Query folding in Power Query in Power BI - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is Query Folding in Power Query?

Which of the following best describes query folding in Power Query?

A The process where Power Query translates steps into native queries executed by the data source.

B A method to manually write SQL queries inside Power Query editor.

C A feature that automatically compresses data files before loading.

D A way to create visual reports directly from Power Query.

💡 Hint

Think about how Power Query optimizes data retrieval from the source.

Query folding means Power Query sends transformations as native queries to the data source, improving performance by reducing data transfer.

📊 Formula Result

intermediate

Identifying Query Folding Status

You apply a filter step in Power Query on a SQL Server table. Which option correctly shows how to check if query folding is still active?

A Right-click the last step and select 'View Native Query'.

B Check the 'Applied Steps' pane for a green checkmark.

C Look for a 'Query Folding Enabled' message in the status bar.

D Open the Advanced Editor and search for 'fold'.

💡 Hint

Power Query provides a direct way to see the native query generated.

Right-clicking the last step and choosing 'View Native Query' shows the SQL query Power Query sends to the source, confirming folding.

❓ Function Choice

advanced

Which Power Query function breaks Query Folding?

Which of these Power Query functions will break query folding when applied?

A Table.Sort

B Table.SelectRows

C Table.RemoveColumns

D Table.TransformColumns with a custom M function

💡 Hint

Consider which function uses custom code that the data source cannot translate.

Using Table.TransformColumns with a custom M function often breaks folding because the data source cannot execute custom M code.

🎯 Scenario

advanced

Optimizing Query Folding in a Complex Query

You have a Power Query that merges two large tables and then adds a custom column with a complex M expression. Which approach will best preserve query folding?

A Avoid merging and instead append the tables.

B Perform the merge first, then add the custom column in a separate step after loading.

C Add the custom column inside the merge step using a native SQL query.

D Add the custom column before merging the tables.

💡 Hint

Think about when custom M code breaks folding and how to isolate it.

Performing the merge first preserves folding for that step. Adding the custom column after loading avoids breaking folding during the merge.

❓ data_analysis

expert

Determining Query Folding Impact on Performance

You notice your Power Query refresh is slow. You suspect query folding is broken. Which of the following observations best indicates that query folding is not happening?

A The query uses only built-in Power Query functions.

B The data preview loads instantly but the final load is slow.

C The query refresh time is much longer than expected and 'View Native Query' is disabled.

D The data source is a local Excel file.

💡 Hint

Consider what disables the 'View Native Query' option and how it relates to performance.

If 'View Native Query' is disabled, it means folding is broken. This usually causes slower refresh because all data is pulled before transformations.