如何转换 sql_variant 使其可以被认为是确定性的

Question

I am trying to create a persisted computed column in a SYSTEM_VERSIONING table dbo.Users as follows:

ALTER TABLE dbo.Users 
    ADD SessionId AS usr.GetSession() PERSISTED 
        CONSTRAINT FK_dboUsers_IdSession 
            FOREIGN KEY REFERENCES dbo.Sessions(IdSession)

Where usr.GetSession() is just retrieving the value stored as BIGINT in SESSION_CONTEXT('IdSession') and converting it to again to BIGINT .

CREATE OR ALTER FUNCTION usr.GetSession()
RETURNS BIGINT WITH SCHEMABINDING
AS
BEGIN
    RETURN CONVERT(BIGINT, SESSION_CONTEXT(N'IdSession'))
END

But getting the following error:

Computed column 'SessionId' in table 'Users' cannot be persisted because the column is non-deterministic.

It is obviously because:

SELECT OBJECTPROPERTY(OBJECT_ID('usr.GetSession'), 'IsDeterministic') AS IsDeterministic;

Is returning 0

A little bit of searching found this about Deterministic and nondeterministic functions

CONVERT
Deterministic unless one of these conditions exists:

• Source type is sql_variant.
• Target type is sql_variant and its source type is nondeterministic.

So, I am understanding that there is no way to make my computed column persisted with a user defined scalar function as sql_variant cannot be handled as deterministic value.

Or there can be some walk around to solve my problem? Or some other solution? Any idea?

Answer 1

No, there is no workaround. You cannot do anything with sql_variant unless you convert it (even implicitly), and as you mention, that is not deterministic.

---

Be that as it may, it seems you are going down the wrong road anyway.

A computed column is the wrong thing here , as in this case it would change every time it was read, whereas it seems you want it changed every time the row is inserted.

Instead you need a DEFAULT

ALTER TABLE dbo.Users 
    ADD SessionId bigint DEFAULT (usr.GetSession())