Sql server's Constant scan - clarification?

Question

I've read this article about Constant scan but still didn't understand when it would be used ( people commented the author that his article still isn't understood):

MSDN :

The Constant Scan operator introduces one or more constant rows into a query. A Compute Scalar operator is often used to add columns to a row produced by a Constant Scan operator.

very unclear.

For example :

why does SELECT TOP 1 GETDATE() produces :

While select getdate() produces nothing. ( in execution plan).

I guess it has to do with constant scan. so :

Question :

• What exactly is constant scan
• Why do it needs to SCAN something if it's a constant , and
• Why does select top 1 getdate() yield different result than plain select getdate()

Answer 1

Because most of the operators that are used in fulfilling a query are specified in terms of one or more input rowsets and one or more output rowsets, it makes sense that if you write a query that requires the use of one of those operators, you'd better make sure that the input that you present to that operator is a rowset.

So the constant scan operator is a special operator that can take one or more scalar inputs and generate a rowset.

It makes composing operators far easier.

---

Why does select top 1 getdate() yield different result than plain select getdate()

Because you've asked to use the TOP operator. And the TOP operator expects to have an input rowset.

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To why I think focussing on the performance is irrelevant:

SET STATISTICS TIME ON
GO
SELECT TOP 1  GETDATE()
GO
SELECT GETDATE()

Messages:

SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.

That is, we can't even get a measure of the amount of time that either statement takes to execute.