Data Dictionary

The Oracle data dictionary is the metadata of the database and contains the names and attributes of all objects in the database. The creation or modification of any object causes an update to the data dictionary that reflects those changes. This information is stored in the base tables that are maintained by the Oracle database, but you access these tables by using predefined views rather than reading the tables directly.

The data dictionary:

• Is used by the Oracle database server to find information about users, objects, constraints,and storage
• Is maintained by the Oracle database server as object structures or definitions are modified
• Is available for use by any user to query information about the database
• Is owned by the SYS user
• Should never be modified directly using SQL

The Data Dictionary consists of two components:

Base Tables: These tables store descriptions of objects in the database.

• These are the first objects created in the data dictionary.These are created when the oracle RDBMS software runs a special script named sql.bsq when a database is created by the CREATE DATABASE command – you do not see the sql.bsq script execute, but it does.
  You should never attempt to write to these tables – never use DML commands to attempt to update base tables directly.
  An example Base Table is IND$ that stores index information.

User-Accessible Views: These views summarize information in the base tables in order to make the information easier for a DBA to use.

• These views are created by the catalog.sql script.
• Using the Oracle Universal Installer to create a database, then the catalog.sql and catproc.sql scripts are run automatically.
• An example data dictionary user-accessible view is TABS – it stores information about tables you create as a system user. TABS is a synonym for the view ALL_TABLES.

The Data Dictionary stores information about all database objects created by system users and information systems professionals including tables, views, indexes, clusters, procedures, functions, synonyms, sequences, triggers and the like. For each object, the Data Dictionary stores:

• Disk space allocation (usually in bytes).
• Integrity constraint information.
• Default values for columns.
• Oracle user names (accounts)
• Privilege and role information for users.
• Auditing information – who has accessed/updated objects.

Data Dictionary Usage

The Data Dictionary is used by the Oracle RDBMS as illustrated in the figure shown below.

DBA: These views display information about objects stored in all schemas (a schema is a logical organization of objects belong to an individual system user). These views are named DBA_xxx where xxx is the object name.Because these views belong to the DBA and were created by the owner SYS so these views not access by public .
Example:
SELECT owner, object_name, object_type
FROM SYS.DBA_OBJECTS;

ALL: These views display "all" information that an individual user of the database is authorized to access – this will include information about your objects as well as information about objects for which you have access permissions.
If you connect to the database, the ALL_xxx views will display all information about objects of all database schemas (if you have access permissions).

SELECT owner, object_name, object_type FROM ALL_OBJECTS;

USER: These views display information that you would most likely want to access.

• These USER_xxx views refer to your own objects in your own schema.
• These views display only rows pertinent to you as a user.
• These views are a subset of the ALL views.
• These rows do not usually display the OWNER column.
  

          SELECT object_name, object_type  FROM USER_OBJECTS;

In general, Data Dictionary Views answer questions about:

• when an object was created,
• Is the object part of another objects,
• who owns the object,
• what privileges do you have as a system user,what restrictions are on an object.

Practice selecting information from the following three views: dba_objects, all_objects, user_objects. You may wish to use the SQL*Plus command DESC to describe the views.Additional queries you may wish to execute to practice accessing parts of the data dictionary:
SELECT table_name, comments
FROM dictionary
WHERE table_name LIKE '%SEGMENTS%';

Dynamic Performance Tables and Views

The Oracle Server records database activity to the Dynamic Performance Tables.
These are complemented by Dynamic Performance Views - virtual tables that exist only in memory when the database is running.

• The tables provide real-time condition information about database operation.
• DBAs use these tables—most ,users should not be able to access these tables.
• The tables cannot be altered – they are fixed and are owned by the user SYS.
• All Dynamic Performance Tables have names that begin with the letters V_$.
• Views of these tables are created along with synonyms that begin with the letters V$.
• Information about the database's datafiles and information about all of the dynamic performance tables and views.

