Oracle Database

So I'm into databases now. The name database suggests that it's a place to store data. However, there are few types of databases. Oracle falls into the category of relational databases. It is Relational Database Management System and is designed to allow simultaneous access to large amounts of stored information. The Oracle database architecture can be described in terms of its logical and physical structures. The physical structure consists of the files reside on the system and the logical structure of logical pieces such as the database schema.


Now for some more explanation!
So what exactly is this Logical structure of Oracle? It consists of 2 elements.


1. Tablespace


It's a logical group of related data and each database has at least one tablespace called the SYSTEM Tablespace. This SYSTEM tablespace is used to store information such as the data dictionary which stores metadata. Metadata are actually the data about data.
E.g.:- table access permissions, information about keys etc.


A tablespace comprises of a collection of one or more Datafiles. Each of these Datafiles consists of ‘Data blocks’, ‘extents’ and ‘segments’.

The 3 terms mentioned above can be easily understood in a diagram as follows.

Data Block

         

Data Block
The smallest level of space allocation/finest level of granularity in Oracle DB architecture is known as a data block. Data is stored in these data block spaces.


Extent
A specific amount of contiguous data blocks are known as an extent. This is used to store a specific type of information.


Segment 
A specific amount of extents are known a segment. This is used to store a specific type of data structure. 
E.g.:- Each table's data is stored in its own data segment; each index's data is stored in its own index segment. 


2. Schema 

Schema objects define how you see the database's data. They are a collection of logical-structure objects. A schema also defines a level of access for the users as well. All the logical objects in oracle are grouped into a schema. 

A schema is a logical grouping of objects such as:

• Tables -consists of a tablename and rows and columns of data; is the basic logical storage unit in the Oracle database
• Clusters -a set of tables physically stored together as one table that shares a common column.
• Indexes -a structure created to help retrieve data more quickly and efficiently 
• Views -a window into one or more tables. A view does not store any data; it presents table data.
• Stored procedures -a predefined SQL query that is stored in the data dictionary.
• Triggers -a procedure that is run automatically when an event occurs.
• Sequences - used to automatically generate a unique sequence of numbers in cache. 

Now comes the second part; the Physical structure of Oracle DB.

The physical structure of the Oracle database consists of 3 types of files:

1. One or more Datafiles
2. Two or more redo log files
3. One or more control files

Datafiles (.dbf files)

These Datafiles store the information contained in the database. The information for a single table can span many datafiles or many tables can share a set of datafiles. This structure helps to improve the DB performance significantly.

Redo log files (.rdo & .arc)

Oracle DB maintains logs of all the transaction against the database and stores them in the Redo Log files. The main purpose of this is to easily recover the information in the event of a system failure.

Control files

These files record control information about all of the files within the database. They maintain internal consistency and guide recovery operation. Control files have the information used to start an instance, such as the location of datafiles and redo log files.