Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
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?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The temperature field is not indexed by default, so the query forces a full scan of all documents.
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
Best answer
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
Distractor review
The time range filter on timestamp cannot be combined with the temperature filter efficiently.
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
Distractor review
The default indexing policy only indexes strings and numbers as range indexes, but temperature is stored as a number and is indexed.
Default indexing does index number fields. The issue is not indexing policy but query structure.
Common exam trap
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.
Technical deep dive
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.
Related practice questions
Related DP-900 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
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Question 2
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Question 3
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Question 4
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Question 5
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Question 6
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FAQ
Questions learners often ask
What does this DP-900 question test?
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, queries that do not include the partition key (deviceId) in the filter will result in a cross-partition query, which scans all partitions and is both slow and expensive. The query for temperature without deviceId cannot be directed to a single partition, so it must fan out across all partitions, consuming many RUs and taking longer.
What should I do if I get this DP-900 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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