Examples Dynamic Performance Table views:
V$CONTROLFILE: Lists the names of the control files
V$DATABASE: Contains database information from the control file.
V$DATAFILE: Contains datafile information from the control file
V$INSTANCE: Displays the state of the current instance
V$PARAMETER: Lists parameters and values currently in effect for the session
V$SESSION: Lists session information for each current session
V$SGA: Contains summary information on the system global area (SGA)
V$SPPARAMETER: Lists the contents of the SPFILE
V$TABLESPACE: Displays tablespace information from the control file
V$THREAD: Contains thread information from the control file
V$VERSION: Version numbers of core library components in the Oracle server

The DUAL Table

Oracle maintains a table named DUAL that is used by Oracle and by user programs to produce a guaranteed known result such as the production of a value through use of an Oracle defined function.
The table has one column named DUMMY and one row containing the value X.
SYS owns the DUAL table, but all users can select from it.
Selecting from DUAL is useful for computing a constant expression with a SELECT statement, since DUAL has only one row, the constant is returned only once.

Example

SQL> select 1+4 from dual;
       1+4
----------
         5

How the Data Dictionary Is Used

Oracle (internally) accesses information about users, schema objects, and storage structures.
This is done to validate a query executed by a system user.
Validates permission and security.
Verifies that referenced objects in queries actually exist.
Oracle modifies the data dictionary for each DDL statement executed.
Oracle users access the data dictionary as a read-only reference.

General Overview
– DICTIONARY, DICT_COLUMNS

• Schema objects
– DBA_TABLES, DBA_INDEXES, DBA_TAB_COLUMNS,
DBA_CONSTRAINTS

• Space allocation
– DBA_SEGMENTS, DBA_EXTENTS

• Database structure
– DBA_TABLESPACES, DBA_DATA_FILES

Modifying the Data Dictionary:

Only Oracle (SYS) should ever modify the data dictionary.During upgrades to new release versions of the Oracle RDBMS, scripts are provided to upgrade the data dictionary.

Row Chaining and Migrating

Whenever  DML operation performed in oracle database ,there are two situations where a data row may not fit into a single data block:

Row chaining The row is too large to fit into one data block when it is first inserted, or the table contains more than 255 columns (the maximum for a row piece).In this case, Oracle stores the data for the row in a chain of data blocks (one or more) reserved for that segment.Row chaining most often occurs with large rows, such as rows that contain a column of datatype LONG or LONG RAW.Row chaining in these cases is unavoidable.Due to  insertion of a large row in a data block. The row is too large for the left block, so the database chains the row by placing the first row piece in the left block and the second row piece in the right block.

Row Migration  A row that originally fit into one data block has one or more columns updated so that the overall row length increases, and the block's free space is already completely filled.
In this case, Oracle migrates the data for the entire row to a new data block, assuming the entire row can fit in a new block.
Oracle preserves the original row piece of a migrated row to point to the new block containing the migrated row.The rowid of a migrated row does not change.The left block contains a row that is updated so that the row is now too large for the block. The database moves the entire row to the right block and leaves a pointer to the migrated row in the left block.

When a row is chained or migrated, I/O performance associated with this row decreases because Oracle must scan more than one data block to retrieve the information for the row.

• Row chaining is typically caused by INSERT operation.
• Row migration is typically caused by UPDATE operation

Overview of Data Blocks

The Database Block or simply Data Block, is the smallest size unit for input/output from/to disk in an Oracle database.Oracle Database manages the logical storage space in the data files of a database in a unit called a data block, also called an Oracle block or page. A data block is the minimum unit of database I/O.
Data Blocks and Operating System Blocks At the physical level, database data is stored in disk files made up of operating system blocks.A data block may be equal to an operating system block in terms of size, or may be larger in size, and should be a multiple of the operating system block. The database requests data in multiples of data blocks, not operating system blocks.

