Personal View #6

Choose a DBMS:discuss how its database structure and entry forms are created.

ORACLE DBMS

Oracle database system comprises at least one instance of the application, along with data storage. An instance comprises a set of operating-system processes and memory-structures that interact with the storage. Typical processes include PMON (the process monitor) and SMON (the system monitor).
The Oracle RDBMS stores data logically in the form of tablespaces and physically in the form of data files. Tablespaces can contain various types of memory segments. For example: Data Segments, Index Segments and others. Segments in turn comprise one or more extents. Extents comprise groups of contiguous data blocks,whileData blocks form the basic units of data storage. At the physical level, data-files comprise one or more data blocks, where the blocksize can vary between data-files.

Oracle database management tracks its computer data storage with the help of information stored in the SYSTEM tablespace. The SYSTEM tablespace contains the data dictionary— and often indexes and clusters. A data dictionary is consists of a special collection of tables that contains information about all user-objects in the database.
Memory Architecture(System Global Area)
Each Oracle instance uses a System Global Area or SGA or a shared-memory area, to store its data and control-information. Each Oracle instance allocates itself an SGA when it starts and de-allocates it at shut-down time.
The SGA contains of the ff. elements:
- database buffer cache: this stores the most recently-used data blocks. These blocks can contain modified data not yet written to disk unmodified blocks, or blocks written to disk since modification.
- redo log buffer: this stores redo entries — a log of changes made to the database.
- shared pool: this area of the SGA stores shared-memory structures such as shared SQL areas in the library cache and internal information in the data dictionary.

Library cache
The library cache stores shared SQL, caching the parse tree and the execution plan for every unique SQL statement.
Data dictionary cache
The data dictionary comprises a set of tables and views that map the structure of the database.
Program Global Area
The Program Global Area or PGA memory-area contains data and control-information for Oracle's server-processes,the size and content of the PGA depends on the Oracle-server options installed.
Process architecture
The Oracle RDBMS typically relies on a group of processes running simultaneously in the background and interacting to further and monitor database operations.