SELECT Output Closure

The SELECT Output Closure transformation examines the outputs of all operators inside the specified SELECT. If any of those outputs are used by operators following the SELECT, the VT is incorrectly formed. To correct the VT, the operator outputs being used improperly are connected to the output of the enclosing branch, an output is added to the SELECT, and that new SELECT output is connected to the operators that previously used the operator output. Outputs are also added to the other branches of the SELECT to maintain consistency (all branches must have the same number of outputs) these outputs become vtbody inputs added specifically for this purpose. 

An example of SELECT Output Closure is shown in Figure 3-12, where the shaded operator causes its enclosing SELECT to be incorrectly formed. After the transformation, the output of that operator has been connected to the branch output, a new vtbody input has been added to the rightmost branch, and an output has been added to the SELECT. 

SELECT Duplication, SELECT-to-SELECT Transfer, and SELECT Output Closure are often used together with the Process Creation and Pipestage Creation transformations described later to form pipestages. 

---