- A
Convert the table to columnar storage using Amazon Redshift
Why wrong: Redshift is a different service, not a design change within RDS.
- B
Add a read replica to offload queries
Why wrong: Read replicas do not improve the primary instance's buffer cache.
- C
Migrate the table to Amazon DynamoDB with DAX
Why wrong: Migrating to a different database type is a major change, not a simple design change.
- D
Increase the instance size to provide more memory
More memory increases buffer cache capacity, reducing disk I/O.
Question 115 of 1,730
Oracle RDS Low Buffer Cache Hit Ratio Improvement
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 running an Oracle database on Amazon RDS. The database has a large table that is frequently accessed by multiple applications. The DBA notices that the table has a high number of index scans but the queries are still slow. Upon investigation, the buffer cache hit ratio is low. Which design change would BEST improve performance?
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
✓
Increase the instance size to provide more memory
The low buffer cache hit ratio indicates that the database's memory (buffer cache) is insufficient to cache frequently accessed data blocks, causing excessive physical I/O. Increasing the instance size provides more memory, which expands the buffer cache and allows more data to be cached, reducing disk reads and improving query performance. This directly addresses the root cause of the slow queries despite efficient index scans.
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.
- ✗
Convert the table to columnar storage using Amazon Redshift
Why it's wrong here
Redshift is a different service, not a design change within RDS.
- ✗
Add a read replica to offload queries
Why it's wrong here
Read replicas do not improve the primary instance's buffer cache.
- ✗
Migrate the table to Amazon DynamoDB with DAX
Why it's wrong here
Migrating to a different database type is a major change, not a simple design change.
- ✓
Increase the instance size to provide more memory
Why this is correct
More memory increases buffer cache capacity, reducing disk I/O.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose a read replica (Option B) thinking it will distribute the load, but they fail to recognize that the low buffer cache hit ratio is a memory constraint on the primary instance, not a CPU or connection bottleneck, so offloading queries does not solve the underlying caching issue.
Detailed technical explanation
How to think about this question
The buffer cache hit ratio is calculated as (logical reads - physical reads) / logical reads; a low ratio (e.g., below 99%) indicates that the working set of data exceeds the available cache memory. In Amazon RDS for Oracle, the buffer cache is part of the SGA (System Global Area), and its size is determined by the instance class's memory allocation; increasing the instance size (e.g., from db.r5.large to db.r5.xlarge) doubles the memory, allowing the SGA to be resized to cache more blocks and reduce physical I/O. This is particularly effective for tables with high index scan activity because index scans often require random I/O, which benefits greatly from caching.
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.
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FAQ
Questions learners often ask
What does this DBS-C01 question test?
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: Increase the instance size to provide more memory — The low buffer cache hit ratio indicates that the database's memory (buffer cache) is insufficient to cache frequently accessed data blocks, causing excessive physical I/O. Increasing the instance size provides more memory, which expands the buffer cache and allows more data to be cached, reducing disk reads and improving query performance. This directly addresses the root cause of the slow queries despite efficient index scans.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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