Describe considerations for working with non-relational data on Azure →mediumMultiple ChoiceObjective-mapped
DP-900 Practice Question: Describe considerations for working with non-relational data on Azure
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?
⚠ Common exam trap
The trap here is that candidates often pick a partition key based on the ORDER BY column (Timestamp) without realizing that the filter column (UserID) should be the partition key to avoid cross-partition queries, and that a composite index is needed to avoid an expensive sort.
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, and create a composite index on (UserID, Timestamp DESC)
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.
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: UserID, and create a composite index on (UserID, Timestamp DESC)
Why this is correct
This design localizes all posts for a user in one partition and uses an index that directly supports the filter and sort order.
- ✗
Partition key: Timestamp, and sort by UserID in the query
Why it's wrong here
Partitioning by Timestamp spreads a single user's posts across many partitions, causing expensive cross-partition queries.
- ✗
Partition key: PostID, and use ORDER BY Timestamp
Why it's wrong here
Partitioning by PostID scatters data randomly; queries for a specific user would hit many partitions, increasing RU consumption.
- ✗
Partition key: UserID, and use ORDER BY PostID
Why it's wrong here
Ordering by PostID does not sort by Timestamp, which is the required order. Additional sorting would be needed.
Go deeper
Related to this question
Learn chapter
Data Roles and Core Concepts
Key term
Index
An index is a data structure that speeds up data retrieval operations on a database table or file, much like a book index helps you find topics quickly.
Key term
Data
Data is raw, unprocessed information, like numbers, words, or measurements, that can be stored, processed, and analyzed by computers.
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