| Database and Instance [message #50062] |
Tue, 26 February 2002 20:52  |
Aravind
Messages: 41
Registered: January 2002
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Hi,
What is the difference between an an instance and a database in Oracle?
Thanks a lot
Aravind
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| Re: Database and Instance [message #50068 is a reply to message #50062] |
Wed, 27 February 2002 00:36  |
Manu Gupta
Messages: 44
Registered: January 2002
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Dear Aravind,
Instance - It is a set of memory structures and background processes. Means, when Oracle start it occupies certain space in RAM or pagefile, you can call it as buffer. Oracle creates a buffer to function smoothly and effeciently. These are called memory structures. And background processes are processes that acts automatically to write database to database from these memory structures, recover data in case of machine crash. This is what an Instance is. You can configure it through init<sid>.ora file.
Database - Database is a set of physical files that actually stores data in encrypted format. Any query that you fire accesses these datafiles. By default Oracle creates few datafiles that stores oracle server configuration called data dictionary.
Database hierarchy can be explained in two ways
1. Physical hierarchy - database files --> Operating System Data Blocks
2. Logical hierarchy - Database --> Tablespace --> Segments --> Extents --> Data Blocks.
You should refer Oracle Doc. or DBA Handbook for more details.
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