SSL Linkage methods
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Chain
●Sequential access
When records are processed in their logical order, a maximum of one d isk access for
each member record is required. The number of accesses can be considerably reduced
by optimizing the placement of the records (see page 165). If the logical order is not
adhered to, UDS/SQL will normally have to search large parts of the set occurrence
which may involve a large number of disk accesses. It should be noted that for reading
a preceding record UDS/SQL must search the set occurrenc e from the beginning
unless backward chaining has been specified.
If the physical placement of the records changes frequently (e.g. if mem ber records for
a different set are stored as a list), it is advisable to perform occasional reorganization
runs in order to keep the PPPs current (see the "Recovery, Information and Reorgani-
zation" manual, BREORG).
●Direct access
– ORDER IS SORTED INDEXED:
Analogous to direct access in pointer array.
– ORDER IS LAST/FIRST/NEXT/PRIOR/SORTED:
On average, UDS/SQL must search half a set occurrence to find a record. A
maximum of one disk access per record is required.
●Insertion
When a record is inserted, it is linked to the preceding an d to the subsequent record by
means of pointers.
For this reason, UDS/SQL updates the records preceding and, in the case of backward
chaining, following the record inserted.
– ORDER IS SORTED [INDEXED]:
UDS/SQL finds the place where the record is insert ed by direct access.
– ORDER IS LAST/FIRST/NEXT:
A pointer points from the owner record to the preceding record, or the owner record
is the preceding record, or the preceding record is immedi ately obtainable by
means of the currency information. Insert ion of the record requires a maximum of
two disk accesses.
– ORDER IS PRIOR:
UDS/SQL must search the set occurrence from the beginning unle ss backward
chaining has been specified. If backward chaining has been specified, in sertion of
a record requires a maximum of three disk accesses.