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Beginning with V11, it is possible to create an index that does not allow an ISAM update to change the key values. On such an index, an ISAM record add can add a new key value and an ISAM record delete can delete a key value, but an ISAM record update cannot change a key value. (Note the special case for a variable-length record: if an ISAM update causes the record to move, the key is allowed to change its record offset from the old offset to the new offset; but the key value itself is not allowed to change.)
To create an index with this feature enabled, OR KTYP_NOISMUPD into the key type (ikeytyp) field of the IIDX structure for that index file before calling CREIFIL() to create the index.
PUTIFIL() can be used to turn this bit on or off. Changes take effect immediately.
An ISAM record update that attempts to change a key value for an index with this property enabled fails with error UKEY_ERR (1001).
