Oracle Data Base Interview Questions Part-4

Define Transaction ?

A Transaction is a logical unit of work that comprises one or more SQL statements executed by a

single user.

When does a Transaction end ?

When it is committed or Rollbacked.

What does COMMIT do ?

COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.

What does ROLLBACK do ?

ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.

What is SAVE POINT ?

For long transactions that contain many SQL statements, intermediate markers or savepoints can be

declared which can be used to divide a transaction into smaller parts. This allows the option of

later rolling back all work performed from the current point in the transaction to a declared

savepoint within the transaction.

What is Read-Only Transaction ?

A Read-Only transaction ensures that the results of each query executed in the transaction are

consistant with respect to the same point in time.

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.

What are the factors that affect OPTIMIZER in choosing an Optimization approach ?

The OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL

parameter of the ALTER SESSION command hints in the statement.

What are the values that can be specified for OPTIMIZER MODE Parameter ?

COST and RULE.

Will the Optimizer always use COST-based approach if OPTIMIZER_MODE is set to “Cost’?

Presence of statistics in the data dictionary for atleast one of the tables accessed by the SQL

statements is necessary for the OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses

RULE-based approach.

What is the effect of setting the value of OPTIMIZER_MODE to ‘RULE’ ?

This value causes the optimizer to choose the rule_based approach for all SQL statements issued to

the instance regardless of the presence of statistics.

What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION

Command ?

CHOOSE,ALL_ROWS,FIRST_ROWS and RULE.

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 effect of setting the value “ALL_ROWS” for OPTIMIZER_GOAL parameter of the ALTER

SESSION command ?

This value causes the optimizer to the cost-based approach for all SQL statements in the session

regardless of the presence of statistics and to optimize with a goal of best throughput.

What is the effect of setting the value ‘FIRST_ROWS’ for OPTIMIZER_GOAL parameter of the ALTER SESSION command ?

This value causes the optimizer to use the cost-based approach for all SQL statements in the session

regardless of the presence of statistics and to optimize with a goal of best response time.

What is the effect of setting the ‘RULE’ for OPTIMIER_GOAL parameter of the ALTER SESSION

Command ?

This value causes the optimizer to choose the rule-based approach for all SQL statements in a

session regardless of the presence of statistics.

What is RULE-based approach to optimization ?

Choosing an executing planbased on the access paths available and the ranks of these access paths.

What is COST-based approach to optimization ?

Considering available access paths and determining the most efficient execution plan based on

statistics in the data dictionary for the tables accessed by the statement and their associated

clusters and indexes.

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.

Oracle Data Base Interview Questions Part-2

What is a Sequence ?

A sequence generates a serial list of unique numbers for numerical columns of a database’s tables.

What is a Synonym ?

A synonym is an alias for a table, view, sequence or program unit.

What are the type of Synonyms?

There are two types of Synonyms Private and Public.

What is a Private Synonyms ?

A Private Synonyms can be accessed only by the owner.

What is a Public Synonyms ?

A Public synonyms can be accessed by any user on the database.

What are synonyms used for ?

Synonyms are used to: Mask the real name and owner of an object.

Provide public access to an object Provide location transparency for tables, views or program units of a remote database. Simplify the SQL statements for database users.

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.

How are Indexes Update ?

Indexes are automatically maintained and used by ORACLE. Changes to table data are automatically incorporated into all relevant indexes.

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 cluster Key?

The related columns of the tables in a cluster are called the Cluster Key.

What is Index Cluster ?

A Cluster with an index on the Cluster Key.

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.

When can Hash Cluster used ?

Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.

What is Database Link ?

A database link is a named object that describes a “path” from one database to another.

What are the types of Database Links ?

Private Database Link, Public Database Link & Network Database Link.

What is Private Database Link ?

Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner’s views or procedures.

What is Public Database Link ?

Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.

What is Network Database link ?

Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.

What is Data Block ?

ORACLE database’s data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.

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 is Row Chaining ?

In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs , the data for the row is stored in a chain of data block (one or more) reserved for that segment.

What is an Extent ?

An Extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.

