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The reserved word EXTERNALLY-DESCRIBED-KEY can specify that the keys for this file are those that are externally described in DDS. The keys are determined by the record formats that are copied by the COPY statement, DDS, DD, DDSR, or DDR format, under the FD for this file.
The key can start at different offsets within the buffer for each format. In this situation, care must be used when changing from one record format to another, using a random READ or START statement. The key must be placed in the record format at the correct offset in the format that will be used in the random access of the file. Unpredictable results can occur if the key for the desired record is based on data that was part of the last record read. This is because the movement of the data to the key field can involve overlapping fields.
The key within a format can be made up of multiple, noncontiguous (not adjacent) fields. Only those record formats copied in within the FD for the file should be referenced by the FORMAT phrase. If a format is referenced that is defined within the file, but that format has not been copied into the program, the key is built using the key fields defined for the first record format that was copied. This can cause unpredictable results.
If a portion of the key is declared in the logical file only as an element of a concatenated item (rather than an independently-declared item), the result of the CONCAT operation must not be a variable-length item.
The reserved word EXTERNALLY-DESCRIBED-KEY cannot be specified with the ALTERNATE RECORD KEY clause.
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