A common table expression node is labeled with the name of the common table expression that is specified in the SQL statement and is, by default displayed, as an arrow polygon.
The DB2® optimizer treats a common table expression as a work file, and DB2 uses a table space scan (TBSCAN) to access it.
A common table expression must be instantiated before it can be accessed. Usually it is instantiated at the beginning of the query. The DB2 optimizer uses the definition of a common table expression to instantiate it. This instantiation is actually a subquery within the SQL statement. This subquery is represented in the access plan graph as one or more query blocks, depending on whether the common table expression definition contains other subqueries.