DEA-C01 Data Operations and Support Practice Question
A company runs a data warehouse on Amazon Redshift. The data engineer notices that some queries are running slowly. Upon reviewing the system tables, the engineer finds that the 'svv_table_info' shows high 'unsorted' percentage for several large tables. What is the MOST effective action to improve query performance?
⚠ Common exam trap
DEA-C01 often tests the distinction between VACUUM (re-sorts and reclaims space) and ANALYZE (updates statistics), tricking candidates into choosing ANALYZE when the symptom is a high unsorted percentage.
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
✓
Run the VACUUM command on the tables.
A high 'unsorted' percentage in svv_table_info means many rows were added after the last sort and are not in sort-key order, forcing Redshift to scan more blocks and slowing range-restricted queries. VACUUM re-sorts the table and reclaims space, restoring the benefits of the sort key. This is the direct, most effective fix for the reported symptom.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Run the ANALYZE command on the tables.
Why it's wrong here
ANALYZE refreshes table statistics for the query planner, which addresses stale or missing statistics, not the high unsorted percentage reported by svv_table_info. That metric reflects rows stored outside sort-key order, so VACUUM SORT is required. ANALYZE would be the right action when queries suffer from poor cardinality estimates.
- ✓
Run the VACUUM command on the tables.
Why this is correct
Running VACUUM re-sorts rows into the table's defined sort key order, directly reducing the high unsorted percentage that svv_table_info reports. Because those large tables were loaded without sorted data, range-restricted scans read far more blocks than necessary; restoring sort order lets Redshift apply zone-map block pruning, cutting I/O for the slow queries.
- ✗
Change the distribution style of the tables to ALL.
Why it's wrong here
Distribution style ALL replicates the entire table to every node, which changes data placement across slices rather than restoring sort-key ordering within each slice; the unsorted percentage remains high. ALL suits small dimension tables joined frequently, not large fact tables with high unsorted percentages.
- ✗
Increase the number of nodes in the Redshift cluster.
Why it's wrong here
Adding nodes increases cluster compute and storage capacity, but it does not re-sort existing rows within each slice, so the high unsorted percentage persists and scans still read unnecessary blocks. Scaling out suits sustained CPU, memory or concurrency pressure, not sort-order degradation on large tables.
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Same concept, more angles
1 more way this is tested on DEA-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company uses Amazon Redshift for its data warehouse. A data engineer notices that queries are running slowly and the system's disk space is nearly full. The engineer runs the STV_PARTITIONS view and sees that many slices have high 'tossed' counts. What does this indicate, and what should the engineer do?
medium- A.The tossed rows are permanent and cannot be reclaimed; the engineer should perform a deep copy to a new table.
- B.The tossed rows indicate that the sort key is not optimal; redefining the sort key will reduce tossed rows.
- C.The tossed rows are due to data skew; redistribute the table on a different distribution key.
- ✓ D.The tossed rows are deleted rows that need to be reclaimed by running VACUUM.
Why D: In Amazon Redshift, the STV_PARTITIONS view shows information about slices, including 'tossed' rows. Tossed rows are rows that were previously deleted but not yet reclaimed by VACUUM. High tossed counts indicate that disk space is being wasted by deleted rows, and running VACUUM will reclaim that space and improve query performance. The other options are incorrect: tossed rows are not permanent, not related to sort keys, and not due to data skew.
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
This DEA-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DEA-C01 exam.