ORACLE Architecture and Terminology


This section will provide a basic understanding of ORACLE including the concepts and terminology of the ORACLE Server. It is important that you read through this section to familiarize yourself with the concepts and terminology to be used throughout this manual. Most of the information contained in this section is DIRECTLY extracted from ``ORACLE7 Server Concepts Manual'' and all credit should be attributed to ORACLE.

Before you can begin to use ORACLE, you must have a basic understanding of the architecture of ORACLE to help you start thinking about an ORACLE database in the correct conceptual manner.

Figure 1 illustrates a typical variation of ORACLE's memory and process structures; some of the memory structures and processes in this diagram are discussed in the following section. For more information on these memory structures and processes, see page 1-15 of ``ORACLE7 Server Concepts Manual.''

Figure 1. ORACLE Architecture

Memory Structures and Processes

The mechanisms of ORACLE execute by using memory structures and processes. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs or tasks that work in the memory of these computers.

Memory Structures

ORACLE creates and uses memory structures to complete several jobs. For example, memory is used to store program code being executed and data that is shared among users. Several basic memory structures are associated with ORACLE: the system global area (which includes the database and redo log buffers, and the shared pool) and the program global area.

System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and control information for one ORACLE instance.

An ORACLE instance contains the SGA and the background processes.

The SGA is allocated when an instance starts and deallocated when the instance shuts down. Each instance that is started has its own SGA.

The Program Global Area (PGA) is a memory buffer that contains data and control information for a server process. A PGA is created by ORACLE when a server process is started. The information in a PGA depends on the configuration of ORACLE.

Processes

A process is a ``thread of control'' or a mechanism in an operating system that can execute a series of steps. Some operating systems use the terms job or task. An ORACLE database system has two general types of processes: user processes and ORACLE processes.

A user process is created and maintained to execute the software code of an application program (such as a PRO*C program) or an ORACLE tool (such as SQL*PLUS). The user process also manages the communication with the server processes. User processes communicate with the server processes through the program interface.

ORACLE processes are called by other processes to perform functions on behalf of the invoking process. ORACLE creates a server process to handle requests from connected user processes. ORACLE also creates a set of background processes for each instance (see ``ORACLE7 Server Concepts Manual'' pages 1-18, 1-19).

Database Structures

The relational model has three major aspects:

Structures
Structures are well-defined objects that store the data of a database. Structures and the data contained within them can be manipulated by operations.

Operations
Operations are clearly defined actions that allow users to manipulate the data and structures of a database. The operations on a database must adhere to a pre-defined set of integrity rules.

Integrity Rule
Integrity rules are the laws that govern which operations are allowed on the data and structures of a database. Integrity rules protect the data and the structures of a database.

An ORACLE database has both a physical and a logical structure. By separating physical and logical database structure, the physical storage of data can be managed without affecting the access to logical storage structures.

Logical Database Structure

An ORACLE database's logical structure is determined by:

The logical storage structures, including tablespaces, segments, and extents, dictate how the physical space of a database is used. The schema objects and the relationships among them form the relational design of a database.


Tablespaces and Data Files

Tablespaces are the primary logical storage structures of any ORACLE database. The usable data of an ORACLE database is logically stored in the tablespaces and physically stored in the data files associated with the corresponding tablespace. Figure 2 illustrates this relationship.

Although databases, tablespaces, data files, and segments are closely related, they have important differences:


databases and tablespaces
An ORACLE database is comprised of one or more logical storage units called tablespaces. The database's data is collectively stored in the database's tablespaces.

tablespaces and data files
Each tablespace in an ORACLE database is comprised of one or more operating system files called data files. A tablespace's data files physically store the associated database data on disk.

databases and data files
A database's data is collectively stored in the data files that constitute each tablespace of the database. For example, the simplest ORACLE database would have one tablespace, with one data file. A more complicated database might have three tablespaces, each comprised of two data files (for a total of six data files).

schema objects, segments, and tablespaces
When a schema object such as a table or index is created, its segment is created within a designated tablespace in the database. For example, suppose a table is created in a specific tablespace using the CREATE TABLE command with the TABLESPACE option. The space for this table's data segment is allocated in one or more of the data files that constitute the specified tablespace. An object's segment allocates space in only one tablespace of a database.

Figure 2. Data Files and Tablespaces

A database is divided into one or more logical storage units called tablespaces. A database administrator can use tablespaces to do the following:

Every ORACLE database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database. You can query these data dictionary tables to obtain pertinent information about the database; for example, the names of the tables that are owned by you or ones to which you have access. See Chapter 3 for more information on how to access data dictionary tables.

Data files associated with a tablespace store all the database data in that tablespace. One or more datafiles form a logical unit of database storage called a tablespace. A data file can be associated with only one tablespace, and only one database.

After a data file is initially created, the allocated disk space does not contain any data; however, the space is reserved to hold only the data for future segments of the associated tablespace - it cannot store any other program's data. As a segment (such as the data segment for a table) is created and grows in a tablespace, ORACLE uses the free space in the associated data files to allocate extents for the segment.