What is a Segment ?

A segment is a set of extents allocated for a certain logical structure.

What are the different type of Segments ?

Data Segment, Index Segment, Rollback Segment and Temporary Segment.

What is a Data Segment ?

Each Non-clustered table has a data segment. All of the table’s data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster’s data segment.

What is an Index Segment ?

Each Index has an Index segment that stores all of its data.

What is Rollback Segment ?

A Database contains one or more Rollback Segments to temporarily store “undo” information.

What are the uses of Rollback Segment ?

Rollback Segments are used :

To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.

What is a Temporary Segment ?

Temporary segments are created by ORACLE when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.

What is a Data File ?

Every ORACLE database has one or more physical data files. A database’s data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.

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 table space.

What is a Redo Log?

The set of Redo Log files for a database is collectively known as the database’s redo log.

Oracle Data Base Interview Questions Part-1

What is data?

Data is a collection of raw, facts and figures that is use in different place we can store anything in database is called data.

What is database?

Collection of organized data which can easily managed access, understand, and update.

What is database management system?

Any programming environment used to create containers, to manage human data, can be conceptualized as a Data Management System

What is table?

It is a set of elements that is consist of rows and columns in which we can store our data. In any table we define specified no, of columns and there are many no. of rows are available in table that is use for store the data row wise.

What is column?

Column is a one element of table that is use logically vertical shape in table. We define specified no. of columns in one table then store data in it.

What is row?

Rows are also second element of table that is use for store the data in table as a horizontally shape. We can enter many rows in one table. In database terms we can say one record to one row in table.

What is primary key?

A primary is uniquely identifies a row of the table this key ensure two things one is that null value does not allow and duplicate value also not allowed in table. A table must always have one and only one primary key. Primary key value is normally employee code, Driving license code, NIC code etc.

What is foreign key?

A foreign key is a references or a link between one table and another. The foreign key can be used to cross reference tables. A foreign has a link with parent table (that have primary key value). Suppose we have one table with primary key and w have second table that have foreign key and have a relation with parent table than we must enter the values in second table that is totally related with parent table.

What is unique key?

Unique key does not accept duplicate values but accept null values in oracle. Unique key is very much related with primary key in primary key we cannot enter NULL value and also duplicate value. But on the other hand in unique key we can enter NULL value but not duplicate value. For example in employee database we set Driving license no. unique because one employee have unique license no. and some employees have not.

What is the difference between primary key and unique key?

Unique key does not accept duplicate values but accept null values and primary key does not accept both two either NULL or duplicate data.

What is Null constraint?

The columns that have NULL constraint must fill with some data. A column in a table can be specified not null. It’s not possible to insert a null in such a column we cannot leave any column empty in tables that have NULL constraint. It should be fill with no matter with duplication of that.

What is the maximum no. of column in oracle table?

The maximum no. of columns is 255.

Is COMMIT statement can be use in database trigger?

We cannot use any transaction control statement (like commit, rollback, savepoint etc.) into the body of a trigger. But oracle provides a facility called Autonomous Transactions through which you can achieve this. You add that piece of the code, which contains commit, in some block and make that block as Autonomous Transaction and call or use that block into your trigger.

What are the components of Physical database structure of Oracle Database?

ORACLE database is comprised of three types of files. One or more Data files, two are more Redo Log files, and one or more Control files.

What are the components of Logical database structure of ORACLE database?

Tablespaces and the Database’s Schema Objects.

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 SYSTEM tablespace and When is it Created?

Every ORACLE database contains a table space named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.

What is schema?

A schema is collection of database objects of a User.

What are Schema Objects ?

Schema objects are the logical structures that directly refer to the database’s data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.

Can objects of the same Schema reside in different tablespaces.?

Yes.

Can a Tablespace hold objects from different Schemes ?

Yes.

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 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.)

Do View contain Data ?

Views do not contain or store data.

Can a View based on another View ?

Yes.

What are the advantages of Views ?

Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.

Hide data complexity.

Simplify commands for the user.

Present the data in a different perpecetive from that of the base table.

Store complex queries.