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SSL Introduction
U929-J-Z125-9-76 131
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
5.1.2 DBTT (Database Key Translation Table)
The DBTT establishes the link between the database key value of a database record and
the physical page address of that record.
Structure of a physical page address
The total space available for storing data in the database is divided into realms. A realm
consists of a number of pages defined at database creation (see the “Creation and Restruc-
turing” manual, “Database creation”). The pages in each realm are consecutively
numbered, as are the realms. UDS/SQL can thus locate data in the database if the number
of the page containing the data and the number of the realm containing the page are known.
The realm reference and page number are therefore combined to form a physical page
address, which UDS/SQL can use as a pointer to the physical position of the data.
Figure 27: Structure of a physical page address
For the main part, data in the database comprises the records and tables defined by the
user. The physical placement of these records and tables can change, but UDS/SQL does
not update all the associated pointers when this happens.
Pointers that must be current at all times are referred to as actual keys (act-key), the
remaining pointers are called Probable Position Pointers (PPP).
PPPs can be updated to the most recent update level by means of the BREORG utility
routine (see the “Recovery, Information and Reorganization” manual, BREORG).
1 byte
3 bytes
REALM-REF PNO
REALM-REF
PNO
= Realm reference
= Page number