Thursday, 28 June 2012

Oracle interview PART 3

Define Transaction ?
A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.

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 a deadlock ? Explain .
Two processes wating to update the rows of a table which are locked by the other process then deadlock arises. In a database environment this will often happen because of not issuing proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.

What is a Schema ?
The set of objects owned by user account is called the schema.

What is a cluster Key ?
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.

What is Parallel Server ?
Multiple instances accessing the same database (Only In Multi-CPU environments)

What are the basic element of Base configuration of an oracle Database ?
It consists of
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
multiple users/schemas
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONito)
PMON (Process MONitor)
LGWR (LoG Write)
DBWR (Data Base Write)
CKPT (Check Point)
User Process with associated PGS

What is clusters ?
Group of tables physically stored together because they share common columns and are often used together is called Cluster.

What is an Index ? How it is implemented in Oracle Database ?
An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table comman (Ver 7.0)

What is a Database instance ? Explain
A database instance (Server) is a set of memory structure and background processes that access a set of database files.
The process can be shared by all users. The memory structure that are used to store most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.

What is the use of ANALYZE command ?
To perform one of these function on an index,table, or cluster:
- To collect statistics about object used by the optimizer and store them in the data dictionary.
- To delete statistics about the object used by object from the data dictionary.
- To validate the structure of the object.
- To identify migrated and chained rows of the table or cluster.

What is default tablespace ?
The Tablespace to contain schema objects created without specifying a tablespace name.

What are the system resources that can be controlled through Profile ?
The number of concurrent sessions the user can establish the CPU processing time available to the user's session the CPU processing time available to a single call to ORACLE made by a SQL statement the amount of logical I/O available to the user's session the amout of logical I/O available to a single call to ORACLE made by a SQL statement the allowed amount of idle time for the user's session the allowed amount of connect time for the user's session.

What is Tablespace Quota ?
The collective amount of disk space available to the objects in a schema on a particular tablespace.

What are the different Levels of Auditing ?
Statement Auditing, Privilege Auditing and Object Auditing.

What is Statement Auditing ?
Statement auditing is the auditing of the powerful system privileges without regard to specifically named objects.

What are the database administrators utilities available ?
SQL * DBA - This allows DBA to monitor and control an ORACLE database. SQL * Loader - It loads data from standard operating system files (Flat files) into ORACLE database tables. Export (EXP) and Import (imp) utilities allow you to move existing data in ORACLE format to and from ORACLE database.

How can you enable automatic archiving ?
Shut the database
Backup the database
Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
Start up the database.

What are roles? How can we implement roles ?
Roles are the easiest way to grant and manage common privileges needed by different groups of database users. Creating roles and assigning provides to roles. Assign each role to group of users. This will simplify the job of assigning privileges to individual users.

What are Roles ?
Roles are named groups of related privileges that are granted to users or other roles.

What are the use of Roles ?
REDUCED GRANTING OF PRIVILEGES - Rather than explicitly granting the same set of privileges to many users a database administrator can grant the privileges for a group of related users granted to a role and then grant only the role to each member of the group.
DYNAMIC PRIVILEGE MANAGEMENT - When the privileges of a group must change, only the privileges of the role need to be modified. The security domains of all users granted the group's role automatically reflect the changes made to the role.
SELECTIVE AVAILABILITY OF PRIVILEGES - The roles granted to a user can be selectively enable (available for use) or disabled (not available for use). This allows specific control of a user's privileges in any given situation.
APPLICATION AWARENESS - A database application can be designed to automatically enable and disable selective roles when a user attempts to use the application.

What is Privilege Auditing ?
Privilege auditing is the auditing of the use of powerful system privileges without regard to specifically named objects.

What is Object Auditing ?
Object auditing is the auditing of accesses to specific schema objects without regard to user.

What is Auditing ?
Monitoring of user access to aid in the investigation of database use.

Wednesday, 27 June 2012

Oracle interview PART 2

Can a view based on another view?

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 perspective from that of the base table.
- Store complex queries.

What is an Oracle 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 types of synonyms?
There are two types of synonyms private and public.

What is a private synonym?
Only its owner can access a private synonym.

What is a public synonym?
Any database user can access a public synonym.

What are synonyms used for?
- 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 Oracle 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 the index updates?
Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes.

What is a Tablespace?
A database is divided into Logical Storage Unit called tablespace. 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 data blocks. 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.

What is Index Cluster ?
A Cluster with an index on the Cluster Key

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.

