18.2 DATABASE STRUCTURES
The general database has three types of environments (referred to as structures or components), including physical, logical, and memory resident. These structures combine to provide the run-time database environment sometimes referred to generically as the database instance.18.2.1 Logical Components
- Schema
A collection of database objects owned by a database user. The schema objects include database structures like tales, views, and indexes. - Table
Contains rows and columns and holds the data. - Views
A customized query of the database that presents data from one or more tables or other views. - Index
Speed up the data retrieval from tables. Similar to the index of a book. - Clusters
Groups of tables that are physically stored together, share common columns, and are used together. - Data block
Corresponds to the bytes of physical space on the disk. This is the finest granularity of data storage. - Extents
An allocation of contiguous data blocks. - Segments
A set of extents allocated for specific use within the logical structure of the database. Segment types include data, index, temporary, and rollback. - Tablespace
A group of related structures that are allocated as a (tablespace) storage unit. The tablespace contains one or more datafiles. The size of the datafile determines the size of the tablespace. The combined storage capacity of the tablespaces determines the total storage capacity of the database. - Other
There are other logical database structures, such partitions, sequences, roles, privileges, snapshots, triggers, packages, functions and links, that are not covered in this section because they are part of the standard database environment.
18.2.2 Physical Components
- Datafiles
Data storage containers assigned to the tablespace. Data in the datafile are read as needed during normal database operation and stored in memory buffers. - Redo log files
Record all changes made to the data. Used for database recovery. Redo log files are overwritten regularly. - Control files
Contain information about the physical structure and status of the database (such as database name, names and locations of the datafiles and redo log files). Control files are critical to the operation of the database. - Archive log files
Store the database transactions (copied from the redo log files) in the event that database recovery becomes necessary. - Parameter file
Used during database startup to allocate and designate storage buffers and file locations and to assign run time database environment variables. - Trace files
Designated files that capture database process information.
18.2.3 Memory Components
- Shared Global Area (SGA)
Shared by all database server processes and client processes and includes the following memory structures: shared pool (dictionary cache and shared SQL cache), database buffer, redo log buffer, and java pool. - Program Global Area (PGA)
Used by single Oracle server process to contain the sort area, session information, cursor state, and stack space. - Shared Pool
Stores data dictionary and shared SQL areas. - Data dictionary cache
Contains table and column definitions and user privilege information. - Library cache
Stores information about the most recently used SQL statements (statement text, parse code and execution plan). - Database buffer cache
Stores the most recently used blocks of database data. - Rollback segment (called Undo segment in Oracle 9.i )
Like tables and indexes, rollback segments exist in data files and parts of them are brought into the database buffer cache when required. Before making any change, the server process saves the old value into a rollback segment. This information is used to undo changes if a transaction is rolled back, ensure that other transactions to not see uncommitted changes, and recover the database to a consistent state in case of failures. - Redo log buffer
Stores log changes made to the database. The redo entries are written to the redo log file, which is used if recovery becomes necessary. Records changes made through the instance: the block that changed, the location of the change, and the new value. Circular buffer: it is reused after it is filled up, but only after all the old redo entries are recorded in the redo log files.
18.2.4 User Processes
The user process is started when the users requests an OpenView session. The user process exits when the user session terminates. User processes facilitate user transactions with the database and sends calls to the dedicated server process. For example, opcuiwww is a JAVA client (user) process.18.2.5 Dedicated Server Processes
Dedicated server processes run on the management server and provide services for the OpenView user process. The dedicated server uses an exclusive PGA. For example, oraopenview is a dedicated server process.18.2.6 Background Processes
The processes shown here make up the database server processes. They must run continuously, otherwise OVO cannot connect to the database. Start or stop the processes with the script /sbin/init.d/ovoracle start (or stop).- Database writer (dbwr)
Writes to datafile, database buffer cache, redo log buffer, and undo segments. - Log writer (lgwr)
Flushes the redo log buffers when and update or commit are executed. - Checkpoint (chkp)
Synchronizes the three major database files and removes uncommitted data from the data file. - System monitor (smon)
Used to clean/free up server resources, such as locks, after a database crash. - Process monitor (pmon)
Cleans up client resource files (for example, after a user process terminates). - Archiver (arch)
Copies the online redo log files to the archival log file. - Dispatcher
(optional) Presents only when shared server configuration is used. Responsible for routing requests from connected user process to shared server processes and returning responses back the user processes. - Recoverer (reco)
(optional) Resolves distributed transactions that are pending to a network or system failure in a distributed database. - Listener
Responsible for receiving incoming client connection requests and directing them to the appropriate service.
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