Deterministic and Nondeterministic Functions

Deterministic functions always return the same result any time they are called with a specific set of input values and given the same state of the database. Nondeterministic functions may return different results each time they are called with a specific set of input values even if the database state that they access remains the same.

All functions are deterministic or nondeterministic:

• Deterministic functions always return the same result any time they are called with a specific set of input values.
  
  
• Nondeterministic functions may return different results each time they are called with a specific set of input values.

Whether a function is deterministic or nondeterministic is called the determinism of the function.

For example, the DATEADD built-in function is deterministic because it always returns the same result for any given set of argument values for its three parameters. GETDATE is not deterministic because it is always invoked with the same argument, yet the value it returns changes each time it is executed.

Earlier versions of Microsoft® SQL Server™ have no functionality that is dependent on the determinism of functions. In Microsoft SQL Server 2000, nondeterministic functions cannot be specified in two types of Transact-SQL expressions:

• An index cannot be created on a computed column if the computed_column_expression references any nondeterministic functions.
  
  
• A clustered index cannot be created on a view if the view references any nondeterministic functions.

One of the properties SQL Server records for user-defined functions is whether the function is deterministic. A nondeterministic user-defined function cannot be invoked by either a view or computed column if you want to create an index on the view or computed column.

User-Defined Function Determinism

Whether a user-defined function is deterministic or nondeterministic depends on how the function is coded. User-defined functions are deterministic if:

• The function is schema-bound.
  
  
• All built-in or user-defined functions called by the user-defined function are deterministic.
  
  
• The body of the function references no database objects outside the scope of the function. For example, a deterministic function cannot reference tables other than table variables that are local to the function.
  
  
• The function does not call any extended stored procedures.

User-defined functions that do not meet these criteria are marked as nondeterministic. Built-in nondeterministic functions are not allowed in the body of user-defined functions.

Built-in Function Determinism

You cannot influence the determinism of any built-in function. Each built-in function is deterministic or nondeterministic based on how the function is implemented by Microsoft SQL Server.

There are three ways to retrieve the current datetime in SQL SERVER.
CURRENT_TIMESTAMP, GETDATE(), {fn NOW()}

CURRENT_TIMESTAMP
CURRENT_TIMESTAMP is a nondeterministic function. Views and expressions that reference this column cannot be indexed. CURRENT_TIMESTAMP can be used to print the current date and time every time that the report is produced.

GETDATE()
GETDATE is a nondeterministic function. Views and expressions that reference this column cannot be indexed. GETDATE can be used to print the current date and time every time that the report is produced.

{fn Now()}
The {fn Now()} is an ODBC canonical function which can be used in T-SQL since the OLE DB provider for SQL Server supports them. {fn Now()} can be used to print the current date and time every time that the report is produced.

If you run following script in Query Analyzer. I will give you same results. If you see execution plan there is no performance difference. It is same for all the three select statement.
SELECT CURRENT_TIMESTAMP
GO
SELECT {fn NOW()}
GO
SELECT GETDATE()
GO

Performance:
There is absolutely no difference in using any of them. As they are absolutely same.

My Preference:
I like GETDATE(). Why? Why bother when they are same!!!