SQL Server Execution Plans - Red Gate Software

Chapter 2: Graphical Execution Plans for Basic Queries
This plan may be a little difficult to read on the printed page. Please execute the
code on your own machine in order to see the plan clearly, and follow along with the
explanations.
Figure 2.34
The physical structures of the tables the query accesses can affect the resulting execution
plans. For example, this table has an IDENTITY column, FOREIGN KEY constraints and a
spatial data column. All these objects will affect the execution plan.
The physical operation of the execution plan starts off, reading right to left, with an
operator that is new to us: Constant Scan. This operator introduces a constant number
of rows into a query. In our case, it's building a row in order for the next two operators to
have a place to add their output. The first of these is a Compute Scalar operator to call a
function called getidentity. This is the point within the query plan when SQL Server
generates an identity value, for the data to follow. Note that this is the first operation
within the plan, which helps explain why, when an INSERT fails, you get a gap in the
identity values for a table.
The next Scalar operation creates a series of placeholders for the rest of the data and
creates the new uniqueidentifier value, and the date and time from the GETDATE
function. You can tell this is what happens by looking at the Property sheet for the
operator and then looking at the Defined Values property. There is an ellipsis next to that
property which, when clicked, will open a window showing what is being built by the
Compute Scalar operation as shown in Figure 2.35.
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