.
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
A partition key determines the logical partition for a document.
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.
What to study next
Got this wrong? Here's your next step.
Review a partition key determines the logical partition for a document., then practise related DP-900 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this DP-900 question test?
Describe considerations for working with non-relational data on Azure — This question tests Describe considerations for working with non-relational data on Azure — A partition key determines the logical partition for a document..
What is the correct answer to this question?
The correct answer is: Partition key: deviceId, Indexing: automatic on all properties — Option A is correct because deviceId is the most frequently filtered attribute (in the WHERE clause), making it an ideal partition key that ensures queries are scoped to a single physical partition, minimizing cross-partition fan-out. Automatic indexing on all properties allows efficient filtering on timestamp within the partition, while the index on deviceId is not strictly needed since the partition key itself routes the query, but it does not harm RU consumption significantly. This combination balances query performance and RU cost for the described workload.
What should I do if I get this DP-900 question wrong?
Review a partition key determines the logical partition for a document., then practise related DP-900 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
A partition key determines the logical partition for a document.
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Last reviewed: Jun 11, 2026
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