Oracle Data Base Interview Questions Part-3

What is the function of Redo Log ?

The Primary function of the redo log is to record all changes made to data.

What is the use of Redo Log Information ?

The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database’s data files.

What does a Control file Contain ?

A Control file records the physical structure of the database. It contains the following information.

Database Name

Names and locations of a database’s files and redolog files.

Time stamp of database creation.

What is the use of Control File ?

When an instance of an ORACLE database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.

What is a Data Dictionary ?

The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database. It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used.

What is an Integrity Constrains ?

An integrity constraint is a declarative way to define a business rule for a column of a table.

Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint ?

No.

Describe the different type of Integrity Constraints supported by ORACLE ?

NOT NULL Constraint - Disallows NULLs in a table’s column.

UNIQUE Constraint – Disallows duplicate values in a column or set of columns.

PRIMARY KEY Constraint - Disallows duplicate values and NULLs in a column or set of columns.

FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table’s UNIQUE or PRIMARY KEY.

CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.

What is difference between UNIQUE constraint and PRIMARY KEY constraint ?

A column defined as UNIQUE can contain NULLs while a column defined as PRIMARY KEY can’t contain Nulls.

Describe Referential Integrity ?

A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data.

What are the Referential actions supported by FOREIGN KEY integrity constraint ?

UPDATE and DELETE Restrict – A referential integrity rule that disallows the update or deletion of referenced data.

DELETE Cascade - When a referenced row is deleted all associated dependent rows are deleted.

What is self-referential integrity constraint ?

If a foreign key reference a parent key of the same table is called self-referential integrity constraint.

What are the Limitations of a CHECK Constraint ?

The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can’t contain subqueries, sequence, the SYSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

What is the maximum number of CHECK constraints that can be defined on a column ?

No Limit.

What constitute an ORACLE Instance ?

SGA and ORACLE background processes constitute an ORACLE instance. (or) Combination of memory

structure and background process.

What is SGA ?

The System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and

control information for one ORACLE instance.

What are the components of SGA ?

Database buffers, Redo Log Buffer the Shared Pool and Cursors.

What do Database Buffers contain ?

Database buffers store the most recently used blocks of database data. It can also contain modified

data that has not yet been permanently written to disk.

What do Redo Log Buffers contain ?

Redo Log Buffer stores redo entries a log of changes made to the database.

What is Shared Pool ?

Shared Pool is a portion of the SGA that contains shared memory constructs such as shared SQL areas.

What is Shared SQL Area ?

A Shared SQL area is required to process every unique SQL statement submitted to a database and

contains information such as the parse tree and execution plan for the corresponding statement.

What is Cursor ?

A Cursor is a handle ( a name or pointer) for the memory associated with a specific statement.

What is PGA ?

Program Global Area (PGA) is a memory buffer that contains data and control information for a server

process.

What is User Process ?

A user process is created and maintained to execute the software code of an application program. It

is a shadow process created automatically to facilitate communication between the user and the

server process.

What is Server Process ?

Server Process handle requests from connected user process. A server process is in charge of

communicating with the user process and interacting with ORACLE carry out requests of the associated

user process.

What are the two types of Server Configurations ?

Dedicated Server Configuration and Multi-threaded Server Configuration.

What is Dedicated Server Configuration ?

In a Dedicated Server Configuration a Server Process handles requests for a Single User Process.

What is a Multi-threaded Server Configuration ?

In a Multi-threaded Server Configuration many user processes share a group of server process.

What is a Parallel Server option in ORACLE ?

A configuration for loosely coupled systems where multiple instance share a single physical database

is called Parallel Server.

Name the ORACLE Background Process ?

DBWR – Database Writer.

LGWR – Log Writer

CKPT – Check Point

SMON – System Monitor

PMON – Process Monitor

ARCH – Archiver

RECO – Recover

Dnnn – Dispatcher, and

LCKn – Lock

Snnn – Server.

What Does DBWR do ?

Database writer writes modified blocks from the database buffer cache to the data files.

When Does DBWR write to the database ?

DBWR writes when more data needs to be read into the SGA and too few database buffers are free. The

least recently used data is written to the data files first. DBWR also writes when CheckPoint

occurs.

What does LGWR do ?

Log Writer (LGWR) writes redo log entries generated in the redo log buffer of the SGA to on-line

Redo Log File.

When does LGWR write to the database?

LGWR writes redo log entries into an on-line redo log file when transactions commit and the log

buffer files are full.

What is the function of checkpoint CKPT)?

The Checkpoint (CKPT) process is responsible for signaling DBWR at checkpoints and updating all the

data files and control files of the database.

What are the functions of SMON ?

System Monitor (SMON) performs instance recovery at instance start-up. In a multiple instance system

(one that uses the Parallel Server), SMON of one instance can also perform instance recovery for

other instance that have failed SMON also cleans up temporary segments that are no longer in use and

recovers dead transactions skipped during crash and instance recovery because of file-read or

off-line errors. These transactions are eventually recovered by SMON when the tablespace or file is

brought back on-line SMON also coalesces free extents within the database to make free space

contiguous and easier to allocate.

What are functions of PMON ?

Process Monitor (PMON) performs process recovery when a user process fails PMON is responsible for

cleaning up the cache and Freeing resources that the process was using PMON also checks on

dispatcher and server processes and restarts them if they have failed.

What is the function of ARCH ?

Archiver (ARCH) copies the on-line redo log files to archival storage when they are full. ARCH is

active only when a database’s redo log is used in ARCHIVELOG mode.

What is function of RECO ?

RECOver (RECO) is used to resolve distributed transactions that are pending due to a network or

system failure in a distributed database. At timed intervals,the local RECO attempts to connect to

remote databases and automatically complete the commit or rollback of the local portion of any

pending distributed transactions.

What is the function of Dispatcher (Dnnn) ?

Dispatcher (Dnnn) process is responsible for routing requests from connected user processes to

available shared server processes and returning the responses back to the appropriate user

processes.

How many Dispatcher Processes are created ?

Atleast one Dispatcher process is created for every communication protocol in use.

What is the function of Lock (LCKn) Process ?

Lock (LCKn) are used for inter-instance locking when the ORACLE Parallel Server option is used.

What is the maximum number of Lock Processes used ?

Though a single LCK process is sufficient for most Parallel Server systems

upto Ten Locks (LCK0,….LCK9) are used for inter-instance locking.