A database is a program that batches data files.

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There are two types of information stored in the Oracle database:

User data is application-specific data (such as customer invoices).

System data is the data that the database system needs to manage by itself (such as the names and storage locations of all data files related to a specific database).

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The database is the cabinet, the drawer in the cabinet is the table space, the folder in the drawer is the data file, the paper in the folder is the table, 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 list spaces are necessary or common in most databases:

The system tablespace stores the information needed to manage the operating system itself and the data stored in it. These table spaces

The name of is fixed.

Temporary tablespaces are areas that are temporarily used in the operating system. When certain events occur, the operating system needs to empty these disks.

Between the management of related affairs.

The tool tablespace stores the objects required by the tool software running on the O or a c l e database.

User-specific database objects are stored in user tablespaces.

. Rollback saves a rollback segment of a database object in a tablespace.

. Data and index tablespaces store user's application data.

. An index is a special type of database object. O r a c l e uses indexes for fast data retrieval.

3) Full-table scanning means that O or a c l e accesses every row of data of a specific object.

4) The recovery (U n d o) information is stored in a special database object called Rollback Segment, which can be used to restore the database object to the old value in case of transaction failure or temporary suspension.

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Redo log is also called transaction log. O or a c l e records database modification operations or transactions in these special operating system files. All modifications to the database are made in memory.

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1) The Oracle database consists of two types of files:

. Data files that are combined to form a tablespace.

. Data files that are combined to form a redo log series.

2) A database must have at least two redo logs.

3) The redo log contains all transaction entries that occurred in the database.

4) Redo logs are usually called transaction logs.

5) In order to prevent data loss, the transaction log must be used. The function of transaction log is to recover data in case of unexpected failure.

6) Oracle database can run in the following two modes:

All transaction logs will be saved in. Archive log mode.

The old transaction log will not be saved in. NOARCHIVELOG mode.

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A dirty data block is a memory area in computer memory, which contains modified data read from the database.

Dirty data block: The original data in the dirty data block stored in memory has been modified, but the modified data has not been written into the database.

Hot blocks store data that needs to be modified frequently.

Recently used at least, recently used at least

A snapshot is a read-only copy of data stored in one or more tables, which is stored from one site (primary site) to another site (remote site).

The function of R o l l b a c k is to restore the data changed by users to the original state.

Revocation information Due to various reasons, when the database needs to restore or roll back the user's operation, revocation information will be generated.

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The control file is a very small file, which contains some key information related to all files in the O or a c l e database.

O r a c l e system maintains the database I n t e g r i t y (I N T E G R I T Y) and decides which redo logs to use when restoring data.

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Each database has at least two control files. We strongly recommend that users generate at least two control files and put them on different disks.

All modifications to the database structure are recorded in the control file.

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There are two types of programs or processes.

One is user (or customer) processes, such as S Q L * P l u s, Oracle Forms, Oracle Reports, etc.

Any tool used to access a database is a user process.

The server process receives requests from the client process, and then meets these requirements through interaction with the database.

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O r a c l e provides many support processes to help user processes communicate with database servers. These support processes are used to:

When there is a checkpoint, write the data back to the data file.

When the checkpoint appears, make sure to write the dirty data block back to disk (C K P T).

. Read and write redo logs.

. Function to run when starting the system to complete database recovery (S M O N).

After the user terminates abnormally during the session, it can be used to release the resources occupied by the user (P M O N).