Thread: Oracle Instance

Background process:

These background processes will exist in oracle instance. Every time an instance is started, a system global area (SGA) is allocated and oracle background processes are started. Background processes perform functions on behalf of the invoking process. They consolidate functions that would otherwise be handled by multiple Oracle programs running for each user. The background processes perform input/output (I/O) and monitor other Oracle processes to provide increased parallelism for better performance and reliability.

Temporary table:
Temporary tables can be created to hold session-private data that exists only for the duration of a transaction or session.
eg: when a large query wan to sort, this is splitted and executed separately in temporary table tables.

Data block:
Its the small unit of storage in database. Oracle data blocks contain:
• Block header: The header contains the data block address, table directory, row directory, and transaction slots that are used when transactions make changes to rows in the block. Block headers grow from the top down.
• Data space: Row data is inserted into the block from the bottom up.
• Free space: The free space in a block is in the middle of the block: thus both the header and the row data space can grow when necessary. The free space in a block is
contiguous initially. However, deletions and updates may fragment the free space in the block. The free space in the block is coalesced by the Oracle server when necessary.

inittrans and maxtrans:
INITRANS and MAXTRANS specify the initial and the maximum number of transaction slots that are created in an index or a data block. The transaction slots are used to store information about transactions that are making changes to the block at a point in time. A transaction uses only one transaction slot, even if it is changing more than one row or index entry.

session:
A session is a specific connection of a user to an Oracle server. The session starts when the user is validated by the Oracle server, and it ends when the user logs out or when there is an abnormal termination.

alert log file:
This is the system generated logs and trace files. This will contain information like Startups and shutdowns of the instance
Messages to the operator console
Errors causing trace files.
Create, alter and drop SQL statements on databases, tablespaces and rollback segments.
Errors when a materialized view is refreshed