New Semantic Model Fallback Even There Is no Difference with Default Semantic Model

[itsmebvk]

Hi Folks,

I got stuck with some strange fallback issue.

I created Lakehouse with small table with two columns (City, ID). This table have only two rows

I created a new semantic model using SQL endpoint including table which we have in default dataset, i didn't make not even one changes at all.everything is same.

when I create a reports using these semantic models the default semantic model is acting as DirectLake mode, where as newly created semantic model acting as Direct Query Mode. 

Using SQl profiler I gathered some traces. Based on traces I have observed that New Semantic model is hitting Lakehouse table as direct query. Please see the screenshot below for more information related to this issue.

New Semantic Model Traces: 

When I compare the datasets with ALM toolkit what I have observed is its changing dataformat and aggregation in new semantic model. Also partition names seems to be different. 

I tried modifying the Datatypes and Aggs, but as per my understnding  it further leads to fallback.

Am I really missing any important point or option? Please suggest.

Thank you.

Regards

BVK

[tackytechtom]

Hi @itsmebvk ,

I do not have a quick solution at hand, but I spontanteously wondered wgether you are querying a table directly or a view (views do not work with Direct Lake)? Another reason could be security (RLS or OLS) that triggers the Direct Query fallback.

Also there is a DMV called TMSCHEMA_DELTA_TABLE_METADATA_STORAGES that has an attribute called FallbackReason that might give you some clue on what's going on 🙂

Hope this helps (at least a bit) 🙂

/Tom
https://www.tackytech.blog/
https://www.instagram.com/tackytechtom/

[itsmebvk]

Hi @tackytechtom ,

Thanks for your quick reply.

To answer your observations. 

1) I am not using views, I am using table

2) I didn't apply security or RLS/OLS either in Lakhouse or Semantic Modle layer.

3) I have already tried DMV you have suggested but its not showing details of object which is leading to fallback however its just showing 4 as fallback reasons for new semantic model.

select * from $SYSTEM.TMSCHEMA_DELTA_TABLE_METADATA_STORAGES

Default Semantic Model (Showing 0 Fallback reasons)

New Semantic Model (Showing 4 Fallback reasons) but no additional details available 

May be I am wrong, I am assuming its highlighting following four difference which happened automatically.

As far as I know I strictly followed MS suggestions to avoid any fallback but still its turning into Directquery.  😞 😞 . 

Any other inputs ?

[itsmebvk · Accepted solution]

@tackytechtom 

Ohooo I think I found the issue 🕺🕺.

Looks like I missed one last suggestion by MS which might leads to Fallback if we don't follow.

https://learn.microsoft.com/en-us/power-bi/enterprise/directlake-overview

When you create New Semantic Model, even though following option (highlighted) enabled on New Semantic Model still we need to refresh manually as highlighted. As soon as I have done that all queries turned into Dirctlakemode and no Fallback is happening.

Before Refresh :

After Refresh :

Please correct me if I am not understanding it properly. Hope this helps someone as well. 

Thanks again for your inputs.