The answer is a single RDS for PostgreSQL instance with a shared schema and Row-Level Security (RLS) policies based on tenant_id. This design is the most cost-effective and operationally efficient because RLS enforces data isolation at the row level using a tenant_id column, eliminating the need for separate schemas or databases per tenant—a critical advantage when managing millions of tenants with small datasets under 100 MB. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of balancing security isolation with cost optimization; a common trap is over-engineering with separate schemas or instances, which drives up cost and operational overhead without performance benefit for small datasets. Remember that RLS in PostgreSQL applies security policies transparently at query time, so you get multi-tenant row-level security without complex application logic. Memory tip: think “RLS = Row-Level Savings” to recall that shared schema with RLS is the leanest path for high-tenant, low-data workloads.
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 designing a multi-tenant SaaS application on Amazon RDS for PostgreSQL. Each tenant's data must be isolated for security and performance. The application has millions of tenants, with most tenants having small datasets (under 100 MB). Which database design pattern is MOST cost-effective and operationally efficient?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a single RDS instance with a shared schema and implement Row-Level Security (RLS) policies based on tenant_id.
Option B is correct because using a single RDS for PostgreSQL instance with Row-Level Security (RLS) allows you to isolate tenant data at the row level based on a tenant_id column, without the overhead of managing millions of separate schemas or tables. This design is both cost-effective (single instance, no per-tenant provisioning) and operationally efficient (simple schema management, no connection pooling issues), while still meeting security and performance isolation requirements for small datasets under 100 MB.
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.
✗
Use Amazon DynamoDB with a separate table per tenant.
Why it's wrong here
DynamoDB is NoSQL; the application may require relational features, and separate tables for millions of tenants is not scalable.
✓
Use a single RDS instance with a shared schema and implement Row-Level Security (RLS) policies based on tenant_id.
Why this is correct
RLS provides tenant isolation with minimal overhead, suitable for many small tenants.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a single RDS instance with a separate schema per tenant.
Why it's wrong here
Managing millions of schemas in one database is inefficient and can cause performance issues.
✗
Use a separate Amazon RDS for PostgreSQL instance per tenant.
Why it's wrong here
This is not cost-effective for millions of small tenants; it would create massive operational overhead and cost.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume separate schemas per tenant (Option C) are the best balance of isolation and cost, but they overlook PostgreSQL's practical limits on the number of schemas and the severe performance degradation from catalog bloat when dealing with millions of tenants.
Detailed technical explanation
How to think about this question
PostgreSQL Row-Level Security (RLS) works by applying a security policy to a table that automatically appends a WHERE clause (e.g., tenant_id = current_setting('app.tenant_id')) to every query, ensuring tenants can only see their own rows. This approach leverages the database engine's query planner to enforce isolation transparently, and with proper indexing on tenant_id, performance remains predictable even with millions of rows. In practice, you must also manage connection pooling carefully (e.g., using PgBouncer) to pass the tenant context securely, and consider using a separate read replica for reporting to avoid contention.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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: Use a single RDS instance with a shared schema and implement Row-Level Security (RLS) policies based on tenant_id. — Option B is correct because using a single RDS for PostgreSQL instance with Row-Level Security (RLS) allows you to isolate tenant data at the row level based on a tenant_id column, without the overhead of managing millions of separate schemas or tables. This design is both cost-effective (single instance, no per-tenant provisioning) and operationally efficient (simple schema management, no connection pooling issues), while still meeting security and performance isolation requirements for small datasets under 100 MB.
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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Question Discussion
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