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Monday, 11 November 2013

DBA Interview Questions with Answer Part 20‎

What is Checkpoint SCN and Checkpoint Count? How we can check it?
Checkpoint is an event when the database writer is going to flush the dirty buffers into the datafiles. This an ongoing activity and in the result checkpoint number constantly incremented in the datafile header and controfile and the background process CKPT take care of this responsibility.
How can you find length of Username and Password?
You can find the length of username with below query. The password is hashed (#) so there is no way to get their length.
You can use special characters ($, #, _) without single quotes and any other characters must be enclosed in single quotation.
Select length (username), username
from dba_users;
The minimum length for password is at least 1 character where as maximum depends on database version. In 10g it is restricted to 17 characters long.
What are the restrictions applicable while creating view?
– A view can be created referencing tables and views only in the current database.
– A view name must not be the same as any table owned by that user.
– You can build view on other view and on procedure that references views.
For More information you can click on the below link: Common Interview Question & Answer
What is difference between Delete/Drop/Truncate?
DELETE is a command that only removes data from the table. It is DML statement. Deleted data can be rollback (when you delete all the data get copied into rollback first then deleted). We can use where condition with delete to delete particular data from the table.
Where as DROP commands remove the table from data dictionary. This is DDL statement. We cannot recover the table before oracle 10g, but flashback feature of oracle 10g provides the facility to recover the drop table.
While TRUNCATE is a DDL command that delete data as well as freed the storage held by this table. This free space can be used by this table or some other table again. This is faster because it performs the deleted operation directly (without copying the data into rollback).
Alternatively you can enable the row movement for that table and can use shrink command while using the delete command.
SQL> Create table test
     Number s1, Number s2
SQL> Select bytes, blocks from user_segments
     where segment_name = ‘test’;
Bytes       block
----------  -------
65536       8
SQL> insert into t select level, level*3
     From dual connect by level <= 3000;
3000 rows created.
SQL> Select bytes, blocks from user_segments
     where segment_name = ‘test’;
Bytes       block
----------  -------
131072      16
SQL> Delete from test;
3000 rows deleted.
SQL> select bytes,blocks from user_segments
     where segment_name = 'test';
Bytes       block
----------  -------
131072      16
SQL> Alter table t enable row movement;
SQL> Alter table t shrink space;
Table altered
SQL> Select bytes,blocks from user_segments 
     where segment_name = 'test';
Bytes       block
----------  -------
65536       8
What is difference between Varchar and Varchar2?
Varchar2 can store upto 4000 bytes where as Varchar can only store upto 2000 bytes. Varchar2 can occupy space for NULL values where as Varchar2 will not specify any space for NULL values.
What is difference between Char and Varchar2?
A CHAR values have fixed length. They are padded with space characters to match the specified length where as VARCHAR2 values have a variable length. They are not padded with any characters.
In which Language oracle has been developed?
Oracle is RDBMS package developed using C language.
What is difference between Translate and Replace?
Translate is used for character by character substitution where as Replace is used to substitute a single character with a word.
What is the fastest query method to fetch data from table?
Using ROWID is the fastest method to fetch data from table.
What is Oracle database Background processes specific to RAC?
LCK0—Instance Enqueue Process
LMS—Global Cache Service Process
LMD—Global Enqueue Service Daemon
LMON—Global Enqueue Service Monitor
Oracle RAC instances use two processes, the Global Cache Service (GCS) and the Global Enqueue Service (GES) to ensure that each oracle RAC database instance obtain the block that it needs to satisfy as query or transaction. The GCS and GES maintain records of the statuses of each data file and each cached block using a Global Resource Directory (GRD). The GRD contents are distributed across all of the active instances.
What is SCAN in respect of oracle RAC?
Single client access name (SCAN) is a new oracle real application clusters (RAC) 11g releases 2 features that provides a single name for client to access an oracle database running in a cluster. The benefit is clients using SCAN do not need to change if you add or remove nodes in the clusters.
Why do we have a virtual IP (VIP) in oracle RAC?
Without VIP when a node fails the client wait for the timeout before getting error where as with VIP when a node fails, the VIP associated with it is automatically failed over to some other node and new node re-arps the world indicating a new MAC address for the IP. Subsequent packets sent to the VIP go to the new node, which will send error RST packets back to the clients. This results in the clients getting errors immediately.
