Why not just always use DUAL instead of IMPORT?

[MikeSeim]

Am I crazy? From all I can tell, DUAL data storage is really "IMPORT v2".  It seems as if it stores the data and is refreshed per user defined schedule. It's clearly NOT DirectQuery as it increases the file size.

So it sure seems that DUAL is IMPORT by another name.

I've seen countless blogs and explainations... I'm too thick skulled. I don't understand. At this point, I feel as if I don't need to. From what I can tell, "DUAL" is merely "IMPORT" with additional benefits that I might one day use (ex. switching to DirectQuery, ex. joining to DirectQuery connections).

So... am I right?  Can anyone confirm that this is the case.

Because if I'm right, it seems the go-forward-direction to my broader team is to always use DUAL instead of IMPORT from now on.

Watcher: @SWon

[lbendlin · Accepted solution]

If I have a crystal ball and can predict with certainty

no need to predict anything - use DAX Studio to see exactly which query type is happening.

Is there any reason NOT TO use DUAL instead of IMPORT?

That's a great question.  Dual Mode tables have the same restrictions as Direct Query mode tables. 

Use storage mode in Power BI Desktop - Power BI | Microsoft Learn

May not be what you want.

[Anonymous · Accepted solution]

Hi @MikeSeim ,

As @lbendlin  answered, dual mode also has some limitations, you can choose the right connection mode according to your needs, please refer to this to understand the advantages and disadvantages of different connection modes.

Best Regards,

Neeko Tang

If this post  helps, then please consider Accept it as the solution  to help the other members find it more quickly. 

[lbendlin]

Look at it from the query engine's perspective. The query engine prefers to join "like"  tables, ie two import mode tables (with all the expanded table baggage) or two Direct Query tables.  What it doesn't like is when it has to combine tables from different modes - that's where the materialization penalty kicks in.

Dual is not Import - unless it's advantageous to the query engine.

Any import table that only ever joins to other import tables doesn't need to be Dual. Same for Direct Query tables that only ever join to other Direct Query tables.

[MikeSeim]

Thanks for your thoughts on the matter.

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Regarding

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Dual is not Import - unless it's advantageous to the query engine

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I'm not convinced. See the image below. I saved a simple PBIX with one data source each three times. Once per DataStorage method. No other data sources. Notice how BOTH the DUAL and IMPORT DataStorage files are large. That would indicate that they are BOTH storing (ie, importing) the data.  Plus, BOTH can be scheduled whereas DirectQuery cannot be scheduled.

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Regarding

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Any import table that only ever joins to other import tables doesn't need to be Dual. Same for Direct Query tables that only ever join to other Direct Query tables.

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Admittedly, that's the heart of my question -- the "only ever" part.  If I have a crystal ball and can predict with certainty whether or not a particular report will ever need to flip from DirectQuery to Import then this wouldn't be a question.

But the reality is, while rare, there are occassions when an IMPORT data source is desired to be switched to a DirectQuery.  When those situations come up it if I have an IMPORT data source I am out of luck. I have to rebuild which is unfortunate.  It would seem with DUAL I can avoid this potential problem.

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Simplier question

So how about this... rather than asking all that I did let me ask the question a different way.

Is there any reason NOT TO use DUAL instead of IMPORT?

[lbendlin · Accepted solution]

If I have a crystal ball and can predict with certainty

no need to predict anything - use DAX Studio to see exactly which query type is happening.

Is there any reason NOT TO use DUAL instead of IMPORT?

That's a great question.  Dual Mode tables have the same restrictions as Direct Query mode tables. 

Use storage mode in Power BI Desktop - Power BI | Microsoft Learn

May not be what you want.