Fujitsu UDS/SQL V2.5 사용자 설명서
SSL
Introduction
U929-J-Z125-9-76
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5
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.
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 “
consists of a number of pages defined at database creation (see the “
” 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.
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).
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 “
routine (see the “
” manual, BREORG).
1 byte
3 bytes
REALM-REF
PNO
REALM-REF
PNO
PNO
= Realm reference
= Page number
= Page number