Why query fails sometimes?
Rollback segments dynamically extent to handle large transactions entry loads. A single transaction may occupy all available free space in rollback segment tablespace. This situation prevents other user using rollback segments. You can monitor the rollback segment status by querying DBA_ROLLBACK_SEGS view.
What is ADPATCH and OPATCH utility? Can you use both in Application?
ADPATCH is a utility to apply application patch and OPATCH is a utility to apply database patch. You have to use both in application for applying in application you have to use ADPATCH and for applying in database you have to use OPATCH.
What is Automatic refresh of Materialized view and how you will find last refresh time of Materialized view?
Since oracle 10g complete refresh of materialized view can be done with deleted instead of truncate. To force the instance to do the refresh with truncate instead of deleted, parameter AUTOMIC_REFRESH must be set to FALSE
When it is FALSE Mview will be faster and no UNDO will be generated and whole data will be inserted.
When it is TRUE Mview will be slower and UNDO will be generated and whole data will be inserted. Thus we will have access of all time even while it is being refreshed.
If you want to find when the last refresh has taken place. You can query with these view: dba_mviews or dba_mview_analysis or dba_mview_refresh_times
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,’YYYY-MM-DD HH24:MI:SS’) from dba_mviews;
SQL> select NAME, to_char(LAST_REFRESH,’YYYY-MM-DD HH24:MI:SS’) from dba_mview_refresh_times;
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,’YYYY-MM-DD HH24:MI:SS’) from dba_mview_analysis;
Why more archivelogs are generated, when database is begin backup mode?
During begin backup mode datafiles headers get freezed so row information can not be retrieved as a result the entire block is copied to redo logs thus more redo log generated or more log switch occur in turn more archivelogs. Normally only deltas (change vector) are logged to the redo logs.
The main reason is to overcome the fractured block. A fractured block is a block in which the header and footer are not consistent at a given SCN. In a user-managed backup, an operating system utility can back up a datafile at the same time that DBWR is updating the file. It is possible for the operating system utility to read a block in a half-updated state, so that the block that is copied to the backup media is updated in its first half, while the second half contains older data. In this case, the block is fractured.
For non-RMAN backups, the ALTER TABLESPACE ... BEGIN BACKUP or ALTER DATABASE BEGIN BACKUP when a tablespace is in backup mode, and a change is made to a data block, the database logs a copy of the entire block image before the change so that the database can re-construct this block if media recovery finds that this block was fractured.
The block that the operating system reads can be split, that is, the top of the block is written at one point in time while the bottom of the block is written at another point in time. If you restore a file containing a fractured block and Oracle reads the block, then the block is considered a corrupt.
Why is UNION ALL faster than UNION?
UNION ALL faster than a UNION because UNION ALL will not eliminate the duplicate rows from the base tables instead it access all rows from all tables according to your query where as the UNION command is simply used to select related distinct information from base tables like JOIN command.
Thus if you know that all the records of your query returns the unique records then always use UNION ALL instead of UNION. It will give you faster results.
How will you find your instance is started with Spfile and Pfile?
You can query with V$spparameter view
SQL> Select isspecified, count(*) from v$spparameter
     Group by isspecified;
------   ----------
FALSE    221
TRUE     39
As isspecified is TRUE with some count we can say that instance is running with spfile. Now try to start your database with pfile and run the previous query again.
SQL> Select isspecified, count(*) from v$spparameter
     Group by isspecified;
------ ----------
FALSE  258
Then you will not find any parameter isspecified in spfile they all come from pfile thus you can say instance is started with pfile.Alternatively you can use the below query
SQL> show parameter spfile;
SQL> Select decode(count(*), 1, 'spfile', 'pfile' )
     from v$spparameter
     where rownum=1 and isspecified='TRUE';
Why we need to enable Maintenance Mode?
To ensure optimal performance and reduce downtime during patching sessions, enabling this feature shuts down the Workflow Business Events System and sets up function security so that Oracle Applications functions are unavailable to users. This provides a clear separation between normal run time operation and system downtime for patching..