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U929-J-Z125-9-76 153
Dokuschablonen 19x24 Version 7.3us für FrameMaker V7.x vom 14.02.2007 © cognitas GmbH 2001-2007
24. Oktober 2007 Stand 08:58.55 Pfad: G:\vogt\fsc\uds\Manuale\en\udsent_e\udsent.k05
5.3.2 Assessing pointer array, list and chain
The time required for the execution of a program depends on the storage modes defined for
the set occurrences.
This section compares retrieval and updating operations when using pointer arrays, lists
and chains. The retrieval operations are subdivided into sequential and direct accesses.
Updating operations dealt with are insertion and deletion.
Pointer arrays
● Sequential access
The pointer array must be present in memory. This is almost always the case whenever
a member record of the set occurrence has already been accessed via the set. One
entry in the pointer array must then be read before the associated record can be read.
The number of disk accesses depends on how records are distributed over the pages
and can be reduced by optimizing the placement of the records (see the sections
“Natural optimization” on page 164 and “PLACEMENT OPTIMIZATION” on page 165).
● Direct access
– ORDER IS SORTED INDEXED: All levels of the pointer array must be in memory.
The higher levels are used to select the number of the page containing the entry of
the record to be found. One disk access is required for each level of the table.
– ORDER IS LAST/FIRST/NEXT/PRIOR: UDS/SQL must search the set occurrence
sequentially until the desired record is found. This may require a considerable
number of disk accesses.
● Insertion
In order to insert an entry, the place of insertion in the pointer array must be determined.
If ORDER IS SORTED INDEXED, this is possible via direct access.
If ORDER IS LAST/FIRST, UDS/SQL can directly access the last or first table page
respectively. If ORDER IS NEXT/PRIOR, the place of insertion is found via the currency
information or by sequentially reading the pointer array.
Writing the record usually requires a further disk access.
● Deletion
The record and its associated entry in the pointer array must be deleted.
● Result
MODE IS POINTER-ARRAY allows speedy sequential and direct access and updating
operations. The response times are largely independent of the order of the sets and the
size of the set occurrences.