How does one create a new database? (for DBA)
One can create and modify Oracle databases using the Oracle "dbca" (Database Configuration Assistant) utility. The dbca utility is located in the $ORACLE_HOME/bin directory. The Oracle Universal Installer (oui) normally starts it after installing the database server software.
One can also create databases manually using scripts. This option, however, is falling out of fashion, as it is quite involved and error prone. Look at this example for creating and Oracle 9i database:

What database block size should I use? (for DBA)
Oracle recommends that your database block size match, or be multiples of your operating system block size. One can use smaller block sizes, but the performance cost is significant. Your choice should depend on the type of application you are running. If you have many small transactions as with OLTP, use a smaller block size. With fewer but larger transactions, as with a DSS application, use a larger block size. If you are using a volume manager, consider your "operating system block size" to be 8K. This is because volume manager products use 8K blocks (and this is not configurable).

What are the different approaches used by Optimizer in choosing an execution plan ?
Rule-based and Cost-based.

What does ROLLBACK do ?
ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.

How does one coalesce free space ? (for DBA)
SMON coalesces free space (extents) into larger, contiguous extents every 2 hours and even then, only for a short period of time.
SMON will not coalesce free space if a tablespace's default storage parameter "pctincrease" is set to 0. With Oracle 7.3 one can manually coalesce a tablespace using the ALTER TABLESPACE ... COALESCE; command, until then use:
SQL> alter session set events 'immediate trace name coalesce level n';
Where 'n' is the tablespace number you get from SELECT TS#, NAME FROM SYS.TS$;
You can get status information about this process by selecting from the SYS.DBA_FREE_SPACE_COALESCED dictionary view.

How does one prevent tablespace fragmentation? (for DBA)
Always set PCTINCREASE to 0 or 100.
Bizarre values for PCTINCREASE will contribute to fragmentation. For example if you set PCTINCREASE to 1 you will see that your extents are going to have weird and wacky sizes: 100K, 100K, 101K, 102K, etc. Such extents of bizarre size are rarely re-used in their entirety. PCTINCREASE of 0 or 100 gives you nice round extent sizes that can easily be reused. E.g.. 100K, 100K, 200K, 400K, etc.

Use the same extent size for all the segments in a given tablespace. Locally Managed tablespaces (available from 8i onwards) with uniform extent sizes virtually eliminates any tablespace fragmentation. Note that the number of extents per segment does not cause any performance issue anymore, unless they run into thousands and thousands where additional I/O may be required to fetch the additional blocks where extent maps of the segment are stored.

Where can one find the high water mark for a table? (for DBA)
There is no single system table, which contains the high water mark (HWM) for a table. A table's HWM can be calculated using the results from the following SQL statements:
Thus, the tables' HWM = (query result 1) - (query result 2) - 1
NOTE: You can also use the DBMS_SPACE package and calculate the HWM = TOTAL_BLOCKS - UNUSED_BLOCKS - 1.

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.

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.

How are extents allocated to a segment? (for DBA)
Oracle8 and above rounds off extents to a multiple of 5 blocks when more than 5 blocks are requested. If one requests 16K or 2 blocks (assuming a 8K block size), Oracle doesn't round it up to 5 blocks, but it allocates 2 blocks or 16K as requested. If one asks for 8 blocks, Oracle will round it up to 10 blocks.
Space allocation also depends upon the size of contiguous free space available. If one asks for 8 blocks and Oracle finds a contiguous free space that is exactly 8 blocks, it would give it you. If it were 9 blocks, Oracle would also give it to you. Clearly Oracle doesn't always round extents to a multiple of 5 blocks.
The exception to this rule is locally managed tablespaces. If a tablespace is created with local extent management and the extent size is 64K, then Oracle allocates 64K or 8 blocks assuming 8K-block size. Oracle doesn't round it up to the multiple of 5 when a tablespace is locally managed.

Can one rename a database user (schema)? (for DBA)
No, this is listed as Enhancement Request 158508. Workaround:
Do a user-level export of user A
create new user B
Import system/manager fromuser=A touser=B
Drop user A

Oracle interview PART 1

What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files.

What are the components of logical database structure of Oracle database?
There are tablespaces and 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 tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.

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 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?


Can a tablespace hold objects from different schemes?

What is Oracle 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 an Oracle 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 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 ?

Can a Tablespace hold objects from different Schemes ?

Can objects of the same Schema reside in different tablespace ?

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.

Oracle interview questions

Explain the difference between trigger and stored procedure.

