Extract, Transform and Load Data in Power BI course with an error in judgement in content of Module1

You are absolutely thinking in the right direction strategically, but the important thing is this:

Dual mode is NOT a general performance optimization switch.
It is a relationship optimization tool in composite models.

Where the misunderstanding usually happens

Your arguments describe business benefits of mixing Import + DirectQuery.

However:

• Dual mode does not automatically decide between Import and DirectQuery per query
• It only applies to dimension tables
• It works only in composite models

What Dual Mode Actually Does

Dual tables:

• Behave as Import when used with Import tables
• Behave as DirectQuery when used with DirectQuery tables

It prevents:

• Limited relationships
• Performance degradation in composite models

It is not meant to:

• Improve scalability alone
• Reduce costs automatically
• Replace Import strategy
• Replace DirectQuery strategy

Where Your Points Need Adjustment
“Dual automatically optimizes query performance”

Not exactly. Power BI doesn’t dynamically “choose best mode per query” for facts.

Dual only affects how dimension tables participate in composite queries.

“Dual handles historical + real-time data”

That’s actually:

• Composite Model (Import + DirectQuery fact tables)
• Dual is just a helper for dimensions.

“Dual reduces cloud query cost”

No. If a query touches DirectQuery fact, it will hit the source regardless.

Dual does not reduce source hits automatically.

When You SHOULD Use Dual

Use Dual when:

• You have DirectQuery fact table
• You have Import aggregation table
• Shared dimension table connects to both
• You want to avoid performance penalty of limited relationships

Classic PL-300 example scenario:

• Sales (DirectQuery)
• Sales Aggregated (Import)
• Date / Customer → Dual

Your reasoning is strategically strong but technically, the transcript likely simplifies or correctly limits Dual’s scope.
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Jaywant Thorat | MCT | Data Analytics Coach
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