ORACLE Interview Questions and Answers

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594. What are the Back ground processes in Oracle and what are they.

This is one of the most frequently asked question.There are basically 9 Processes but in a general system we need to mention the first five background processes.They do the house keeping activities for the Oracle and are common in any system.
The various background processes in oracle are
a) Data Base Writer(DBWR) :: Data Base Writer Writes Modified blocks from Database buffer cache to Data Files.This is required since the data is not written whenever a transaction is commited.
b)LogWriter(LGWR) :: LogWriter writes the redo log entries to disk. Redo Log data is generated in redo log buffer of SGA. As transaction commits and log buffer fills, LGWR writes log entries into a online redo log file.
c) System Monitor(SMON) :: The System Monitor performs instance recovery at instance startup.This is useful for recovery from system failure
d)Process Monitor(PMON) :: The Process Monitor peforms process recovery when user Process fails. Pmon Clears and Frees resources that process was using.
e) CheckPoint(CKPT) :: At Specified times, all modified database buffers in SGA are written to data files by DBWR at Checkpoints and Updating all data files and control files of database to indicate the most recent checkpoint
f)Archieves(ARCH) :: The Archiver copies online redo log files to archival storal when they are busy.
g) Recoveror(RECO) :: The Recoveror is used to resolve the distributed transaction in network
h) Dispatcher (Dnnn) :: The Dispatcher is useful in Multi Threaded Architecture
i) Lckn :: We can have upto 10 lock processes for inter instance locking in parallel sql.

595. How many types of Sql Statements are there in Oracle

There are basically 6 types of sql statments.They are
a) Data Defination Language(DDL) :: The DDL statments define and maintain objects and drop objects.
b) Data Manipulation Language(DML) :: The DML statments manipulate database data.
c) Transaction Control Statements :: Manage change by DML
d) Session Control :: Used to control the properties of current session enabling and disabling roles and changing .e.g :: Alter Statements,Set Role
e) System Control Statements :: Change Properties of Oracle Instance .e.g:: Alter System
f) Embedded Sql :: Incorporate DDL,DML and T.C.S in Programming Language.e.g:: Using the Sql Statements in languages such as 'C', Open,Fetch, execute and close

596. What is a Transaction in Oracle

A transaction is a Logical unit of work that compromises one or more SQL Statements executed by a single User. According to ANSI, a transaction begins with first executable statment and ends when it is explicitly commited or rolled back.

597. Key Words Used in Oracle

The Key words that are used in Oracle are ::
a) Commiting :: A transaction is said to be commited when the transaction makes permanent changes resulting from the SQL statements.
b) Rollback :: A transaction that retracts any of the changes resulting from SQL statements in Transaction.
c) SavePoint :: For long transactions that contain many SQL statements, intermediate markers or savepoints are declared. Savepoints can be used to divide a transactino into smaller points.
d) Rolling Forward :: Process of applying redo log during recovery is called rolling forward.
e) Cursor :: A cursor is a handle ( name or a pointer) for the memory associated with a specific stament. A cursor is basically an area allocated by Oracle for executing the Sql Statement. Oracle uses an implicit cursor statement for Single row query and Uses Explcit cursor for a multi row query.
f) System Global Area(SGA) :: The SGA is a shared memory region allocated by the Oracle that contains Data and control information for one Oracle Instance.It consists of Database Buffer Cache and Redo log Buffer.
g) Program Global Area (PGA) :: The PGA is a memory buffer that contains data and control information for server process.
g) Database Buffer Cache :: Databese Buffer of SGA stores the most recently used blocks of datatbase data.The set of database buffers in an instance is called Database Buffer Cache.
h) Redo log Buffer :: Redo log Buffer of SGA stores all the redo log entries.
i) Redo Log Files :: Redo log files are set of files that protect altered database data in memory that has not been written to Data Files. They are basically used for backup when a database crashes.
j) Process :: A Process is a 'thread of control' or mechansim in Operating System that executes series of steps.

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