Query folding in Power Query in Power BI - Dashboard Guide

Dashboard Mode - Query folding in Power Query

Goal

Understand how query folding improves data refresh speed by pushing transformations to the data source in Power Query.

Sample Data

OrderID	Customer	Region	OrderDate	Sales
1001	Alpha Co	East	2024-01-10	250
1002	Beta LLC	West	2024-02-15	450
1003	Gamma Inc	East	2024-03-20	300
1004	Delta Ltd	North	2024-04-05	500
1005	Epsilon	South	2024-05-12	150

Dashboard Components

• KPI Card: Total Sales
  Formula: = List.Sum(Source[Sales])
  Shows total sales amount from all orders.
• Table: Filtered Orders (Region = East)
  Formula: = Table.SelectRows(Source, each [Region] = "East")
  Shows only orders from the East region.
• Chart: Sales by Region
  Formula: = Table.Group(Source, {"Region"}, {{"TotalSales", each List.Sum([Sales]), type number}})
  Aggregates sales per region for visualization.
• Query Folding Indicator
  Shows if the applied filter and grouping steps are folded to the data source for efficiency.

Dashboard Layout

+----------------------+-----------------------+
|      Total Sales      |   Sales by Region     |
|      (KPI Card)       |       (Chart)         |
+----------------------+-----------------------+
|       Filtered Orders (Table)                 |
+----------------------------------------------+
|           Query Folding Indicator             |
+----------------------------------------------+

Interactivity

User can select a region filter. This filter updates the Filtered Orders table and the Sales by Region chart. The Total Sales KPI updates to show sales only for the selected region. Query folding status updates to show if the filter is pushed to the data source.

Self Check

If you add a filter for Region = "West", which components update and how?

• Total Sales KPI shows sum of sales for West region only (450).
• Filtered Orders table shows only orders from West region (OrderID 1002).
• Sales by Region chart updates to show sales only for West region.
• Query Folding Indicator shows if the filter step is folded to the source.

Key Result

Dashboard shows total sales, sales by region, filtered orders by region, and query folding status in Power Query.

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