What are the DMVs and how you can use them?
The DMVs are queries that allow you to retrieve valuable metadata information about your data model, server operations, and server health.
What are the DMVs
The DMVs queries are an interface to schema rowsets based on SQL and use a SELECT statement and the $System schema with an XML/A schema rowset, however, they do not support the full syntax of a SELECT statement as: JOIN, GROUP BY, LIKE, CAST, and CONVERT.
Apply to: SQL Server Analysis Services, Azure Analysis Services, Power BI Desktop and Power BI Premium
Example of a DMV query:
SELECT * FROM $System.DISCOVER_CALC_DEPENDENCY
WHERE OBJECT_TYPE = ‘MEASURE’
ORDER BY TABLE ASC
For more information visit the official documentation: https://docs.microsoft.com/en-us/analysis-services/instances/use-dynamic-management-views-dmvs-to-monitor-analysis-services?view=asallproducts-allversions
What are they for?
- The DMVs provide information about the active sessions and connections, objects that consume the most CPU or memory at a specific point in time, such as:
| DMV | Description |
| $system.discover_commands | Provides resource usage and activity information about the currently executing or last executed commands in the opened connections on the server. |
| $system.discover_sessions | This query reports on active sessions, including session user and duration. |
| $system.discover_connections | List the currently opened connections on the server. |
| $system.discover_memoryusage | Lists all memory consumption by object. Most information here: https://www.kasperonbi.com/new-ssas-memory-usage-report-using-power-bi/ |
- To document your Power BI or SSAS data model, the main queries DMVs available for model documentation are:
- TMSCHEMA_TABLES: List the Column objects in each table.
- TMSCHEMA_COLUMNS: List the Columns objects in each table.
- TMSCHEMA_MEASURES: List the Measure objects in each table.
- TMSCHEMA_RELATIONSHIPS: List the Relationship objects in the model.
- TMSCHEMA_ROLES: Shows information about the Role objects in the model.
- TMSCHEMA_PARTITIONS: Provides information about the Partition objects in each table.
In the past I used to create my own documenters using the DMVs, but I recently discovered the excellent and very complete tool “Model Documenter” created by Marc Lelijveld (MVP).
This tool generates the documentation about your data model in an easy and quick way, you can get more information and download the tool in the page: https://data-marc.com/model-documenter/
DISCOVER_CALC_DEPENDENCY
With this DMV you can obtain the list of the dependencies among the objects in your model and extract the DAX expressions. This allows you to have more control over your data models as you can measure the impact of modify or remove any object.
I created the tool “Tabular model cleaner” using this DMV, which helps you to obtain all the dependences of the data model in an efficient and automated way and the usability of the model objects in the reports connected to the Power BI or SSAS Data model.
For more information visit: Tabular model cleaner
How to run the DMVs
1. DAX Studio: Open the Power BI Dataset that you want to document. Open DAX Studio and choose the “PBI / SSDT Model” data source option, then click on Connect:
If you have Power BI Premium you can connect to the dataset, to do this select the workspace, go to “Workspace settings” and copy the Workspace Connection and paste it into the “Tabular Server” option in the DAX Studio.
Go to DMV pane and click on the query you want consult:
2. From Power BI Desktop: You can run DMVs queries from DAX Studio but that will return you a table with the data, you should connect to the DMVs using Power BI, to view and analyze the information more easily.
– Open Power BI Desktop.
– Choose the “SQL Server Analysis Services database” source.
– Include the Server and Database.
Consider if you are connected to a local data model, the database name will change every time you open the Power BI Desktop file, so you will have to edit the connection parameter when you need to refresh the data.
