Oracle Architecture

RDBMS:

          The Oracle Relational Database Management System, or RDBMS, is designed to allow simultaneous access to large amounts of stored information. The RDBMS consists of the database (the information) and the instance (the embodiment of the system). The database contains the physical files that reside on the system and the logical pieces such as the database schema. These database files take various forms, as described in the following section. The instance is the method used to access the data and consists of processes and system memory.

The Oracle Database:

The Oracle database has a logical layer and a physical layer. The physical layer consists of the files that reside on the disk; the components of the logical layer map the data to these physical components.

The Physical Layer:

The physical layer of the database consists of three types of files:

• One or more datafiles--Datafiles store the information contained in the database. You can have as few as one datafile or as many as hundreds of datafiles. The information for a single table can span many datafiles or many tables can share a set of datafiles. Spreading tablespaces over many datafiles can have a significant positive effect on performance. The number of datafiles that can be configured is limited by the Oracle parameter MAXDATAFILES.
• Two or more redo log files--Redo log files hold information used for recovery in the event of a system failure. Redo log files, known as the redo log, store a log of all changes made to the database. This information is used in the event of a system failure to reapply changes that have been made and committed but that might not have been made to the datafiles. The redo log files must perform well and be protected against hardware failures (through software or hardware fault tolerance). If redo log information is lost, you cannot recover the system.
• One or more control files--Control files contain information used to start an instance, such as the location of datafiles and redo log files; Oracle needs this information to start the database instance. Control files must be protected. Oracle provides a mechanism for storing multiple copies of control files.

The Logical Layer:

The logical layer of the database consists of the following elements:

• One or more tablespaces.

• The database schema, which consists of items such as tables, clusters, indexes, views, stored procedures, database triggers, sequences, and so on.

Tablespaces and Datafiles:

The database is divided into one or more logical pieces known as tablespaces. A tablespace is used to logically group data together. For example, you can create one tablespace for accounting and a separate tablespace for purchasing. Segmenting groups into different tablespaces simplifies the administration of these groups. Tablespaces consist of one or more datafiles. By using more than one datafile per tablespace, you can spread data over many different disks to distribute the I/O load and improve performance.

The relationship between the database, tablespaces, and datafiles.

As part of the process of creating the database, Oracle automatically creates the SYSTEM tablespace for you. Although a small database can fit within the SYSTEM tablespace, it's recommended that you create a separate tablespace for user data. The SYSTEM tablespace is where the data dictionary is kept. The data dictionary contains information about tables, indexes, clusters, and so on.

The Database Schema:

The database schema is a collection of logical-structure objects, known as schema objects, which define how you see the database's data. These schema objects consist of structures such as tables, clusters, indexes, views, stored procedures, database triggers, and sequences.

• Table--A table, which consists of a table name and rows and columns of data, is the basic logical storage unit in the Oracle database. Columns are defined by name and data type. A table is stored within a tablespace; often, many tables share a tablespace.
• Cluster--A cluster is a set of tables physically stored together as one table that shares a common column. If data in two or more tables is frequently retrieved together based on data in the common column, using a clustered table can be quite efficient. Tables can be accessed separately even though they are part of a clustered table. Because of the structure of the cluster, related data requires much less I/O overhead if accessed simultaneously.
• Index--An index is a structure created to help retrieve data more quickly and efficiently (just as the index in this book allows you to find a particular section more quickly). An index is declared on a column or set of columns. Access to the table based on the value of the indexed column(s) (as in a WHERE clause) will use the index to locate the table data.
• View--A view is a window into one or more tables. A view does not store any data; it presents table data. A view can be queried, updated, and deleted as a table without restriction. Views are typically used to simplify the user's perception of data access by providing limited information from one table, or a set of information from several tables transparently. Views can also be used to prevent some data from being accessed by the user or to create a join from multiple tables.
• Stored procedure--A stored procedure is a predefined SQL query that is stored in the data dictionary. Stored procedures are designed to allow more efficient queries. Using stored procedures, you can reduce the amount of information that must be passed to the RDBMS and thus reduce network traffic and improve performance.
• Database trigger--A database trigger is a procedure that is run automatically when an event occurs. This procedure, which is defined by the administrator or developer, triggers, or is run whenever this event occurs. This procedure could be an insert, a deletion, or even a selection of data from a table.
• Sequence--The Oracle sequence generator is used to automatically generate a unique sequence of numbers in cache. By using the sequence generator you can avoid the steps necessary to create this sequence on your own such as locking the record that has the last value of the sequence, generating a new value, and then unlocking the record.