The data in the segments of objects (data segments, index segments, rollback segments, and so on) in a tablespace are physically stored in one or more of the data files that constitute the tablespace. Note that a schema object does not correspond to a specific data file; rather, a data file is a repository for the data of any object within a specific tablespace. The extents of a single segment can be allocated in one or more data files of a tablespace (see Figure 3); therefore, an object can ``span'' one or more data files. The database administrator and end-users cannot control which data file stores an object.


Data Blocks, Extents, and Segments

ORACLE allocates database space for all data in a database. The units of logical database allocations are data blocks, extents, and segments. Figure 3 illustrates the relationships between these data structures.

Data Blocks
At the finest level of granularity, an ORACLE database's data is stored in data blocks (also called logical blocks, ORACLE blocks, or pages). An ORACLE database uses and allocates free database space in ORACLE data blocks. Figure 4 illustrates a typical ORACLE data block.

Extents
The next level of logical database space is called an extent. An extent is a specific number of contiguous data blocks that are allocated for storing a specific type of information.

Segments
The level of logical database storage above an extent is called a segment. A segment is a set of extents which have been allocated for a specific type of data structure, and all are stored in the same tablespace. For example, each table's data is stored in its own data segment, while each index's data is stored in its own index segment. ORACLE allocates space for segments in extents. Therefore, when the existing extents of a segment are full, ORACLE allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk, and may or may not span files. An extent cannot span files, though.
Figure 3. The Relationship Among Segments, Extents and Data Blocks

ORACLE manages the storage space in the data files of a database in units called data blocks. A data block is the smallest unit of I/O used by a database. A data block corresponds to a block of physical bytes on disk, equal to the ORACLE data block size (specifically set when the database is created - 2048). This block size can differ from the standard I/O block size of the operating system that executes ORACLE.

The ORACLE block format is similar regardless of whether the data block contains table, index, or clustered data. Figure 4 shows the format of a data block.

Figure 4. Data Block Format

Header (Common and Variable)
The header contains general block information, such as block address, segment type, such as data, index, or rollback. While some block overhead is fixed in size (about 107 bytes), the total block overhead size is variable.

Table Directory
The table directory portion of the block contains information about the tables having rows in this block.

Row Directory
This portion of the block contains row information about the actual rows in the block (including addresses for each row piece in the row data area). Once the space has been allocated in the row directory of a block's header, this space is not reclaimed when the row is deleted.

Row Data
This portion of the block contains table or index data. Rows can span blocks.

Free Space
Free space is used to insert new rows and for updates to rows that require additional space (e.g., when a trailing null is updated to a non-null value). Whether issued insertions actually occur in a given data block is a function of the value for the space management parameter PCTFREE and the amount of current free space in that data block.

Space Used for Transaction Entries
Data blocks allocated for the data segment of a table, cluster, or the index segment of an index can also use free space for transaction entries.

Two space management parameters, PCTFREE and PCTUSED, allow a developer to control the use of free space for inserts of and updates to the rows in data blocks. Both of these parameters can only be specified when creating or altering tables and clusters (data segments). In addition, the storage parameter PCTFREE can also be specified when creating or altering indicies (index segments).

The PCTFREE parameter is used to set the percentage of a block to be reserved (kept free) for possible updates to rows that already are contained in that block. For example, assume that you specify the following parameter within a CREATE TABLE statement:

pctfree 20

This states that 20\% of each data block used for this table's data segment will be kept free and available for possible updates to the existing rows already within each block.

After a data block becomes full, as determined by PCTFREE, the block is not considered for the insertion of new rows until the percentage of the block being used falls below the parameter PCTUSED. Before this value is achieved, the free space of the data block can only be used for updates to rows already contained in the data block. For example, assume that you specify the following parameter within a CREATE TABLE statement:

pctused 40

In this case, a data block used for this table's data segment is not considered for the insertion of any new rows until the amount of used space in the blocks falls to 39\% or less (assuming that the block's used space has previously reached PCTFREE).

No matter what type, each segment in a database is created with at least one extent to hold its data. This extent is called the segment's initial extent.

If the data blocks of a segment's initial extent become full and more space is required to hold new data, ORACLE automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or incremented size of the previous extent in that segment.

Every non-clustered table in an ORACLE database has a single data segment to hold all of its data. The data segment for a table is indirectly created via the CREATE TABLE/SNAPSHOT command.

Storage parameters for a table, snapshot, or cluster control the way that a data segment's extents are allocated. Setting these storage parameters directly via the CREATE TABLE/SNAPSHOT/CLUSTER or ALTER TABLE/SNAPSHOT/CLUSTER commands affects the efficiency of data retrieval and storage for that data segment.

For more information on Data Blocks, Segments and Extents, see ``ORACLE7 Server Concepts Manual.''

Physical Database Structure

An ORACLE database's physical structure is determined by the operating system files that constitute the database. Each ORACLE database is comprised of these types of files: one or more data files, two or more redo log files, and one or more control files. The files of a database provide the actual physical storage for database information. For more information on these physical storage files, see ``ORACLE7 Server Concepts Manual.''

Figure 5. Maintaining the Free Space of Data Blocks with PCTFREE and PCTUSED