Trigger in act which is performed automatically before or after a event occur
Stored procedure is a set of functionality which is executed when it is explicitly invoked.

Explain Row level and statement level trigger.

Row-level: - They get fired once for each row in a table affected by the statements.
Statement: - They get fired once for each triggering statement.

Advantage of a stored procedure over a database trigger

Firing of a stored procedure can be controlled whereas on the other hand trigger will get fired whenever any modification takes place on the table.

What are cascading triggers?

A Trigger that contains statements which cause invoking of other Triggers are known as cascading triggers. Here’s the order of execution of statements in case of cascading triggers:
·         Execute all BEFORE statement triggers that apply to the current statement.
·         Loop for each row affected statement.
·         Execute all BEFORE row triggers that apply to the current statement in the loop.
·         Lock and change row, perform integrity constraints check; release lock.
·         Execute all AFTER row triggers that apply to the current statement.
·         Execute all AFTER statement triggers that apply to the current statement.

What is a JOIN? Explain types of JOIN in oracle.

A JOIN is used to match/equate different fields from 2 or more tables using primary/foreign keys. Output is based on type of Join and what is to be queries i.e. common data between 2 tables, unique data, total data, or mutually exclusive data.
Types of JOINS:
Simple JOIN
SELECT p.last_name, t.deptName
FROM person p, dept t
Find name and department name of students who have been allotted a department
Inner/Equi/Natural JOIN

SELECT * from Emp INNER JOIN Dept WHERE Emp.empid=Dept.empid
Extracts data that meets the JOIN conditions only. A JOIN is by default INNER unless OUTER keyword is specified for an OUTER JOIN.
Outer Join

SELECT distinct * from Emp LEFT OUTER JOIN Dept Where Emp.empid=Dept.empid
It includes non matching rows also unlike Inner Join.

SELECT, from emp a, emp b WHERE
Joining a Table to itself.

What is object data type in oracle?

New/User defined objects can be created from any database built in types or by their combinations. It makes it easier to work with complex data like images, media (audio/video). An object types is just an abstraction of the real world entities. An object has:
·         Name
·         Attributes
·         Methods
Create type MyName as object (first varchar2(20), second varchar2(20));

Now you can use this datatype while defining a table below: 

Create table Emp (empno number(5),Name MyName);
One can access the Atributes as Emp.Name.First and Emp.Name.Second

What is composite data type?

Composite data types are also known as Collections .i.e RECORD, TABLE, NESTED TABLE, VARRAY.
Composite data types are of 2 types:
PL/SQL Collections- Table, Varray, Nested Table

Differences between CHAR and NCHAR in Oracle

NCHAR allow storing of Unicode data in the database. One can store Unicode characters regardless of the setting of the database characterset

Differences between CHAR and VARCHAR2 in Oracle

CHAR is used to store fixed length character strings where as Varchar2 can store variable length character strings. However, for performance sake Char is quit faster than Varchar2.
If we have char name[10] and store “abcde”, then 5 bytes will be filled with null values, whereas in case of varchar2 name[10] 5 bytes will be used and other 5 bytes will be freed.

Differences between DATE and TIMESTAMP in Oracle

Date is used to store date and time values including month, day, year, century, hours, minutes and seconds. It fails to provide granularity and order of execution when finding difference between 2 instances (events) having a difference of less than a second between them.
TimeStamp datatype stores everything that Date stores and additionally stores fractional seconds.
Date: 16:05:14
Timestamp: 16:05:14:000

Define CLOB and NCLOB datatypes.

CLOB: Character large object. It is 4GB in length.
NCLOB: National Character large object. It is CLOB datatype for multiple character sets , upto 4GB in length.

What is the BFILE datatypes?

It refers to an external binary file and its size is limited by the operating system.

What is Varrays?

Varrays are one-dimensional, arrays. The maximum length is defined in the declaration itself. These can be only used when you know in advance about the maximum number of items to be stored.
For example: One person can have multiple phone numbers. If we are storing this data in the tables, then we can store multiple phone numbers corresponding to single Name. If we know the maximum number of phone numbers, then we can use Varrays, else we use nested tables.

What is a cursor? What are its types?

Cursor is used to access the access the result set present in the memory. This result set contains the records returned on execution of a query.
They are of 2 types:
1.       Explicit
2.       Implicit

Explain the attributes of implicit cursor

  1. %FOUND - True, if the SQL statement has changed any rows.
  2. %NOTFOUND - True, if record was not fetched successfully.
  3. %ROWCOUNT - The number of rows affected by the SQL statement.
  4. %ISOPEN - True, if there is a SQL statement being associated to the cursor or the cursor is open.

