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Oracle Interview Questions
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- What is Log Switch? - The
point at which ORACLE ends writing to one online redo log file and begins
writing to another is called a log switch.
- What is On-line Redo Log? - The
On-line Redo Log is a set of tow or more on-line redo files that record
all committed changes made to the database. Whenever a transaction is
committed, the corresponding redo entries temporarily stores in redo log
buffers of the SGA are written to an on-line redo log file by the
background process LGWR. The on-line redo log files are used in cyclical
fashion.
- Which parameter specified
in the DEFAULT STORAGE clause of CREATE TABLESPACE cannot be altered after
creating the tablespace? - All the
default storage parameters defined for the tablespace
can be changed using the ALTER TABLESPACE command. When objects are
created their INITIAL and MINEXTENS values cannot be changed.
- What are the steps involved
in Database Startup? - Start an instance, Mount the Database and
Open the Database.
- What are the steps involved
in Instance Recovery? - Rolling forward to recover data that has
not been recorded in data files, yet has been
recorded in the on-line redo log, including the contents of rollback
segments. Rolling back transactions that have been explicitly rolled back
or have not been committed as indicated by the rollback segments
regenerated in step a. Releasing any resources (locks) held by
transactions in process at the time of the failure. Resolving any pending
distributed transactions undergoing a two-phase commit at the time of the
instance failure.
- Can Full Backup be
performed when the database is open? - No.
- What are the different
modes of mounting a Database with the Parallel Server? -
Exclusive Mode If the first instance that mounts a database does so in
exclusive mode, only that Instance can mount the database. Parallel Mode
If the first instance that mounts a database is started in parallel mode,
other instances that are started in parallel mode can also mount the
database.
- What are the advantages of
operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG
mode? - Complete database recovery from disk failure is
possible only in ARCHIVELOG mode. Online database backup is possible only
in ARCHIVELOG mode.
- What are the steps involved
in Database Shutdown? - Close the Database, Dismount the Database
and Shutdown the Instance.
- What is Archived Redo Log? -
Archived Redo Log consists of Redo Log files that have archived before
being reused.
- What is Restricted Mode of
Instance Startup? - An instance can be started in (or later
altered to be in) restricted mode so that when the database is open
connections are limited only to those whose user accounts have been
granted the RESTRICTED SESSION system privilege.
- What is Partial Backup? - A
Partial Backup is any operating system backup short of a full backup,
taken while the database is open or shut down.
- What is Mirrored on-line
Redo Log? - A mirrored on-line redo log consists of copies of
on-line redo log files physically located on separate disks,
changes made to one member of the group are made to all members.
- What is Full
Backup? - A full backup is an operating system backup of
all data files, on- line redo log files and control file that constitute
ORACLE database and the parameter.
- Can a View based on another
View? - Yes.
- Can a Tablespace
hold objects from different Schemes? - Yes.
- Can objects of the same
Schema reside in different tablespaces? - Yes.
- 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.
- Do View
contain Data? - Views do not contain or store 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 are
the type of Synonyms? - There are two types of Synonyms
Private and Public
- What is a Redo Log? - The set
of Redo Log files YSDATE,UID,USER or USERENV SQL
functions, or the pseudo columns LEVEL or ROWNUM.
- What is an Index Segment? - Each
Index has an Index segment that stores all of its data.
- Explain the relationship
among Database, Tablespace and Data file.? - Each databases logically
divided into one or more tablespaces one or more
data files are explicitly created for each tablespace
- What are
the different type of Segments? - Data Segment, Index
Segment, Rollback Segment and Temporary Segment.
- What are Clusters? -
Clusters are groups of one or more tables physically stores together to
share common columns and are often used together.
- What is an Integrity
Constrains? - An integrity constraint is a declarative way to
define a business rule for a column of a table.
- What is an Index? - An
Index is an optional structure associated with a table to have direct
access to rows, which can be created to increase the performance of data
retrieval. Index can be created on one or more columns of a table.
- What is an Extent? - An
Extent is a specific number of contiguous data blocks, obtained in a
single allocation, and used to store a specific type of information.
- What is a View? - A view
is a virtual table. Every view has a Query attached to it. (The Query is a
SELECT statement that identifies the columns and rows of the table(s) the
view uses.)
- What is Table? - A table
is the basic unit of data storage in an ORACLE database. The tables of a
database hold all of the user accessible data. Table data is stored in
rows and columns.
- What is a Synonym? - A
synonym is an alias for a table, view, sequence or program unit.
- What is a Sequence? - A
sequence generates a serial list of unique numbers for numerical columns
of a database’s tables.
- What is a Segment? - A
segment is a set of extents allocated for a certain logical structure.
- What is schema? - A
schema is collection of database objects of a User.
- 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 is Hash Cluster? - A row
is stored in a hash cluster based on the result of applying a hash
function to the row’s cluster key value. All rows with the same hash key
value are stores together on disk.
- What is a Private Synonyms? - A
Private Synonyms can be accessed only by the owner.
- What is Database Link? - A
database link is a named object that describes a “path” from one database
to another.
- What is a Tablespace? - A database is divided into
Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical
structures together
- What is Rollback Segment? - A
Database contains one or more Rollback Segments to temporarily store
“undo” information.
- What are the
Characteristics of Data Files? - A data file can be associated
with only one database. Once created a data file can’t change size. One or
more data files form a logical unit of database storage called a tablespace.
- How to define Data Block
size? - A data block size is specified for each ORACLE
database when the database is created. A database users and allocated free
database space in ORACLE datablocks. Block size
is specified in INIT.ORA file and can’t be changed latter.
- 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 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. 47.What is Index
Cluster? - A Cluster with an index on the Cluster Key 48.When does a
Transaction end? - When it is committed or Rollbacked.
- What is the effect of
setting the value “ALL_ROWS” for OPTIMIZER_GOAL parameter of the ALTER
SESSION command? - What are the factors that affect OPTIMIZER in
choosing an Optimization approach? - Answer The
OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary
the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the
statement.
- What is the effect of
setting the value “CHOOSE” for OPTIMIZER_GOAL, parameter of the ALTER
SESSION Command? - The Optimizer chooses Cost_based
approach and optimizes with the goal of best throughput if statistics for atleast one of the tables accessed by the SQL
statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.
- What is the function of
Optimizer? - The goal of the optimizer is to choose the most
efficient way to execute a SQL statement.
- What is Execution Plan? - The combinations of the steps the optimizer chooses
to execute a statement is called an execution plan.
- What are the different
approaches used by Optimizer in choosing an execution plan? -
Rule-based and Cost-based.
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