Power Pivot Principles: The A to Z of DAX Functions – INFO.EXPRESSIONS

In our long-established Power Pivot Principles articles, we continue our series on the A to Z of Data Analysis eXpression (DAX) functions.  This week, we look at INFO.EXPRESSIONS.

The INFO.EXPRESSIONS function

Dynamic Management Views (DMVs) are specialised queries provided by SQL Server Analysis Services (SSAS), Azure Analysis Services (AAS), and Power BI that offer an administrative view into the internal state of these systems.  DMVs are used to retrieve metadata, monitor health and performance, and diagnose problems within the database or data model.  They serve as a powerful tool for administrators and developers to gain insights into the workings of the database engine and the tabular data model, covering aspects like performance metrics, configuration settings and the structure of database objects.

The $System schema DMVs in SQL Server Analysis Services (SSAS), Azure Analysis Services (AAS), and Power BI are categorised into four [4] types, each serving specific purposes:

• DISCOVER: requires admin privileges and provides information about the model, including details on connected sessions and environment configuration
• DMSCHEMA: focused on data mining, offering insights for predictive analytics and pattern recognition, mainly used in SSAS/AAS
• MDSCHEMA: targets multidimensional models, delivering metadata and structure from an MDX perspective, relevant for OLAP cubes and dimensions
• TMSCHEMA: designed for tabular models, it provides detailed metadata about tables, columns, measures, etc., using Tabular Model Scripting Language (TMSL) information, crucial for Power BI and tabular SSAS/AAS models.

In the past, if we wanted to query those $System schema DMVs we used external tools like Tabular Editors or DAX Studio to query them:

Now, we just need to write a simple DAX syntax to query those TMSCHEMA directly in DAX query view.  In this instance, we are using INFO.EXPRESSIONS function to query the TMSCHEMA_EXPRESSIONS.

The INFO.EXPRESSIONS function is one of the system functions.  It employs the following syntax:

This function has no parameters.

Based upon the ‘[MS-SSAS-T]: SQL Server Analysis Services Tabular Protocol’ from Microsoft (which you may access here), the Expression object represents a named expression that can be used by one or more partitions.  It is a child of a Model object and requires compatibility level 1400 or higher.

We can write this INFO.EXPRESSIONS function on DAX query view to get the same information on the TMSCHEMA_EXPRESSIONS:

It will query $SYSTEM.TMSCHEMA_EXPRESSIONS and return an entire table with 12 columns:

• ID: this is a reference to the object
• ModelID: this is an ID-based reference to a Model object
• Name: this is the name of the object
• Description: this is the description of the object
• Kind: this is the kind of the expression.  The possible values are as follows:
• M (0) – an expression that is based on M (Power Query Formula Language)
• Expression: this represents the descriptive text of the expression
• ModifiedTime: this is the time that the object was last modified
• QueryGroupID: this is an ID-based reference to a QueryGroup object.  Compatibility level 1500 or higher is required
• ParameterValuesColumnID: this is an ID-based reference to a Column object.  Compatibility level 1600 or higher is required
• MAttributes: this is a set of optional properties that can be used to define the literal attributes on the section members of the M queries.  Compatibility level 1600 or higher is required
• LineageTag: this is an optional tag that can be used to define the lineage of an expression across different versions of a model.  Compatibility level 1600 or higher is required
• SourceLineageTag: An optional tag that can be used to define the lineage of a referenced expression across different versions of a model.  As opposed to LineageTag, SourceLineageTag can be used to define the lineage of a referenced expression rather than an expression itself.  SourceLineageTag is useful when a model references other models by using a Direct Query connection.  Compatibility level 1600 or higher is required.

It should be noted that:

• it is used for querying the DMV (Dynamic Management Views) from the $System schema
• sometimes querying DMVs may fail if we do not have the appropriate permission.

Come back next week for our next post on Power Pivot in the Blog section.  In the meantime, please remember we have training in Power Pivot which you can find out more about here.  If you wish to catch up on past articles in the meantime, you can find all of our Past Power Pivot blogs here.