- A
The temperature field is not indexed by default, so the query forces a full scan of all documents.
Why wrong: By default, all fields in Cosmos DB are automatically indexed, including temperature. The issue is not lack of indexing but the absence of the partition key in the filter.
- B
The query does not specify the partition key (deviceId), causing a cross-partition query that scans every physical partition.
Cosmos DB uses partition key (deviceId) to distribute data. When a query does not include the partition key, it must fan out to all partitions, which is slow and expensive in RUs.
- C
The time range filter on timestamp cannot be combined with the temperature filter efficiently.
Why wrong: Cosmos DB can efficiently use composite indexes or multiple filters. The main problem is lack of partition key specification, not the combination of filters.
- D
The default indexing policy only indexes strings and numbers as range indexes, but temperature is stored as a number and is indexed.
Why wrong: Default indexing does index number fields. The issue is not indexing policy but query structure.
Quick Answer
The answer is that the query does not specify the partition key (deviceId), forcing a cross-partition query that scans every physical partition. In Azure Cosmos DB, the partition key determines how data is distributed; without it in the filter, the query must fan out to all partitions, reading every document to find temperature readings above 50 in the last hour, which is why cross-partition queries are slow and consume excessive RUs. On the DP-900 exam, this tests your understanding of partition key design and query efficiency—a common trap is assuming any filter works equally well, but only partition-key-inclusive queries target a single partition. Remember the memory tip: “No key, full sweep; with key, one leap.”
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 smart home company stores device telemetry in Azure Cosmos DB using the NoSQL API. Each document contains: deviceId (string), timestamp (datetime), temperature (float), humidity (float). The most common query retrieves all documents for a specific deviceId within a time range, ordered by timestamp descending. This query performs well. However, a new query that finds devices with temperature > 50 in the last hour (without specifying deviceId) is extremely slow and consumes many request units (RUs). What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 query does not specify the partition key (deviceId), causing a cross-partition query that scans every physical partition.
In Azure Cosmos DB NoSQL API, the partition key (deviceId) determines data distribution across physical partitions. Queries that do not include the partition key in the filter become cross-partition queries, which must fan out to every physical partition, scanning all documents. This is extremely slow and consumes many RUs, especially in large containers. The original query specifying deviceId performs well because it targets a single partition.
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 temperature field is not indexed by default, so the query forces a full scan of all documents.
Why it's wrong here
By default, all fields in Cosmos DB are automatically indexed, including temperature. The issue is not lack of indexing but the absence of the partition key in the filter.
- ✓
The query does not specify the partition key (deviceId), causing a cross-partition query that scans every physical partition.
Why this is correct
Cosmos DB uses partition key (deviceId) to distribute data. When a query does not include the partition key, it must fan out to all partitions, which is slow and expensive in RUs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The time range filter on timestamp cannot be combined with the temperature filter efficiently.
Why it's wrong here
Cosmos DB can efficiently use composite indexes or multiple filters. The main problem is lack of partition key specification, not the combination of filters.
- ✗
The default indexing policy only indexes strings and numbers as range indexes, but temperature is stored as a number and is indexed.
Why it's wrong here
Default indexing does index number fields. The issue is not indexing policy but query structure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the slowness is due to a missing index on temperature, but Azure Cosmos DB automatically indexes all fields by default, so the real issue is the cross-partition query caused by omitting the partition key.
Detailed technical explanation
How to think about this question
Azure Cosmos DB uses a hash-based partition key to distribute data across physical partitions. When a query does not include the partition key, the query engine must issue a parallel fan-out query to all partitions, each of which may have its own index, but the aggregation of results and the need to scan every partition dramatically increases RU consumption and latency. The indexing policy can be customized, but by default all fields are indexed, so the temperature field is not the bottleneck.
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.
What to study next
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The query does not specify the partition key (deviceId), causing a cross-partition query that scans every physical partition. — In Azure Cosmos DB NoSQL API, the partition key (deviceId) determines data distribution across physical partitions. Queries that do not include the partition key in the filter become cross-partition queries, which must fan out to every physical partition, scanning all documents. This is extremely slow and consumes many RUs, especially in large containers. The original query specifying deviceId performs well because it targets a single partition.
What should I do if I get this DP-900 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This DP-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DP-900 exam.
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