Instance Overview in Oracle Database

Oracle database is one of the world's leading relational database management systems. It provides powerful functions and flexibility and is widely used in enterprise-level systems. In Oracle database, instance is a very important concept. It carries the running environment and memory structure of the database and is the key to connecting with users and performing SQL operations.

What is an Oracle database instance?

Oracle database instance is a collection of processes created when the database is started, including the memory structure and background processes of the database instance. There is a one-to-one correspondence between instances and databases, and each database instance is composed of a separate set of processes. When a user connects to the database, the instance allocates memory and resources to the user and is responsible for parsing SQL statements, querying data, managing transactions and other operations.

Components of an instance

An Oracle database instance consists of the following key parts:

1. SGA (System Global Area): SGA is a shared memory area that contains important information about the database instance, such as buffer pools, log buffers, etc. SGA is a memory area shared by all users and is used to improve database performance and reduce IO operations.
2. Background processes: Background processes are some special processes created when the database instance is started. They are used to process user requests and maintain the status of the database. Common background processes include PMON, SMON, LGWR, DBWR, etc.
3. PGA (Program Global Area): PGA is a memory area allocated when each user connects to a database instance. It is used to store user session information, SQL execution plans, etc. Each user's PGA is independent and not shared.

Creating and starting an Oracle database instance

Creating and starting an Oracle database instance requires the use of management tools provided by Oracle, such as SQL*Plus or SQL Developer. Here is a simple example that demonstrates how to create and start an Oracle database instance:

1. First, log in to the database using SQL*Plus:

sqlplus / as sysdba

2. Create A new database instance:

CREATE DATABASE mydb
  USER SYS IDENTIFIED BY oracle
  USER SYSTEM IDENTIFIED BY oracle
  LOGFILE GROUP 1 ('/u01/oracle/mydb/redo01a.log') SIZE 100M,
          GROUP 2 ('/u01/oracle/mydb/redo02a.log') SIZE 100M,
          GROUP 3 ('/u01/oracle/mydb/redo03a.log') SIZE 100M
  MAXLOGFILES 5
  MAXLOGMEMBERS 5
  MAXLOGHISTORY 1
  MAXDATAFILES 100
  MAXINSTANCES 1
  CHARACTER SET AL32UTF8
  NATIONAL CHARACTER SET AL16UTF16
  EXTENT MANAGEMENT LOCAL
  DATAFILE '/u01/oracle/mydb/system01.dbf' SIZE 325M REUSE
  SYSAUX DATAFILE '/u01/oracle/mydb/sysaux01.dbf' SIZE 325M REUSE
  DEFAULT TABLESPACE users
     DATAFILE '/u01/oracle/mydb/users01.dbf'
     SIZE 500M REUSE AUTOEXTEND ON MAXSIZE UNLIMITED
  DEFAULT TEMPORARY TABLESPACE temp
     TEMPFILE '/u01/oracle/mydb/temp01.dbf'
     SIZE 20M REUSE
  UNDO TABLESPACE undotbs
     DATAFILE '/u01/oracle/mydb/undotbs01.dbf'
     SIZE 200M REUSE AUTOEXTEND ON MAXSIZE UNLIMITED;

3. Start the database instance:

STARTUP

Through the above steps, we created an Oracle database named "mydb" instance and started the instance. Users can connect to the instance through SQL*Plus or other connection tools and start operating on the database.

Summary

Oracle database instance is an important concept in the database management system, which carries the running environment and memory structure of the database. Properly understanding and using database instances can improve database performance and ensure data security and stability. I hope this article has given you a preliminary understanding of Oracle database instances and demonstrated the steps to create and start an instance through code examples.

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