We don't store the System.Description field in the warehouse because of its size. You can use the work item tracking web view to get the full history of your work items. You can add links to the web views to your reports.
If your datasource is pointing to the cube, the [Measures].[Work Item URL] measure will return you the link to the web view if the [Work Item].[System_Id].[System_Id] dimension is on the row axis.
If you are using relational datasource, the [TFSWarehouse].[dbo].[Tool Artifact Display Url] table contains the base URL, which you can use to construct the full link by appending the work item id to it.
Here's how the link would look like: http://<Your AT Name>:8080/WorkItemTracking/WorkItem.aspx?artifactMoniker=<Your Bug ID>
Hope this helps.
-Jim
I do not recommend accessing this data directly as the schema might change in later versions without notice. The DB schema is not a publicly supported interface of the system.
If you do need the data directly from the backend, the System.Description data would be stored in the TFSWorkitemtracking DB in the WorkitemLongTexts table. You would need to pick up rows with FldID = 52, the field id of the System.Description table.
Thanks,
Amit
Dev, Team Foundation Server