Data Blocks and Operating System Blocks

1. Oracle data block: Can be based on one or more operating system blocks, but is the same for the entire database.
2. Operating system blocks: The operating system block is the smallest unit of operating system I/O.

Question: Is there a difference between data blocks and system blocks in Oracle?
In Oracle Database environments, the terms "data blocks" and "system blocks" are used to describe distinct types of storage units that serve specific purposes. Both types of blocks play crucial roles in the proper functioning and management of the database. Understanding their differences is essential for database administrators and developers working with Oracle systems.

Data blocks, also referred to as database blocks or logical blocks, are the smallest units of storage in an Oracle database. They store the actual data within the database, including tables, indexes, and other database objects. Data blocks are typically organized into extents, which are contiguous sets of blocks that belong to a specific segment (such as a table or an index).
Data blocks have the following key characteristics:

1. Size: The default size of a data block is determined by the database block size parameter (DB_BLOCK_SIZE) set during database creation. Common sizes include 2, 4, 8, 16, or 32 KB, depending on the specific requirements of the database.
2. Structure: Each data block contains a block header, table directory, row directory, and the actual data. The block header stores metadata about the block, such as the block address and transaction information. The table and row directories maintain information about the tables and rows stored within the block.
3. Management: Data blocks are managed by the buffer cache in the System Global Area (SGA). The buffer cache ensures that frequently accessed data blocks are readily available in memory, reducing the need for time-consuming disk I/O operations.

System blocks, on the other hand, are blocks that belong to the system tablespaces and store metadata and internal information related to the Oracle database. This includes data dictionary tables, internal tables, and indexes that help manage the database itself.
System blocks have the following key characteristics:

1. Purpose: These blocks store critical information about the database structure, schema objects, privileges, and other metadata required for the database to operate correctly. They do not store user data.
2. Location: System blocks reside in the SYSTEM and SYSAUX tablespaces. The SYSTEM tablespace is mandatory for every Oracle database and contains the data dictionary, while the SYSAUX tablespace is optional and stores auxiliary metadata for various Oracle options and features.
3. Access: System blocks are accessed and managed by Oracle's internal processes and mechanisms. While database administrators can query and manipulate the metadata stored in these blocks using SQL and PL/SQL, direct access to the system blocks themselves is not permitted.

In summary, data blocks and system blocks serve distinct purposes in an Oracle database environment. Data blocks store user data and are organized into extents and segments, while system blocks store metadata and internal information required for the database's proper functioning. Both types of blocks are essential for the smooth operation and management of the Oracle Database.

Index segments are objects designed to increase the speed of data accessed in a table. Using a library analogy, think about how a card catalog works.
Suppose we want to find where the book, "Professional Oracle" is located in the library. We can wander around the library and hope to stumble upon this book or we can go to the card catalog to find its location. The card catalog is an index of all the books available in the library including their locations and we can jump to a listing of library books starting with the letter "O" to quickly find our book. Once our book has been located with the card catalog, we can look up the unique decimal number of the book that provides a path to the physical location of the book we want.
Back to index segments, an index contains the value for one or more columns in a table. Furthermore, a ROWID is used to identify values located in any corresponding columns. What this means is that Oracle can access data directly from a table by first looking up the ROWID in an index. Indexes are not always the most efficient way for retrieving data and this particularly holds true for smaller tables that require less than 100 data blocks. Oracle 8i provides many types of index, of which the most commonly used are:

1. B*-Tree,
2. reverse key,
3. bitmapped, and
4. function-based.