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Understanding the Vertipaq Engine

Hello

 

I am doing a deep dive into the Vertipaq engine and I have a few questions regarding a concrete use case.

 

I have a .pbix file with two Fact Tables:

- Raw (300k rows and 17 columns) with the following structure:

LocationABCDOP
London644 544417 834961 806367 504 83 035893 560
Vancouver717 302153 883616 675997 484 779 110672 422
Copenhagen643 32917 978215 068935 608 270 478827 192
London279 920224 229774 350764 692 770 781929 427
Vancouvert158 089104 762209 37822 990 938 29593 513
Vancouver436 20670 139223 948482 096 136 600930 017
Copenhagen290 725986 045260 513714 680 491 096293 309
       

 

- Unpiv which is the unpivoted version of the 'Raw' table above (4.8 Million rows and 3 columns). So it has the following structure:

LocationTypeSales
LondonA644 544
LondonB417 834
LondonC961 806
LondonD367 504
LondonO83 035
LondonP893 560
VancouverA717 302
VancouverB153 883
VancouverC616 675
  
CopenhagenA643 329

 

Obviously, I used the super Vertipaq analyzer from @marcorusso & @AlbertoFerrari to understand data compression.

Not surprinsingly, because Vertipaq uses a columnar storage, my 'Unpiv' Table is ~45% smaller than 'Raw' table:

Result from the Vertipaq analyserResult from the Vertipaq analyser

We clearly see (i) the impact of the cardinality of the columns on their total size (ii) the relevance of unpivoting the table. Fair.

 

Here are my questions :

1. As we can see above, the size difference between 'Raw' and 'Unpiv' is mainly due to the "Column Hierarchies". What is it about ? I found no real explanation on the web or on Marco Russo's book. I read somewhere that it has to do with MDX structure...?!

 

2.  How is the size of Column Hierarchy computed? How can it be estimated or perhaps turned off? I understand the logic of value encoding , dictionnary encoding and RLE encoding. Is it something similar ?

 

3. What is the unit of the measures displayed by the Vertipaq analyser in the pivot table above ? When I save my .pbix file on my disk, its total size is: 74MB. I have no relationships between 'Raw' & 'Unpiv' and 0 graph on my report. so the data model itself is ~ 73MB (extracted by zipping the .pbix file).

--> As a result, I would expect the Total Table size from the Vertipaq analyzer of my two tables above (49 391 934) to be similar to the 73 MB of the data model. Where does this difference come from ?

 

Thanks in advance for your answers !

Tristan

@Datatouille

 

1 ACCEPTED SOLUTION

  1. Columns Hierarchies are internal structures storing a sorted textual version of the content of each column. It is required for MDX, and it is now used also internally for optimizing certain queries (in other words - we cannot get rid of it, even for columns we never use un MDX)
  2. See above - even numbers are converted into strings
  3. I don't understand the question - but VertiPaq Analyzer reports the memory consumption of the data model, whereas the size of the files could be different (serialization and additional structures).

 

I hope it helps!

 

Marco Russo - SQLBI

View solution in original post

5 REPLIES 5

  1. Columns Hierarchies are internal structures storing a sorted textual version of the content of each column. It is required for MDX, and it is now used also internally for optimizing certain queries (in other words - we cannot get rid of it, even for columns we never use un MDX)
  2. See above - even numbers are converted into strings
  3. I don't understand the question - but VertiPaq Analyzer reports the memory consumption of the data model, whereas the size of the files could be different (serialization and additional structures).

 

I hope it helps!

 

Marco Russo - SQLBI

View solution in original post

Dear Marco,

 

I hear you in this video sayng that is possible to disable Hier Size in SSAS, when not used, of course.

And now I can also see the green tickbox just on top, which makes me think that is really possible.

Could you be so kind to address me to some specific article where I can see how to do that?

Also let me thank you for all the availability, support and videos.

You are making a great job among PBI and Dax world! A Big thank you.

You should set the "Available In MDX" property to False. The internal property name is "isAvailableInMdx" and it is described here: Chris Webb's BI Blog: How The New IsAvailableInMDX Property For Analysis Services Tabular Can Reduce...

This property is available only in Tabular Editor and Visual Studio, you cannot modify it from Power BI Desktop.

Disabling it in Power BI Desktop is not supported - it could work, I never tried it. It is supported for Analysis Services (Azure and SSAS). It should work on Power BI Service as well through XMLA endpoint.

Don't disable this feature on columns that could be used as filter or exposed as visible columns to Excel. Disable only when a column is only used in an aggregation like SUM (...) and it is never visible to user.

I will definitely follow your recommendations. Thanks a lot for replying quickly.

Hopefully I will get the best from VertyPaq analyzer and improve performance of the Tabular Model which we have.

Thanks again. 

For the folks on this thread, there is a new 'Available in MDX' feature in Azure Analysis Services, SSAS 2017 (and soon Power BI Premium) which answers my questions n°1 and n°2 above: https://blog.crossjoin.co.uk/2018/07/02/isavailableinmdx-ssas-tabular/

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