Does lightweight pooling disable built-in CLR facilities? Running SQL Server in fibre mode (lightweight pooling) disables the use of SQL CLR:


  Common language runtime (CLR) execution is not supported under lightweight pooling. Disable one of two options: "clr enabled" or "lightweight pooling". Features that rely upon CLR and that do not work properly in fiber mode include the hierarchy data type, replication, and Policy-Based Management.


On the other hand, disabling SQL CLR alone (without enabling lightweight pooling) does not disable the built-in CLR types like geometry, and geography (though hierarchyid is mentioned above), as shown in How can &quot;HierarchyID&quot; type work when &quot;CLR&quot; is disabled?

Now some new language features rely on the CLR, for example the FORMAT function:


  FORMAT relies on the presence of the .NET Framework Common Language Runtime (CLR).


Does running SQL Server in fibre mode disable the FORMAT function and/or using the CLR types?
Yes.
When running in fibre mode, everything that needs the CLR is unavailable.
For example, both:
SELECT FORMAT(GETDATE(), N'd', N'en-us');

and:
DECLARE @g geometry;
SELECT @g;

...return the (slightly misleading) error message:

Msg 5846, Level 16, State 2, Line 13
Common language runtime (CLR) execution is not supported under lightweight pooling.
Disable one of two options: &quot;clr enabled&quot; or &quot;lightweight pooling&quot;.

It is the lightweight pooling that is the problem, no setting of the clr enabled option will allow CLR execution of any kind under lightweight pooling.