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Workload-Specific Database Design →mediumMultiple ChoiceObjective-mapped

Redshift Join Performance — Distribution Key Mismatch

This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A company uses Amazon Redshift for data warehousing. They run a query that joins a large fact table (10 billion rows) with a small dimension table (1 million rows). The query is slow. The distribution style of the fact table is AUTO, and the dimension table has DISTSTYLE ALL. The join key is user_id. What is the MOST likely reason for the poor performance?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The dimension table does not have a sort key on user_id
Why wrong: Sort key helps with range queries but not necessarily with hash joins.
B
The fact table's distribution key is not user_id, causing redistribution
AUTO may distribute by another key, leading to large data movement during join.
C
The dimension table uses DISTSTYLE ALL, which is inefficient for joins
Why wrong: DISTSTYLE ALL is efficient for small dimension tables.
D
The fact table should have column compression disabled for the join key
Why wrong: Compression is always beneficial and does not affect join performance negatively.

Full breakdown with real-world context →

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 fact table's distribution key is not user_id, causing redistribution

Option B is correct because when the fact table uses DISTSTYLE AUTO, Redshift may choose a distribution key that is not user_id. When the fact table is distributed on a different key, joining on user_id requires Redshift to redistribute the fact table rows across nodes to match the dimension table's distribution, causing significant network traffic and slower performance. The dimension table with DISTSTYLE ALL is already replicated to all nodes, so the bottleneck is the fact table's distribution mismatch.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 dimension table does not have a sort key on user_id
Why it's wrong here
Sort key helps with range queries but not necessarily with hash joins.
✓
The fact table's distribution key is not user_id, causing redistribution
Why this is correct
AUTO may distribute by another key, leading to large data movement during join.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The dimension table uses DISTSTYLE ALL, which is inefficient for joins
Why it's wrong here
DISTSTYLE ALL is efficient for small dimension tables.
✗
The fact table should have column compression disabled for the join key
Why it's wrong here
Compression is always beneficial and does not affect join performance negatively.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume DISTSTYLE ALL is always inefficient for joins, but in this scenario it is actually beneficial, while the real culprit is the fact table's distribution key not matching the join key due to AUTO assignment.

Detailed technical explanation

How to think about this question

Under the hood, Redshift's query planner evaluates distribution styles and chooses a redistribution or broadcast strategy. With DISTSTYLE AUTO, Redshift may assign a distribution key based on table size or workload patterns, but if it differs from the join key, the planner must redistribute the larger fact table across the cluster's slices, which is expensive. In contrast, if the fact table were distributed on user_id, the join could be performed locally on each slice without data movement, leveraging collocated joins.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The fact table's distribution key is not user_id, causing redistribution — Option B is correct because when the fact table uses DISTSTYLE AUTO, Redshift may choose a distribution key that is not user_id. When the fact table is distributed on a different key, joining on user_id requires Redshift to redistribute the fact table rows across nodes to match the dimension table's distribution, causing significant network traffic and slower performance. The dimension table with DISTSTYLE ALL is already replicated to all nodes, so the bottleneck is the fact table's distribution mismatch.

What should I do if I get this DBS-C01 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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