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Example 1
To retrieve a customer description to be displayed when processing an order:

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By including the product price (Predetermined Join Field) and product value (virtual field) when defining a select command in RDML, the arithmetic will be performed without any additional RDML code.
Comments/Warnings
Although the Predetermined Join Field processing is coded within the I/O module the source files are only accessed when one of the Predetermined Join Fields on a particular access route is used in the function calling the I/O module.
This means that, unless the Predetermined Join Field is explicitly specified in the I/O operation (for example FETCH or SELECT), it would not be retrieved and calculated from the source file (not even when it is derived from a Virtual Field).
If the function has been DBOPTIMIZED the Predetermined Join Field processing is included in the I/O module logic which is embedded in the function.
If the file to be accessed by the access route is defined as a high speed table the source information for the Predetermined Join Fields will be retrieved from the high speed table thereby reducing I/Os.
Warning: Read the appropriate sections on high speed tables before attempting to use them.
When defining Predetermined Join Fields careful thought should be given to their use. They are very efficient for looking up code files for descriptions when used with a KEEP LAST value or accessing a high speed table, but if they are used excessively each file accessed will affect the number of files used in the I/O module and DBOPTIMIZSED functions (see Using *DBOPTIMIZE / *DBOPTIMIZE_Batch for a further explanation). Also they could greatly increase the number of I/Os performed by the I/O module where there are 1 to many relationships.
As a guide, do not access more than 10 to 15 files.
 
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