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Databanks


A Mimer SQL database consists of the Mimer SQL system databanks and a number of user databanks.

Each databank is a single physical file in the host file system.

Mimer SQL System Databanks

The Mimer SQL system databanks are fundamental to the functioning of a Mimer SQL database and they are created during the process of installing a database.

System databanks are not used for storing user-defined information and cannot be updated directly by users.

If any one of the system databanks is damaged or missing, attempts to log on to Mimer SQL will fail. Backup and restore procedures for the system databanks are described in Backing-up and Restoring Data.

The Mimer SQL system databanks are:

SYSDB

SYSDB is the most important system databank as it stores the tables that make up the data dictionary, see The Data Dictionary.

Among other things, the data dictionary holds information about the other databanks that make up the database, the tables each user databank contains, the users (idents) that are known to the Mimer SQL system and the access rights each ident has.

TRANSDB

TRANSDB stores information that allows the database server to bring the database into a consistent state after a system failure, see Transaction Control.

LOGDB

LOGDB records all write operations performed within transactions on SYSDB and user databanks which have been defined with the LOG option.

The information in this databank is used by the backup and restore facilities, see Backing-up and Restoring Data, to restore the contents of a database in the event of a system failure.

READLOG facility is provided, see READLOG, to allow the information in this databank to be examined.

SQLDB

SQLDB is used by the transaction handling mechanism to store row data read from the database, see Transaction Control, and is used by Mimer SQL for temporary storage of result tables.

User Databanks

User databanks contain the tables in the database created by the users of the system. Typically these databanks are created by the system administrator and TABLE privilege is granted to the users of the system.

The CREATE DATABANK statement is used to create user databanks, see the Mimer SQL Reference Manual, Chapter 12, SQL Statement Descriptions.

Except at the point when tables are created, the existence of databanks is transparent to users and application programs. When access is requested to a table, information in the data dictionary is used by Mimer SQL to locate the table and make it available, if permissible.

The division of a database into databanks is made on the basis of file handling considerations from the operating system viewpoint and on the basis of transaction control considerations from the database viewpoint. The use of databanks allows considerable flexibility in the physical placement of data on the computer system.



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