Explain the attributes of explicit cursor.

  1. %FOUND - True, if the SQL statement has changed any rows.
  2. %NOTFOUND - True, if record was not fetched successfully.
  3. %ROWCOUNT - The number of rows affected by the SQL statement.
  4. %ISOPEN - True, if there is a SQL statement being associated to the cursor or the cursor is open.

What is the ref cursor in Oracle?

REF_CURSOR allows returning a recordset/cursor from a Stored procedure.
It is of 2 types:
Strong REF_CURSOR: Returning columns with datatype and length need to be known at compile time.
Weak REF_CURSOR: Structured does not need to be known at compile time.
Syntax till Oracle 9i
create or replace package REFCURSOR_PKG as
Procedure returning the REF_CURSOR:
create or replace procedure test( p_deptno IN number , p_cursor OUT 
open p_cursor FOR 
select *
from emp
where deptno = p_deptno;
end test;
Since Oracle 9i we can use SYS_REFCURSOR
create or replace procedure test( p_deptno IN number,p_cursor OUT SYS_REFCURSOR)
open p_cursor FOR 
select *
from emp
where deptno = p_deptno;
end test;
For Strong
create or replace procedure test( p_deptno IN number,p_cursor OUT REFCURSOR_PKG.STRONG 
open p_cursor FOR 
select *
from emp
where deptno = p_deptno;
end test;

What are the drawbacks of a cursor?

Cursors allow row by row processing of recordset. For every row, a network roundtrip is made unlike in a Select query where there is just one network roundtrip. Cursors need more I/O and temp storage resources, thus it is slower.

What is a cursor variable?

In case of a cursor, Oracle opens an anonymous work area that stores processing information. This area can be accessed by cursor variable which points to this area. One must define a REF CURSOR type, and then declare cursor variables of that type to do so.
/* Create the cursor type. */
TYPE company_curtype IS REF CURSOR RETURN company%ROWTYPE;
/* Declare a cursor variable of that type. */
company_curvar company_curtype;

What is implicit cursor in Oracle?

PL/SQL creates an implicit cursor whenever an SQL statement is executed through the code, unless the code employs an explicit cursor. The developer does not explicitly declare the cursor, thus, known as implicit cursor.
In the following UPDATE statement, which gives everyone in the company a 20% raise, PL/SQL creates an implicit cursor to identify the set of rows in the table which would be affected.
SET salary = salary * 1.2;

Can you pass a parameter to a cursor? Explain with an explain

Parameterized cursor:
/*Create a table*/
create table Employee(
First_Name VARCHAR2(10 BYTE)
/*Insert some data*/
Insert into Employee (ID, First_Name) values (‘01’,’Harry’);
/*create cursor*/
cursor c_emp(cin_No NUMBER)is select count(*) from employee where id=cin_No;
v_countEmp NUMBER;
open c_emp (v_deptNo);
fetch c_emp into v_countEmp;
close c_emp;

/*Using cursor*/
Open c_emp (10);

What is a package cursor?

A Package that returns a Cursor type is a package cursor.
Create or replace package pkg_Util is
    cursor c_emp is select * from employee;
    r_emp c_emp%ROWTYPE;
/*Another package using this package*/
Create or replace package body pkg_aDifferentUtil is
    procedure p_printEmps is
        open pkg_Util.c_emp;
            fetch pkg_Util.c_emp into pkg_Util.r_emp;
            exit when pkg_Util.c_emp%NOTFOUND;
        end loop;
        close pkg_Util.c_emp;

Explain why cursor variables are easier to use than cursors.

Cursor variables are preferred over a cursor for following reasons:
A cursor variable is not tied to a specific query.
One can open a cursor variable for any query returning the right set of columns. Thus, more flexible than cursors.
A cursor variable can be passed as a parameter.
A cursor variable can refer to different work areas.

What is locking, advantages of locking and types of locking in oracle?

Locking is a mechanism to ensure data integrity while allowing maximum concurrent access to data. It is used to implement concurrency control when multiple users access table to manipulate its data at the same time.

Advantages of locking:
a.       Avoids deadlock conditions
b.      Avoids clashes in capturing the resources
Types of locks:
a.       Read Operations: Select
b.      Write Operations:  Insert, Update and Delete

What are transaction isolation levels supported by Oracle?

