A database is a group of programs that deal with data files.
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There are two types of information stored in an Oracle database:
. User data is application-specific data (for example, customer shipment orders).
. System data is the data that the database system needs to manage itself (for example, the names and locations of all the data files associated with a particular database).
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The database is the cabinet, the drawers in the cabinet are the tablespaces, the folders in the drawers are the data files, the paper in the folders are the tables, and the information written on the paper is the data.
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1) A tablespace is a collection of one or more data files.
2) The following tablespaces are required or common in most databases:
. System tablespaces hold the information needed to manage the O r a c l e system itself and the data stored in it. The names of these tablespaces
are fixed.
. Temporary tablespaces are areas in O r a c l e that are used temporarily. When a specific event occurs, O r a c l e needs to use these disk spaces
to manage the transaction in question.
. The tools tablespace holds those objects that are required by the tools software running on the O r a c l e database.
. The user tablespace holds user-specific database objects.
. The rollback tablespace holds the rollback segments for database objects.
. The data and index tablespace holds the user's application data.
. Indexes are a special type of database object.O r a c l e Use indexes for fast data retrieval.
3) Full Table Scan means O r a c l e accesses every row of data of a particular object.
4) Recovery ( U n d o ) information is stored in a specialized database object called the Rollback Segment (Rollback Segment), which can be used to return the database object to its old values in the event of a failure or temporary abort of a transaction.
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Redo Log, also known as Transaction Log, is a special operating system file in which O r a c l e logs modification operations or transactions against the database. modification operations or transactions performed against the database. All modifications made to the database are made in memory.
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1) An Oracle database consists of two types of files:
. Data files that are combined together to form a tablespace.
. Data files that are combined together to form the redo log series.
2) A database must have at least two redo logs.
3) The redo log contains entries for all transactions that occurred in the database.
4) The redo log is often also called the transaction log.
5) To prevent data loss, you must use the transaction log, which serves the function of recovering data in the event of an unexpected failure.
6) Oracle database can run in the following two modes:
. ARCHIVELOG mode will keep all transaction logs.
. NOARCHIVELOG mode does not save old transaction logs.
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A dirty data block is an area of memory in a computer's memory that contains data read from a database that has been modified.
Dirty Data Block: The original data stored in the dirty data block in memory has been modified, and the modified data has not been written to the database.
Hot Data Block: Stores data that is modified frequently.
L R U (Least Recently Used)
Snapshot is a read-only copy of data stored in one or more tables, deposited from one site (the primary site) to another site (the remote site).
The function of a rollback ( R o l l b a c k ) is to restore user-altered data to the state in which it originally resided.
Recovery Information ( Undo Information) Recovery information is generated when the database needs to recover or roll back user operations for various reasons.
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A control file is a very small file that holds some key information related to all the files in the O r a c l e database.
The control file is used by the O r a c l e system to maintain the integrity of the database ( I n t e g r i t y ) as well as to determine which redo logs are used when recovering data.
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. Each database has at least two control files. It is highly recommended that users generate a minimum of two control files and place them on separate disks.
. All modifications to the database structure are logged in the control files.
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There are two types of programs or processes.
. One type is a user (or client) process, such as S Q L * P l u s , Oracle Forms, Oracle Reports, and any
other tools used to access the database are user processes.
. The server process receives requests from client processes and then fulfills those requests by interacting with the database.
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O r a c l e provides a number of support processes to help user processes communicate with the database server. These support processes are used to:
. Write data back to the data file in the event of a checkpoint ( D B W R ) .
. Ensure that dirty data blocks are written back to disk when a checksum point occurs ( C K P T ) .
. Read and write the redo log ( L G W R ).
. Runs at startup of the system to complete the database recovery ( S M O N ).
. Available to release resources occupied by a user after the user terminates abnormally during a session ( P M O N ) .