Query folding in Power Query in Power BI - Cell-by-Cell Formula Trace

Sample Data

This table shows products with their IDs, names, categories, and prices.

Cell	Value
A1	ProductID
B1	ProductName
C1	Category
D1	Price
A2	101
B2	Chair
C2	Furniture
D2	50
A3	102
B3	Desk
C3	Furniture
D3	150
A4	103
B4	Pen
C4	Stationery
D4	5
A5	104
B5	Notebook
C5	Stationery
D5	10

Formula Trace

Table.SelectRows(Source, each [Category] = "Furniture" and [Price] > 40)

Step 1: Source

Step 2: each [Category] = "Furniture"

Step 3: each [Price] > 40

Step 4: each [Category] = "Furniture" and [Price] > 40

Step 5: Table.SelectRows(Source, condition)

Cell Reference Map

     A          B           C           D
1 | ProductID | ProductName | Category  | Price
2 |    101   |   Chair    | Furniture |  50  
3 |    102   |   Desk     | Furniture |  150 
4 |    103   |   Pen      | Stationery|  5   
5 |    104   | Notebook   | Stationery|  10  

Formula references columns C (Category) and D (Price) for filtering.

The formula uses the Category column (C) and Price column (D) to filter rows.

Result

     A          B           C           D
1 | ProductID | ProductName | Category  | Price
2 |    101   |   Chair    | Furniture |  50  
3 |    102   |   Desk     | Furniture |  150 

Only rows with Category 'Furniture' and Price > 40 remain after filtering.

The filtered table shows only furniture products priced above 40.

Sheet Trace Quiz - 3 Questions

Which rows does the filter condition keep?

A Rows where Category is 'Furniture' and Price > 40

B Rows where Category is 'Stationery' and Price > 40

C Rows where Price < 40

D All rows

Key Result

Table.SelectRows filters rows by applying a condition to each row.

Practice

1. What is query folding in Power Query?

easy

A. It pushes data transformations back to the data source for faster processing.

B. It copies all data locally before applying transformations.

C. It disables all data refresh options.

D. It automatically creates charts from data.

Solution

1. Step 1: Understand the concept of query folding

   Query folding means Power Query sends transformation steps back to the data source to run there.

2. Step 2: Compare options with this definition

   Only It pushes data transformations back to the data source for faster processing. describes pushing transformations to the source for speed.

3. Final Answer:

   It pushes data transformations back to the data source for faster processing. -> Option A

4. Quick Check:

   Query folding = pushing steps to source [OK]

Hint: Query folding means running steps at the source, not locally [OK]

Common Mistakes:

• Thinking query folding copies data locally first
• Confusing query folding with data visualization
• Believing query folding disables refresh

2. Which of the following Power Query steps supports query folding?

easy

A. Adding a custom column with complex M code referencing external files

B. Creating a manual index column

C. Merging queries with a local Excel table

D. Filtering rows based on a column value

Solution

1. Step 1: Identify steps that can be folded

   Simple filters on source columns usually fold back to the database.

2. Step 2: Analyze other options

   Custom columns with external references, merges with local tables, and manual indexes break folding.

3. Final Answer:

   Filtering rows based on a column value -> Option D

4. Quick Check:

   Filters fold, complex custom steps don't [OK]

Hint: Simple filters fold; complex or local steps usually don't [OK]

Common Mistakes:

• Assuming all steps fold automatically
• Thinking merges with local tables fold
• Believing manual indexes fold

3. Given this Power Query M code, which step breaks query folding?

let
  Source = Sql.Database("Server", "DB"),
  FilteredRows = Table.SelectRows(Source, each [Status] = "Active"),
  AddedCustom = Table.AddColumn(FilteredRows, "Custom", each Text.Length([Name]))
in
  AddedCustom

medium

A. Source step

B. FilteredRows step

C. AddedCustom step

D. None, all steps fold

Solution

1. Step 1: Check each step for folding support

   Source connects to SQL, supports folding. Filtering rows on a column also folds.

2. Step 2: Analyze the AddedCustom step

   Adding a custom column with a function like Text.Length is done locally and breaks folding.

3. Final Answer:

   AddedCustom step -> Option C

4. Quick Check:

   Custom columns break folding [OK]

Hint: Custom columns usually break folding; filters don't [OK]

Common Mistakes:

• Thinking all steps fold if source is SQL
• Assuming filters break folding
• Ignoring custom column impact

4. You notice your Power Query refresh is slow. Which fix helps restore query folding?

medium

A. Replace a custom column step with a simple filter step

B. Add a manual index column before filtering

C. Merge with a local Excel table before filtering

D. Disable the data source privacy settings

Solution

1. Step 1: Identify what breaks folding

   Custom columns and merges with local tables break folding and slow refresh.

2. Step 2: Choose a fix that restores folding

   Replacing custom columns with simple filters allows folding to resume.

3. Final Answer:

   Replace a custom column step with a simple filter step -> Option A

4. Quick Check:

   Simple filters restore folding, custom steps break it [OK]

Hint: Use filters instead of custom columns to keep folding [OK]

Common Mistakes:

• Adding manual indexes early breaks folding
• Merging with local tables breaks folding
• Thinking privacy settings affect folding

5. You want to optimize a Power Query that merges a SQL table with a large local Excel table. How can you maximize query folding?

hard

A. Load the local Excel table to SQL before merging

B. Filter the SQL table first, then merge with the local Excel table

C. Merge first, then filter the combined table

D. Disable query folding to avoid errors

Solution

1. Step 1: Understand folding with merges

   Merging with local tables breaks folding, but filtering source first keeps folding for that part.

2. Step 2: Choose the best approach

   Filtering SQL table before merging reduces data and preserves folding on the source side.

3. Final Answer:

   Filter the SQL table first, then merge with the local Excel table -> Option B

4. Quick Check:

   Filter source before merge to keep folding [OK]

Hint: Filter source data before merging with local tables [OK]

Common Mistakes:

• Merging before filtering breaks folding early
• Assuming disabling folding helps performance
• Not considering loading local data to SQL