The correct answer is that the SUMX iterator can be slow on large tables, so you should consider using CALCULATE instead. This is because SUMX is an iterator function that evaluates an expression row by row across the entire filtered table, and with a Sales table containing 10 million rows, this row-by-row processing creates a significant performance bottleneck. On the Microsoft Power BI Data Analyst PL-300 exam, this question tests your understanding of iterator versus filter context performance, often appearing as a trap where candidates assume FILTER is the culprit—but FILTER itself is not inherently slow; the issue is the iterative nature of SUMX. A common memory tip is to remember that iterators like SUMX are "row-by-row tortoises," while CALCULATE with a filter argument is a "set-based hare" that processes the entire table in one pass, making it far more efficient for large datasets.
PL-300 Model the data Practice Question
This PL-300 practice question tests your understanding of model the data. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
```
DAX Measure:
Total Sales =
SUMX(
FILTER(
Sales,
Sales[Quantity] > 10
),
Sales[Amount]
)
```
You have a DAX measure that sums the Amount column for rows where Quantity is greater than 10. The Sales table has 10 million rows. What is a potential performance issue with this measure?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SUMX iterator can be slow on large tables; consider using CALCULATE instead
Option D is correct because SUMX iterates over each row in the filtered table, which can be slow for large tables. It is generally more efficient to use CALCULATE with a filter argument. Option A is wrong because the FILTER function does not inherently cause table scans; it creates a filtered table. Option B is wrong because the measure uses a simple condition, not complex. Option C is wrong because the measure is not context-based in a way that would cause issues.
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 SUMX iterator can be slow on large tables; consider using CALCULATE instead
Why this is correct
CALCULATE with a filter is often more efficient than SUMX for simple aggregations.
Related concept
Read the scenario before looking for a memorised answer.
✗
The FILTER function will cause a full table scan each time the measure is evaluated
Why it's wrong here
FILTER does scan the table, but the issue is more about iteration.
✗
The measure uses context transition which can cause performance issues
Why it's wrong here
Context transition is not used here; SUMX is an iterator.
✗
The measure uses a complex condition that cannot be optimized
Why it's wrong here
The condition is simple (Quantity > 10).
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Identify which PL-300 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Model the data — This question tests Model the data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The SUMX iterator can be slow on large tables; consider using CALCULATE instead — Option D is correct because SUMX iterates over each row in the filtered table, which can be slow for large tables. It is generally more efficient to use CALCULATE with a filter argument. Option A is wrong because the FILTER function does not inherently cause table scans; it creates a filtered table. Option B is wrong because the measure uses a simple condition, not complex. Option C is wrong because the measure is not context-based in a way that would cause issues.
What should I do if I get this PL-300 question wrong?
Identify which PL-300 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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