The space available property is not computed correctly

The Database.SpaceAvailable property is not computed correctly. The reason for this issue is that the input used in the calculation of this property, as seen in the source code, relies on a join operation between the sys.allocation_units and sys.partitions DMVs. However, this inner join is incompatible with columnstore indexes, as clearly documented in the official documentation. Consequently, the size is inaccurately computed for databases using columnstores. The solution is straightforward: remove this incompatible join operation from the calculation.

Start in src/Microsoft/SqlServer/Management/SqlEnum/PostProcessDatabaseInsideAttribs.cs at the linked calculation of Database.SpaceAvailable. Review how sys.allocation_units and sys.partitions are joined, then verify the result for a database using columnstore indexes. Done means the property reports the correct available space without the incompatible join.