7.2 Query Folding Explained

Key Concepts

Query Folding
Data Source Compatibility
Performance Benefits
Implementation in Power Query
Troubleshooting Query Folding

Query Folding

Query Folding is a feature in Power Query that allows transformations and filtering to be pushed down to the data source, rather than being processed in memory. This means that the data source (e.g., SQL Server, Excel, etc.) performs the transformations, which can significantly improve performance and reduce memory usage.

Data Source Compatibility

Not all data sources support query folding. Compatibility depends on the capabilities of the data source and the specific transformations being applied. Common data sources that support query folding include SQL Server, Oracle, and Azure SQL Database.

Performance Benefits

Query folding offers several performance benefits:

Reduced Data Transfer: Only the necessary data is transferred from the source, reducing network load.
Faster Processing: Transformations are handled by the data source, which is often more efficient than in-memory processing.
Lower Memory Usage: By offloading transformations to the data source, less memory is required on the client side.

Implementation in Power Query

To implement query folding in Power Query, follow these steps:

1. Connect to your data source (e.g., SQL Server).
2. Apply transformations (e.g., filtering, grouping) in the Power Query Editor.
3. Check if query folding is supported by reviewing the M code generated.
4. If query folding is supported, the transformations will be pushed down to the data source.

Troubleshooting Query Folding

If query folding is not working as expected, consider the following troubleshooting steps:

Check Data Source: Ensure the data source supports the transformations being applied.
Review M Code: Examine the M code generated in Power Query to see if the transformations are being pushed down to the data source.
Simplify Transformations: Break down complex transformations into simpler steps to see if query folding is supported for each step.

Examples and Analogies

Example: Filtering Data with Query Folding

Imagine you have a large dataset in SQL Server and you want to filter it to include only records from the year 2023. With query folding, the filtering operation can be pushed down to SQL Server, which will execute the filter directly on the server side.

let
    Source = Sql.Database("ServerName", "DatabaseName"),
    FilteredRows = Table.SelectRows(Source, each [Year] = 2023)
in
    FilteredRows

Analogy: Query Folding as a Remote Assistant

Think of query folding as having a remote assistant who handles tasks for you. Instead of bringing all the data to your desk and sorting through it yourself, you instruct the assistant to perform the sorting and filtering at their location. This not only saves you time but also reduces the amount of work you need to do.