Effect on User Defined Attribute Instances if Definition is Changed

In all that follows it should be remembered that no change to a UDA instance (apart from deletion of the UDA definition element) will occur until the COMPILE command has been given in Lexicon.

Where an instance of a UDA exists, then changes to the definition may cause changes to the instance.

If the UDA type (UTYPE attribute) is altered, the current instances will be inaccessible. Unless UDA instances are purged (see next subsection), the process is reversible, that is, resetting the type will re-enable access to the instances.

If the UDA length (ULEN attribute) is decreased, latter parts of numeric arrays or text strings will become inaccessible. This process is reversible, that is, if ULEN is subsequently increased, the values may again be accessed.

If an element type is removed from an Element List (ELELIST), instances of these element types will be inaccessible. Again, this is reversible.

This indirect alteration of the apparent contents of the database could be disturbing for users and it is recommended that the System Administrator should avoid modifying UDA definitions once they are in use.