The answer is to use AVERAGEX over the Products table and sum the quantity for each product. This approach optimizes DirectQuery measure performance by iterating over the smaller dimension table (10,000 rows) rather than the massive fact table (50 million rows), drastically reducing the number of queries sent to SQL Server. In DirectQuery, every row iteration translates to a database round trip, so minimizing the iteration set is critical for speed. On the Microsoft Power BI Data Analyst PL-300 exam, this question tests your understanding of iterator functions versus aggregator functions in DirectQuery mode—a common trap is assuming CALCULATE with FILTER on the fact table is efficient, but it forces row-by-row evaluation over millions of rows. Remember the memory tip: “Iterate the small, not the tall”—always scan the dimension, not the fact, when using AVERAGEX or SUMX in DirectQuery.
PL-300 Model the data Practice Question
This PL-300 practice question tests your understanding of model the data. 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.
You have a Power BI data model that uses a DirectQuery connection to a SQL Server database. The model contains a fact table 'OrderDetails' with 50 million rows and a dimension table 'Products' with 10,000 rows. You need to create a measure that calculates the average order quantity per product. What is the best approach to optimize performance?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use AVERAGEX over the Products table and sum the quantity for each product
Option C is correct because using AVERAGEX with the Products table reduces the number of rows processed; it calculates the average per product, which is more efficient than iterating over millions of rows. Option A is wrong because CALCULATE with FILTER on the fact table can be slow. Option B is wrong because calculated columns in DirectQuery are evaluated row by row and can degrade performance. Option D is wrong because creating an aggregated table in the source is not always feasible and adds complexity.
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.
✗
Use CALCULATE with a filter on the Products table
Why it's wrong here
This may still cause the fact table to be scanned.
✗
Create a calculated column in the Products table that sums the quantity
Why it's wrong here
Calculated columns in DirectQuery are evaluated row by row and can be slow.
✓
Use AVERAGEX over the Products table and sum the quantity for each product
Why this is correct
AVERAGEX iterates over products (10,000 rows) rather than order details (50 million rows).
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a summarized table in the source database and import it
Why it's wrong here
This would change the model from DirectQuery to import, which may not meet requirements.
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: Use AVERAGEX over the Products table and sum the quantity for each product — Option C is correct because using AVERAGEX with the Products table reduces the number of rows processed; it calculates the average per product, which is more efficient than iterating over millions of rows. Option A is wrong because CALCULATE with FILTER on the fact table can be slow. Option B is wrong because calculated columns in DirectQuery are evaluated row by row and can degrade performance. Option D is wrong because creating an aggregated table in the source is not always feasible and adds complexity.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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