- A
Partition key: PostID; Index: range on Timestamp
Why wrong: Incorrect - Partitioning by PostID distributes posts randomly across partitions. Queries for a specific UserID will hit every partition (cross-partition query), increasing RU.
- B
Partition key: UserID; Index: range on Timestamp
Correct - UserID as partition key keeps each user's posts together. A range index on Timestamp enables efficient in-partition sorting, resulting in low RU.
- C
Partition key: Timestamp; Index: range on UserID
Why wrong: Incorrect - Partitioning by Timestamp scatters posts for the same user across many partitions. Filtering by UserID then becomes a costly cross-partition query.
- D
Partition key: UserID; Index: composite on (UserID, PostID)
Why wrong: Incorrect - While UserID is a good partition key, the composite index on (UserID, PostID) does not support ordering by Timestamp, requiring a sort after retrieval, which consumes more RU.
Quick Answer
The correct answer is UserID as the partition key with a range index on Timestamp. This strategy minimizes Request Unit consumption because the query filters on UserID, so using UserID as the partition key ensures all posts for a given user reside in the same physical partition, eliminating costly cross-partition queries. The range index on Timestamp then allows Cosmos DB to sort results in descending order directly from the index without scanning or sorting documents, which would otherwise add significant RU overhead. On the DP-900 exam, this scenario tests your understanding of how partition key selection directly impacts query efficiency and how indexing supports sorted queries—a common trap is choosing a unique key like PostID, which scatters data across partitions and forces fan-out queries. Remember the memory tip: “Filter first, sort second—partition on the filter, index on the sort.”
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 social media application stores user posts in Azure Cosmos DB. Each document contains fields: PostID (unique), UserID, Timestamp, Content, LikesCount. The most common query retrieves all posts by a specific UserID ordered by Timestamp descending. Which partition key and indexing strategy minimizes Request Unit (RU) consumption?
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: UserID; Index: range on Timestamp
Option B is correct because the query filters on UserID, so setting UserID as the partition key ensures all posts for a user are in the same physical partition, avoiding cross-partition queries. Adding a range index on Timestamp allows efficient sorting without additional RU overhead, as Cosmos DB can use the index to return results in descending order directly.
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.
- ✗
Partition key: PostID; Index: range on Timestamp
Why it's wrong here
Incorrect - Partitioning by PostID distributes posts randomly across partitions. Queries for a specific UserID will hit every partition (cross-partition query), increasing RU.
- ✓
Partition key: UserID; Index: range on Timestamp
Why this is correct
Correct - UserID as partition key keeps each user's posts together. A range index on Timestamp enables efficient in-partition sorting, resulting in low RU.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Partition key: Timestamp; Index: range on UserID
Why it's wrong here
Incorrect - Partitioning by Timestamp scatters posts for the same user across many partitions. Filtering by UserID then becomes a costly cross-partition query.
- ✗
Partition key: UserID; Index: composite on (UserID, PostID)
Why it's wrong here
Incorrect - While UserID is a good partition key, the composite index on (UserID, PostID) does not support ordering by Timestamp, requiring a sort after retrieval, which consumes more RU.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose a composite index (Option D) thinking it optimizes both filter and sort, but Cosmos DB's indexing engine can satisfy the ORDER BY with a simple range index on the sort column alone, and a composite index would only add unnecessary write RU cost.
Detailed technical explanation
How to think about this question
Under the hood, Cosmos DB uses the partition key to route requests to the correct physical partition; a single-partition query costs at most 10 RUs, while a cross-partition query costs at least 2.5 RUs per partition plus additional RU for merging results. The range index on Timestamp enables the query engine to use an in-memory sort or index scan without loading all documents, and descending order is handled natively by the index without extra cost.
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
Got this wrong? Here's your next step.
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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: Partition key: UserID; Index: range on Timestamp — Option B is correct because the query filters on UserID, so setting UserID as the partition key ensures all posts for a user are in the same physical partition, avoiding cross-partition queries. Adding a range index on Timestamp allows efficient sorting without additional RU overhead, as Cosmos DB can use the index to return results in descending order directly.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Same concept, more angles
1 more ways this is tested on DP-900
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A social media application stores user posts in Azure Cosmos DB. Each post has fields: PostID (unique), UserID, Timestamp, Content, LikesCount. The application frequently queries for all posts by a specific UserID ordered by Timestamp descending. To minimize Request Unit (RU) consumption, which partition key and indexing strategy should be used?
medium- ✓ A.Partition key: UserID, and create a composite index on (UserID, Timestamp DESC)
- B.Partition key: Timestamp, and sort by UserID in the query
- C.Partition key: PostID, and use ORDER BY Timestamp
- D.Partition key: UserID, and use ORDER BY PostID
Why A: Option A is correct because UserID is the most frequently filtered attribute, making it an ideal partition key to distribute data evenly and avoid cross-partition queries. Adding a composite index on (UserID, Timestamp DESC) allows the query to be served from a single physical partition with an index seek, minimizing RU consumption by avoiding a full scan or sort operation.
Last reviewed: Jun 11, 2026
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