5.4.4 Predetermined Join Fields
A Predetermined Join Field is used to hold the result of various operations performed on information retrieved from a file nominated by the access route.
These operations can be TOTAL, MAXIMUM, MINIMUM, AVERAGE, COUNT, and LOOKUP. The type of operations which can be performed depend on the relationship between the files.
When the relationship is 1:1, using the key defined on the access route, a single value will be retrieved and inserted into the predetermined join field. For example, a product description can be retrieved for a product code (provided an access route has been defined between an order file and the product description file). A "keep last" value can be nominated to reduce I/Os, but only for 1:1 relationships.
When the relationship is 1:many (number of records is greater than 1) the predetermined join field will hold the result of the selected operation. For example the total of the line values from an order line file could be retrieved for display with the order header file.
As a guide, do not access more than 10 to 15 files for predetermined join fields.
If the file to be accessed is a high speed table, the high speed table will be used, thus reducing I/Os.
Refer to Access Routes to other Files in the or attend the , to learn about predetermined join fields and access routes and the steps involved in defining, reviewing or changing them.
The is also available in a self study format and is inlcuded with the LANSA documentation.
Access routes should not be confused with IBM i access paths.