Oracle supports 3 transaction isolation levels:
a.       Read committed (default)
b.      Serializable transactions
c.       Read only

What is SQL*Loader?

SQL*Loader is a loader utility used for moving data from external files into the Oracle database in bulk. It is used for high performance data loads.

What is Program Global Area (PGA)?

The Program Global Area (PGA): stores data and control information for a server process in the memory. The PGA consists of a private SQL area and the session memory.

What is a shared pool?

The shared pool is a key component. The shared pool is like a buffer for SQL statements.  It is to store the SQL statements so that the identical SQL statements do not have to be parsed each time they're executed. 

38. What is snapshot in oracle?

A snapshot is a recent copy of a table from db or in some cases, a subset of rows/cols of a table. They are used to dynamically replicate the data between distributed databases. 

What is a synonym?

A synonym is an alternative name tables, views, sequences and other database objects.

What is a schema?

A schema is a collection of database objects. Schema objects are logical structures created by users to contain data. Schema objects include structures like tables, views, and indexes. 

What are Schema Objects?

Schema object is a logical data storage structure. Oracle stores a schema object logically within a tablespace of the database.

What is a sequence in oracle?

Is a column in a table that allows a faster retrieval of data from the table because this column contains data which uniquely identifies a row. It is the fastest way to fetch data through a select query. This column has constraints to achieve this ability. The constraints are put on this column so that the value corresponding to this column for any row cannot be left blank and also that the value is unique and not duplicated with any other value in that column for any row.

Difference between a hot backup and a cold backup

Cold backup: It is taken when the database is closed and not available to users.  All files of the database are copied (image copy).  The datafiles cannot be changed during the backup as they are locked, so the database remains in sync upon restore. 

Hot backup: While taking the backup, if the database remains open and available to users then this kind of back up is referred to as hot backup.  Image copy is made for all the files.  As, the database is in use the entire time, so there might be changes made when backup is taking place. These changes are available in log files so the database can be kept in sync

What are the purposes of Import and Export utilities?

Export and Import are the utilities provided by oracle in order to write data in a binary format from the db to OS files and to read them back.

These utilities are used:
·         To take backup/dump of data in OS files.
·         Restore the data from the binary files back to the database.
·         move data from one owner to another

Difference between ARCHIVELOG mode and NOARCHIVELOG mode

Archivelog mode is a mode in which backup is taken for all the transactions that takes place so as to recover the database at any point of time.
Noarichvelog   mode is in which the log files are not written. This mode has a disadvantage that the database cannot be recovered when required. It has an advantage over archivelog mode which is increase in performance.
What are the original Export and Import Utilities?

SQL*Loader, External Tables

What are data pump Export and Import Modes?

It is used for fast and bulk data movement within oracle databases. Data Pump utility is faster than the original import & export utilities.

What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?

SQLCODE: It returns the error number for the last encountered error.
SQLERRM: It returns the actual error message of the last encountered error. 

Explain user defined exceptions in oracle.

A User-defined exception has to be defined by the programmer. User-defined exceptions are declared in the declaration section with their type as exception. They must be raised explicitly using RAISE statement, unlike pre-defined exceptions that are raised implicitly. RAISE statement can also be used to raise internal exceptions. 


userdefined  EXCEPTION;

<Condition on which exception is to be raised>
RAISE userdefined;

WHEN userdefined THEN
<task to perform when exception is raised>

Explain the concepts of Exception in Oracle. Explain its type.

Exception is the raised when an error occurs while program execution. As soon as the error occurs, the program execution stops and the control are then transferred to exception-handling part.
There are two types of exceptions:
1.       Predefined : These types of exceptions are raised whenever something occurs beyond oracle rules. E.g. Zero_Divide
2.       User defined: The ones that occur based on the condition specified by the user. They must be raised explicitly using RAISE statement, unlike pre-defined exceptions that are raised implicitly. 

How exceptions are raised in oracle?

There are four ways that you or the PL/SQL runtime engine can raise an exception:
·         Exceptions are raised automatically by the program.
·         The programmer raises a user defined exceptions. 
·          The programmer raises pre defined exceptions explicitly.

What is tkprof and how is it used?

tkprof  is used for diagnosing performance issues.  It formats a trace file into a more readable format for performance analysis. It is needed because trace file is a very complicated file to be read as it contains minute details of program execution.

What is Oracle Server Autotrace?

It is a utility that provides instant feedback on successful execution of any statement (select, update, insert, delete).  It is the most basic utility to test the performance issues.