DEA-C01 Data Store Management Practice Question
A data engineer is troubleshooting an Amazon Redshift cluster that has been experiencing slow query performance. The engineer checks the system tables and finds that many queries are waiting on 'wlm_queued' time. The cluster has 10 nodes and uses automatic WLM. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often confuse 'wlm_queued' with resource contention (like CPU or I/O), but WLM queuing specifically indicates a concurrency limit, not a performance bottleneck during execution.
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 number of concurrent queries exceeds the available query slots.
When queries show 'wlm_queued' time in Amazon Redshift, it indicates they are waiting in the Workload Management (WLM) queue before execution. With automatic WLM, the system dynamically manages concurrency, but if the number of concurrent queries exceeds the available query slots (determined by the cluster's memory and concurrency scaling settings), queries will be queued. This is the most direct cause of 'wlm_queued' wait events, as WLM queues queries when all slots are occupied.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Network bandwidth saturation between nodes.
Why it's wrong here
wlm_queued measures time spent waiting in a workload management queue for a query slot, so the cause is concurrency limits or slot allocation, not inter-node bandwidth. Network saturation would appear as high bytes transferred or redistribution delays; it is tempting because a 10-node cluster moves data between nodes during joins.
- ✗
Sorting operations are too expensive.
Why it's wrong here
wlm_queued time indicates queries waiting for a WLM queue slot, not sorting spill; expensive sorts surface as high scan or sort step times instead. Sorting optimisation would be the answer where query plans show large unsorted joins or missing sort keys, but queue contention is the actual bottleneck here.
- ✓
The number of concurrent queries exceeds the available query slots.
Why this is correct
High 'wlm_queued' wait time indicates queries are sitting in a workload management queue rather than executing. With automatic WLM, concurrency scaling is not configured here, so when simultaneous queries exceed the query slots automatic WLM allocates per queue, they queue. This directly matches the stem's observed wait event.
- ✗
Insufficient disk space on the cluster.
Why it's wrong here
Disk space exhaustion surfaces as disk-based query or write activity, not as wlm_queued wait time, which specifically counts time queries spend waiting in an automatic WLM queue for a query slot. Insufficient storage would be the cause when queries fail with disk-full errors or spill to disk during large sorts and hash joins.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.