What is Data Dictionary

What is Data Dictionary

Each Oracle database contains a collection called "metadata", or data used to describe the database's data structure. Tables and views that contain the metadata are called ORACLE data dictionaries.
Tables with the prefix v $ or GV $ in the data dictionary are dynamic tables, which are constantly updated to reflect the current status of the Oracle database. The static data dictionary table has a prefix, such as DBA _, all _, or user _, indicating the object range listed in this view.

Database components tables and views in the database dictionary
Database v $ Database
Tablespace dba_tablespace, dba_data_files,
Dba_free_space
Control File v $ controlfile, V $ parameter,
V $ controlfile_record_section
Data File v $ datafile, V $ datafile_header, V $ filestat,
Dba_data_files
Segment dba_segments
Data range: dba_extents
Log thread, log group, and V $ thread, V $ log, V $ logfile
Log serial number
Archive status v $ database, V $ log, V $ archived_log,
V $ archive_dest
Database instance v $ instance, V $ parameter,
V $ system_parameter
Memory Structure v $ SGA, V $ sgastat, V $ db_object_cache, V $ SQL,
V $ sqltext, V $ sqlarea
Background process v $ bgprocess, V $ session

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A data dictionary is a document that describes the field name, field length, field description, and other information of a database table;
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The data dictionary is a description of fields in the database.
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A data dictionary is a collection of basic elements in a database! It can store basic table data structures, stored procedures, and so on...
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A data dictionary is a table that records information about some databases.
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Each programming environment has two basic components:ProgramLogic-work description required by the Program-and data. Data can be temporarily stored in the program or permanently and stably in the external environment as variables, such as files or databases. Traditional file structures require developers to understand and describe the file structure so that programs can access and process file fields or records as appropriate. However, in Oracle relational databases, the entire database structure (including its tables, views, indexes, users, and security rules) is stored in a special set of tables and views, it is called a data dictionary.

The Oracle Data Dictionary is owned by a special user account and is called sys. Sys accounts are the same as root superusers in UNIX systems. Usually, developers do not need to connect to the shared database as privileged accounts. Database administrators are not even encouraged to use the sys account for any work, but only a few operations require this level of capability. If you are both a developer and DBA, you may need to understand the potential crisis of interactive work when connecting as sys on the personal copy of the Oracle database.

Another common entry about Oracle Data Dictionary stores its tables in some special tablespaces (such as system and sysaux. In these special tablespaces, no other application database objects are created.

The key to Oracle Data Dictionary is that you can use the same SQL language to access the data structure of the application within the database. Generally, direct access to the dictionary is limited to select statements (insert, update, or delete statements are never released for dictionary objects ). The data dictionary is indirectly modified using special SQL statements, which is usually called the data processing language (DML) command. DML statements, such as create table, drop index, and Grant, require special permission. They are generally executed by the DBA in the Development Organization, which allocates responsibilities between the dba and the development personnel. To establish and maintain your own database environment, students or developers who work independently will have to learn the slq DML command.

Concepts in data dictionary in SQL Server

There are two types of data dictionaries:
A dbms records the stored procedure of the system field definition relation trigger view to some sys tables in SQL ......

Second, the user data dictionary is equivalent to processing on the system and describing and managing user objects.

Information about various objects in the database, such
DBO. syscolumns records the columns in the table, the parameters in the stored procedure, and the columns in the index.
DBO. syscomments records creation statements of stored procedures and views.
DBO. sysdepends records the dependency between objects.
DBO. sysfilegroups records file group information
DBO. sysfiles records data file information
DBO. sysfiles1 .....
DBO. sysforeignkeys
DBO. sysfulltextcatalogs records full-text index information
DBO. sysfulltextnotify .....
DBO. sysindexes records the index and related table information
DBO. sysindexkeys .....
DBO. sysmembers .....
DBO. sysobjects records all database objects
DBO. syspermissions
DBO. sysproperties
DBO. sysprotects
DBO. sysreferences
DBO. policypes records information of all data types in the database
DBO. sysusers records information of all users in the database