Oracle System Overview-Data Dictionary

* Data dictionary is an important logical structure in Oracle databases. It is composed of read-only basic tables and views. In these basic tables, all emails from the Oracle server are saved.

* Data dictionary is an important logical structure in Oracle databases. It is composed of read-only basic tables and views. In these basic tables, all emails from the Oracle server are saved.

1. Concepts of Data Dictionary

* Data dictionary is an important logical structure in Oracle databases. It is composed of read-only basic tables and views. All information about the Oracle server is stored in these basic tables. The information mainly includes:

* Definitions of the physical and logical structures of databases, including data files on servers, control files and redo log files, as well as tables, views, indexes, synonyms, sequences, stored procedures, packages, trigger and other schema objects.

* Database storage space allocation information, such as how much space is allocated to an object in Oracle and how much space is used.

* Integrity constraints of data.

* Database security information, including user information, permissions granted to users and role information.

* Audit information, such as the database objects accessed or modified by a Jilin user.

* Performance and Statistics during database running.

* Other basic information about the database.

Related reading:

Oracle System Overview-Data Dictionary

Oracle System Overview-instances

Oracle System Overview-memory structure

Overview of Oracle Server Architecture

The data in the data dictionary is organized in the form of tables and views. The data reflects the basic information of each component in the Oracle architecture and the information of each State in the Oracle operating process, it is a collection of data for Oracle to manage and control its normal operation.

The data dictionary is the core data of Oracle and is stored in the system tablespace. Only Oracle can manage and maintain these tables and views in the data dictionary. Any database user, DBAs can only read data in the data dictionary, but do not have the right to modify the data.

2. Data dictionary structure

A data dictionary is a logical structure consisting of two parts:

(1) base table. The data in the data dictionary is stored in the basic table. Only Oracle can modify the basic table. Oracle users cannot directly access the basic table, and cannot modify or delete the data. Data storage in basic tables is generally encrypted.

(2) User View. The user view is a view provided by the Oracle server for users. The Oracle server connects the basic table through tables and forms different views based on the user's query needs. The data in the view is the data in the decrypted basic table.

3. Usage of Data Dictionary

A data dictionary is usually created when a database is installed. The data in the dictionary is updated by Oracle in response to certain operations. For example, you have added, modified, and deleted schema objects in the Oracle database. These operations have all modified the composition of the Oracle server, in this case, Oracle records the structure information of the modified schema object in the data dictionary. Generally, after each DDL statement is executed, Oracle must modify the relevant information in the data dictionary. The data dictionary is extremely important for running the server. The server relies on the data dictionary to maintain, test, and process ongoing operations.

You can use the SELECT statement to query the data in the basic table in the user view to obtain information about the Oracle server. For users, the data dictionary is like the Oracle server reference manual. All information about the Oracle server can be obtained by querying data fields.

The data dictionary is mainly used as follows:

* Oracle obtains information about users, schema objects, and other storage structures by accessing the data dictionary to control and manage server work.

* Any database user can obtain information about the database from the data dictionary to develop, modify, and maintain the database.

* DBA can obtain various parameter information and running status information of the database from the data dictionary for effective database management.

4. User View

A user view is also called a static view. Because the content in the view is data in the basic table, the content of the view changes only after the basic table changes.

Oracle uses the SELECT statement to access the user view to obtain data in the data dictionary. However, due to database security considerations, not every user can access any data in the data dictionary. Generally, users can only query their own data information, only DBA or users with select anytable permission can query all data information in the data dictionary.

User views can be roughly divided into three categories, each of which is differentiated by different prefixes, as shown in Table 1-1.

Table 1-1

5. Dynamic Performance Table

During the running process, the Oracle server records the running status of the current server in some "virtual" tables in the memory. These "virtual" tables are called dynamic performance tables. These dynamic performance tables can only exist in the memory. As the Oracle instance is created, these tables are also deleted when Oracle is disabled.

The dynamic performance table belongs to SYS users. Only SYS users and SYSDBA users can query the dynamic performance table. A Dynamic Performance Table is usually prefixed with V $.

The most commonly used dynamic performance view is shown in Table 1-2:

Table 1-2

In addition to the above user view and dynamic performance view, the Oracle Data Dictionary also has many user views and dynamic performance views. As a DBA, you must have a deep understanding of the Oracle architecture. by querying data fields, you can obtain the Oracle server's organizational structure and running status, such as the database surplus and SGA, user Permissions and buffer usage.