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154 U929-J-Z125-9-76
List
● Sequential access
If MODE IS LIST, the records are grouped together in a contiguous storage area. This
storage mode offers fastest sequential processing. The number of accesses when
processing large numbers of records depends on the record length.
● Direct access
If ORDER IS SORTED INDEXED, all levels of the list must be in memory. The higher
levels are used to select the number of the page containing the record. One disk access
is required for each level of the table. The number of levels depends on the record
length.
If ORDER IS SORTED INDEXED has not been defined, UDS/SQL must search the set
occurrence from the beginning. The number of disk accesses required for this depends
to a large degree on the record length and the size of the set occurrence.
● Insertion
If ORDER IS SORTED INDEXED, UDS/SQL finds the place where the record is to be
inserted by means of direct access. Otherwise UDS/SQL requires a maximum of two
disk accesses.
Insertion of a record may necessitate re-storage of a number of records by UDS/SQL.
● Deletion
In general, UDS/SQL need modify only the page from which the record is to be deleted.
● Result
MODE IS LIST offers the fastest sequential access mode. If an updating operation
causes a change in the record sequence, this may result in the physical relocation of
records. The PPPs in SEARCH key tables pointing to these records are then no longer
valid.
The access behavior depends to a large degree on the order and size of the set occur-
rences.