The answer is an undersized buffer cache due to insufficient DB instance memory. After migrating to Aurora PostgreSQL, complex queries that previously performed well become slower because the working set of data no longer fits in the buffer cache, forcing frequent reads from the distributed storage layer and increasing I/O latency. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how Aurora’s architecture separates compute and storage: the buffer cache lives entirely in the DB instance’s memory, so choosing an instance class with too little RAM directly impacts query performance. A common trap is to suspect network latency or connection pooling, but the core issue is that Aurora’s storage is remote and fast only when the cache is warm. Memory tip: think “cache miss = disk fetch” — if your queries slow down after migration, the buffer cache likely can’t hold the hot data set.
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 company is migrating a PostgreSQL database to Amazon Aurora PostgreSQL. The current database has complex queries that join multiple tables and performs well. After migration, the same queries are slower on Aurora. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DB instance class does not have enough memory for the buffer cache.
Option A is correct because after migrating to Aurora PostgreSQL, the same complex queries are slower, which often indicates that the buffer cache is too small to hold the working set of data. Aurora uses a distributed storage system where the buffer cache is managed by the DB instance's memory; if the instance class lacks sufficient memory, frequently accessed data pages must be read from storage more often, increasing I/O latency. This is a common performance bottleneck when migrating from on-premises PostgreSQL, where the buffer cache might have been larger or the working set fit entirely in memory.
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 DB instance class does not have enough memory for the buffer cache.
Why this is correct
Aurora's buffer cache is in memory; insufficient memory leads to more disk reads.
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.
✗
Aurora PostgreSQL does not support complex joins; the queries must be rewritten.
Why it's wrong here
Aurora PostgreSQL is fully compatible with PostgreSQL and supports all join types.
✗
Aurora PostgreSQL does not support indexes on joined columns.
Why it's wrong here
Aurora supports indexes, including on joined columns.
✗
The default DB parameter group is optimized for write-heavy workloads, not read-heavy.
Why it's wrong here
Default parameter groups are generic; but the main issue is likely memory.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume Aurora PostgreSQL has inherent limitations with joins or indexes (options B and C), when in fact the most common post-migration performance issue is insufficient memory for the buffer cache, not a missing feature.
Detailed technical explanation
How to think about this question
Aurora PostgreSQL uses a shared storage architecture where the buffer cache is local to the DB instance, and the default buffer cache size is determined by the instance class (e.g., db.r5.large has ~8 GB of buffer cache). If the working set exceeds the buffer cache, the database must perform more physical reads from the distributed storage layer, which adds latency even though Aurora's storage is SSD-backed. The `shared_buffers` parameter can be tuned, but it is capped by instance memory; choosing an instance class with more memory (e.g., db.r5.xlarge or higher) often resolves the issue.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The DB instance class does not have enough memory for the buffer cache. — Option A is correct because after migrating to Aurora PostgreSQL, the same complex queries are slower, which often indicates that the buffer cache is too small to hold the working set of data. Aurora uses a distributed storage system where the buffer cache is managed by the DB instance's memory; if the instance class lacks sufficient memory, frequently accessed data pages must be read from storage more often, increasing I/O latency. This is a common performance bottleneck when migrating from on-premises PostgreSQL, where the buffer cache might have been larger or the working set fit entirely in memory.
What should I do if I get this DBS-C01 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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Question Discussion
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