DBS-C01 Workload-Specific Database Design Practice Question
A company is migrating an on-premises PostgreSQL database to Amazon RDS for PostgreSQL. The database has a large table that is frequently accessed by reporting queries. The reporting queries filter on a column that has a high cardinality but low selectivity. To optimize query performance on this table, which design choice should the database specialist recommend?
⚠ Common exam trap
Many exam-takers choose partitioning (Option A) for any large table with filtering, but fail to recognize that low selectivity means partitioning offers no pruning benefit, while a covering index directly reduces I/O by avoiding heap access.
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
✓
Create a covering index on the filter column
A covering index includes all columns needed by the reporting queries, allowing PostgreSQL to satisfy the query entirely from the index without accessing the heap (table) pages. This eliminates the overhead of random I/O for row lookups, which is especially beneficial when filtering on a high-cardinality, low-selectivity column where many rows match but the index scan alone can return the required data. In Amazon RDS for PostgreSQL, this reduces read IOPS consumption and improves query latency.
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 table by the filter column
Why it's wrong here
Partitioning helps with large table management but doesn't optimize queries filtering on low-selectivity columns.
- ✗
Use a read replica to offload reporting queries
Why it's wrong here
Read replicas scale read traffic but do not optimize the performance of an individual query.
- ✗
Increase the provisioned read IOPS for the RDS instance
Why it's wrong here
Provisioned IOPS improves I/O performance but does not directly optimize query execution plans.
- ✓
Create a covering index on the filter column
Why this is correct
A covering index includes all columns needed, allowing query results to be returned from the index alone.
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