Question 10 of 1,663
DBS-C01 Management and Operations Practice Question
A database administrator is troubleshooting a slow-running query on an Amazon RDS for PostgreSQL DB instance. The query plan shows a sequential scan on a large table. The table has a primary key and an index on the column used in the WHERE clause. Why might the query optimizer choose a sequential scan over an index scan?
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
✓
The query is expected to return a large percentage of rows.
When a query is expected to return a large percentage of rows, a sequential scan is often more efficient than an index scan due to the overhead of random I/O from the index. Option B is wrong because the scenario includes a WHERE clause, so SELECT * without WHERE does not apply. Option C is wrong because outdated statistics could cause the optimizer to choose a sequential scan, but the question asks for a reason the planner might choose sequential scan; outdated stats are a possible cause but not the direct reason given in A. Option D is wrong because the index order does not automatically force its use; the optimizer decides based on selectivity.
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 query is expected to return a large percentage of rows.
Why this is correct
Correct: When the query returns a large percentage of rows, a sequential scan is more efficient than random I/O from an index.
- ✗
The query is a SELECT * without a WHERE clause.
Why it's wrong here
Incorrect: The scenario describes a query with a WHERE clause, so this option does not apply.
- ✗
The table's statistics are outdated.
Why it's wrong here
Incorrect: Outdated statistics can cause suboptimal plans, but the question asks for why the optimizer might choose sequential scan; selectivity is the key factor.
- ✗
The index on the column is not being used because it is a composite index with a different column order.
Why it's wrong here
Incorrect: The composite index order may affect usability, but the optimizer still decides based on row selectivity.
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Last reviewed: Jun 20, 2026
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