What Is a Temporary Field?

Reference:

A temporary field is a field whose value is not stored in the data source, but can be calculated from the data that is there, or assigned an absolute value. A temporary field takes up no storage space in the data source, and is created only when needed.

When you create a temporary field, you determine its value by writing an expression. You can combine fields, constants, and operators in an expression to produce a single value. For example, if your data contains salary and deduction amounts, you can calculate the ratio of deductions to salaries using the following expression: 

deduction / salary

You can specify the expression yourself, or you can use one of the many supplied functions that perform specific calculations or manipulations. In addition, you can use expressions and functions as building blocks for more complex expressions, as well as use one temporary field to evaluate another.

Reference: Types of Temporary Fields

You can use two types of temporary fields (a virtual field and a calculated value), which differ in how they are evaluated:

A virtual field (DEFINE) is evaluated as each record that meets the selection criteria is retrieved from the data source. The result of the expression is treated as though it were a real field stored in the data source.

A calculated value (COMPUTE) is evaluated after all the data that meets the selection criteria is retrieved, sorted, and summed. Therefore, the calculation is performed using the aggregated values of the fields.

Reference: Evaluation of Temporary Fields

The following illustration shows how a request processes, and when each type of temporary field is evaluated:

processing against the internal matrix

Example: Distinguishing Between Virtual Fields and Calculated Values

In the following example, both the DRATIO field (virtual field) and the CRATIO (calculated value) use the same expression DELIVER_AMT/OPENING_AMT, but do not return the same result. The value for CRATIO is calculated after all records have been selected, sorted, and aggregated. The virtual field DRATIO is calculated for each retrieved record. 

DEFINE FILE SALES
DRATIO = DELIVER_AMT/OPENING_AMT;
END
TABLE FILE SALES
SUM DELIVER_AMT AND OPENING_AMT AND DRATIO
COMPUTE CRATIO = DELIVER_AMT/OPENING_AMT;
END

The output is:

distinguishing virtual fields and calculated values

Reference: Selecting a Temporary Field

The following is to help you choose the kind of temporary field you need.

Choose a virtual field when you want to:

  • Use the temporary field to select data for your report. You cannot use a calculated value, since it is evaluated after data selection takes place.
  • Use the temporary field to sort on data values. A calculated value is evaluated after the data is sorted. With the BY TOTAL phrase, you can sort on this type of field.

Choose a calculated value when you want to:

  • Evaluate the temporary field using total values or prefix operators (which operate on total values). You cannot use a virtual field, since it is evaluated before any totaling takes place.
  • Evaluate the temporary field using fields from different paths in the data structure. You cannot use a virtual field, since it is evaluated before the relationship between data in the different paths is established.