The Compute Scalar operator is used to compute new values based on other columns in the same row. These new values are then added as extra column(s) in the output rows.
The expressions used to compute the new values can only refer to constant values and to columns in the input rows of the Compute Scalar operator. Other than that, there are, to my knowledge, no restrictions. The expressions can vary from very simple to extremely complex. The expressions can even include references to scalar user-defined functions, to CLR user-defined functions, and to built-in CLR functions.
One rather unique behavior of the Compute Scalar operator is that it very often does not actually execute. Depending on the expressions specified in the Defined Values property, SQL Server can and will often defer the evaluation of the expressions to other operators. When this happens, the Compute Scalar operator has no work to do; in this case it doesn’t execute at all. This is unfortunately not directly visible in execution plans, although you can find evidence of this in an execution plan plus run-time statistics, or predict that it’s the case based on the expressions used in the Defined Values property.
Visual appearance in execution plans
Depending on the tool being used, a Compute Scalar operator is displayed in a graphical execution plan as shown below:
SSMS and ADS
Paste The Plan
The basic algorithm for the Compute Scalar operator is as shown below:
Note that this flowchart is a simplification. It doesn’t show that execution stops whenever a row is returned, and resumes where it was upon next being called.
Also note that when the Compute Scalar doesn’t execute at all (as explained above and below), then this flowchart is not used at all, since the operator is never called.
If evaluation is not deferred, Compute Scalar computes, for every expression in the Defined Values property, the result value based on the values in the input row. It then stores the results of these computations, assuming no run-time error occurs, in the appropriate column(s) in the output row.
When evaluation is deferred, Compute Scalar is only the container for the definition of the expression. Another operator evaluates the expression, when its result is needed. That operator then uses the result of the expression, but doesn’t store it. This is different from other operators having a Defined Values property of their own: when that’s the case, that other operator will evaluate those expressions and store them in the output row.
Evaluation of a scalar user-defined function, CLR user-defined function, or built-in CLR function is never deferred. These are always evaluated by the Compute Scalar. This evaluation starts by invoking the SQL or CLR code of the function, with the values from the current row passed in as its arguments; Compute Scalar then resumes work when the call to the function returns its result.
Most of the expressions that are used in the Defined Values property use standard SQL operators and keywords, though excessive use of parentheses, brackets, and fully qualified column names can make it hard to read the expressions. There also are several built-in functions that are not valid in SQL, but that are supported and understood by Compute Scalar. Most of these are also supported by other operators, either for deferred evaluation or as part of a Defined Values property on that other operator itself. Please check the Scalar Functions page for a full list of all currently known internal scalar functions.
Actual execution versus deferred evaluation
In most (but not all!) cases where you see a Compute Scalar operator in an execution plan, it doesn’t actually execute; instead the operator merely functions as a placeholder to store the definitions of columns that can be computed through scalar expressions. The actual evaluation is done by other operators, as and when they need the expression result; this also means that the actual number of times the expression is evaluated is not related to the number of rows flowing through the Compute Scalar. See Paul White’s post for a more detailed explanation.
This does not always happen. There are also cases where the Compute Scalar operator does actually execute, to compute the Defined Values expression(s) for each row that passes through and add the results as extra columns in the rows returned. This seems to be triggered by the actual functions used in the Defined Values property. So far, actual execution of Compute Scalar as opposed to deferred evaluation by other operators seems to be triggered by the use of the GetRangeThroughConvert or GetRangeWithMismatchedTypes functions (see table above), or the use of any scalar user-defined function in the Defined Values property.
Though a Compute Scalar can define more than one column in its Defined Values property, it appears that it never mixes expressions for which the evaluation can be deferred with those that it has to compute during actual execution. If a query uses for example both a scalar user-defined function and a simple expression, the execution plan will contain two Compute Scalar operators; one that will actually execute (for the user-defined function) and one that is merely a container for the definition of a deferred evaluated expression.
In an execution plan plus run-time statistics, it is easy to tell whether a Compute Scalar was actually executed, or whether evaluation was deferred. In the case of deferred evaluation, none of the run-time statistics normally associated with an execution plan plus run-time statistics will be reported for this Compute Scalar. A Compute Scalar that actually executes will report all these properties as normal.
When a Compute Scalar runs in batch mode execution, it always evaluates all expressions as part of its operator. Deferred evaluation is not supported for batch mode.
The properties below are specific to the Compute Scalar operator, or have a specific meaning when appearing on it. For all other properties, see Common properties. Properties that are included on the Common properties page but are also included below for their specific meaning for the Segment operator are marked with a *.
|Actual everything *||As mentioned in the main text, Compute Scalar often only defines expressions that qualify for deferred evaluation; the actual evaluation is then done in other operators when the result is needed. In that case the Compute Scalar does not execute, hence it will not return any of the properties for run-time statistics.|
|Defined Values *||A Compute Scalar operator always has a Defined Values property. In some cases, the definition of the expression uses functions that Management Studio is unable to display. In such cases, the Defined Values property will appear empty. Looking at the execution plan XML is the only way to find the actual definition in these cases.|
|Estimated CPU Cost *||For Compute Scalar, the Estimated CPU Cost is always equal to the Estimated Number of Rows, multiplied by 0.0000001 (one millionth). Neither the number of expressions in Defined Values, nor their complexity has any effect on this. Even if scalar user-defined functions are invoked with a known higher cost, they do not affect this cost estimate.|
|Estimated I/O Cost *||For Compute Scalar, the Estimated I/O Cost is always equal to zero. Even if scalar user-defined functions are invoked that are known to do I/O, they do not affect this cost estimate.|
|Number of Executions *||As mentioned in the main text, Compute Scalar often only defines expressions that qualify for deferred evaluation; the actual evaluation is then done in other operators when the result is needed. In that case the Compute Scalar does not execute, hence it will not return any of the properties for run-time statistics.|
This table below lists the behavior of the implicit properties for the Compute Scalar operator.
|Batch Mode enabled||The Compute Scalar operator supports both row mode and batch mode execution. In batch mode, deferred evaluation is not supported.|
|Blocking||The Compute Scalar operator is non-blocking.|
|Memory requirement||The Compute Scalar operator does not have any special memory requirement.|
|Order-preserving||The Compute Scalar operator is fully order-preserving.|
|Parallelism aware||The Compute Scalar operator is not parallelism aware.|
|Segment aware||The Compute Scalar operator is not segment aware.|