DBS-C01 Workload-Specific Database Design Practice Question
A company runs an OLTP workload on an RDS for MySQL instance. The database has a table with 50 million rows. The application frequently runs queries that join this table with a small lookup table (1000 rows) using a foreign key. The queries are slow. Which design change would most improve performance?
⚠ Common exam trap
The DBS-C01 exam often tests the misconception that partitioning or read replicas can fix join performance issues, but the real bottleneck is typically a missing index on the join column in the large table.
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
✓
Add an index on the foreign key column in the large table.
The query joins a large table (50M rows) with a small lookup table (1000 rows) on a foreign key column. Without an index on the foreign key column in the large table, MySQL must perform a full table scan for each join, leading to slow performance. Adding an index on that column allows MySQL to use an index lookup (e.g., B-tree) to quickly locate matching rows, dramatically reducing query time.
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 the large table by the foreign key column.
Why it's wrong here
Partitioning can improve data management but may not speed up joins without proper indexing.
- ✗
Scale up the RDS instance to a larger size.
Why it's wrong here
Scaling up may improve throughput but does not address the root cause of missing indexes.
- ✓
Add an index on the foreign key column in the large table.
Why this is correct
An index on the join column allows the database to quickly find matching rows, dramatically improving join performance.
- ✗
Create a read replica and direct all read queries to it.
Why it's wrong here
Read replicas help with read scaling but do not optimize individual query execution.
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