When the database requests a data block, the operating system translates this operation into a requests for data in permanent storage. The logical separation of data blocks from operating system blocks has the following implications:

• Applications do not need to determine the physical addresses of data on disk.
• Database data can be striped or mirrored on multiple physical disks etc.

Oracle Data Block

Data Blocks is smallest logical unit to store Oracle Data.

• One data block represent specific number of bytes on physical hard disk
• Data Block in Oracle is also called as  logical blocks, Oracle blocks, or pages
• Data Block size is usually multiple of operating system block size
• You can have multiple block sizes with in single database
• Block Size is specified by initialization parameter DB_BLOCK_SIZE

Architecture of Oracle Data Block

Every data block has a format or internal structure that enables the database to track the data and free space in the block. This format is similar whether the data block contains table, index, or table cluster data.

Data Block Overhead

Oracle Database uses the block overhead to manage the block itself. The block overhead is not available to store user data.The data block header, table directory, and row directory are referred to collectively as overhead. Some block overhead is fixed in size; the total block overhead size is variable. On average, the fixed and variable portions of data block overhead total 84 to 107 bytes.

Header (Common and Variable) part contains general information about the block, including disk address and segment type(for example, data or index). For blocks that are transaction-managed, the block header contains active and historical transaction information
Table Directory portion of the data block contains information about the table having rows in this block it also contains metadata about tables whose rows are stored in this block and In a table cluster, multiple tables can store rows in the same block.
Row Directory  describes the location of rows in the data portion of the block.The row directory entry contains a address(pointer) to the location of the row on the data block .
Free space is allocated for insertion of new rows and for updates to rows that require additional space (for example, when a trailing null is updated to a nonnull value).
Row Data  part of the block contains the actual data, such as table rows or index key entries

Logical Database Structure

Oracle Database allocates logical space for all data in the database. The logical units of database space allocation are data blocks, extents, segments, and tablespaces. At a physical level, the data is stored in data files on disk. The data in the data files is stored in operating system blocks.
Oracle database logically divided into tow or more tablespaces ,a database is divided into logical storage units called tablespaces, which group related data blocks, extents, and segments.
The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.Every Oracle database contains a SYSTEM tablespace and a SYSAUX tablespace. Oracle Database creates them automatically when the database is created.
Below diagram for physical and logical storage

Tablespaces :

Tablespaces logically organize data that are physically stored in datafiles.

• A tablespace belongs to only one database, and has at least one datafile that is used to store data for the associated tablespace.
• The term "tablespaces" is misleading because a tablespace can store tables, but can also store many other database objects such as indexes, views, sequences, etc.
• Datafiles are always assigned to only one tablespace and, therefore, to only one database.
• Tablespace is contain one or more segments (table,index etc).

Logical Storage Hierarchy:

A segment contains one or more extents, each of which contains multiple data blocks.

Data Block

A data block is the smallest logical unit of data storage in Oracle Database.
One logical data block corresponds to a specific number of bytes of physical disk space, for example, 2 KB. Data blocks are the smallest units of storage that Oracle Database can use or allocate.

Extent

An extent is a set of logically contiguous data blocks allocated for storing a specific type of information .In the preceding graphic, the 24 KB extent has 12 data blocks, while the 72 KB extent has 36 data blocks.

Segment

A segment is a set of extents allocated for a specific database object, such as a table.
For example, the data for the employees table is stored in its own data segment, whereas each index for employees is stored in its own index segment. Every database object that consumes storage consists of a single segment.
A tablespace is a database storage unit that contains one or more segments.

• A database is made up of one or more tablespaces
• A tablespace is made up of one or more data files, a tablespace contains segments
• A segment (table, index, etc) is made up of one or more extents. A segment exists in a tablespace but may have data in many data files within a tablespace.
• An extent is a continuous set of blocks on a disk. An extent is in a single tablespace and is always in a single file within that tablespace.
• A block is the smallest unit of allocation in the database. A block is the smallest unit of i/o used by the database.