Title Notes 1.pdf ✅

Unit -1 Introduction to Oracle RDBMS Overview of Oracle Database Architecture Oracle Database is an object-relational database management system developed and marketed by Oracle Corporation. Oracle Database is commonly referred to as Oracle RDBMS or simply Oracle. An Oracle Database consists of a database and at least one instance. An instance, or database instance, is the combination of memory and processes that are a part of a running installation and a database is a set of files that store data. The following picture illustrates the Oracle Database server architecture. Fig: Oracle Server Architecture Sometimes, a database instance is referred to as an entire running database. However, it is important to understand the distinctions between the two. • First, you can start a database instance without having it accessing any database files. This is how you create a database, starting an instance first and creating the database from within the instance. • Second, an instance can access only one database at a time. When you start an instance, the next step is to mount that instance to a database. And an instance can mount only one database at a single point in time. • Third, multiple database instances can access the same database. In a clustering environment, many instances on several servers can access a central database to enable high availability and scalability. • Finally, a database can exist without an instance. However, it would be unusable because it is just a set of files.
Physical storage structures The physical storage structures are simply files that store data. When you execute a CREATE DATABASE statement to create a new database, Oracle creates the following files: • Data files: data files contain real data, e.g., sales order and customer data. The data of logical database structures such as tables and indexes are physically stored in the data files. • Control files: every database has a control file that contains metadata. The metadata describes the physical structure of the database including the database name and the locations of data files. • Online redo log files: every database has an online redo log that consists of two or more online redo log files. An online redo log is made up of redo entries that record all changes made to the data. Besides these files, an Oracle database includes other important files such as parameter files, network files, backup files, and archived redo log files for backup and recovery. Fig: Oracle Database Physical Structure Logical Storage Structures Oracle Database uses a logical storage structure for fine-grained control of disk space usage. The following are logical storage structures in an Oracle Database: • Data blocks: a data block corresponds to a number of bytes on the disk. Oracle stores data in data blocks. Data blocks are also referred to as logical blocks, Oracle blocks or pages. • Extents: An extent is a specific number of logically contiguous data blocks used to store the particular type of information. • Segments: a segment is a set of extents allocated for storing database objects, e.g., a table or an index.