MOST IMPORTANT DBA & APPS DBA INTERVIEW QUESTIONS

Hi ,
Below is the list of favourite interview questions and answers:

1)Describe the hot backup mechanism?Why we say more redos generate during hot backup?
(SONY,TESCO,ROBERT BOSCH
Ans: When we give the below command:

SQL>Alter tablespace/database TS_NAME begin backup;
The following things happen:

1)A Checkpoint occurs
2)A timestamp is started.
3)Datafile header gets freezed but the DBWR process can write the data or the server process reads the data to the other portion of the datafile.


Here we come across some interesting things happening:

1)As Users can continue to work on the database when the database is in hotbackup mode,at the same time we say more redos are generated.

Let me tell you how we confirm the tablespace is in hot backup mode:

SQL>select status from v$backup;

SQL> select file#||' '||status||' '||time from v$backup;

If the status = active,It is in hotbackup mode.
If the status = not active,It is not in hotbackup mode.

We say when the tablespace is in hot backup mode more redos are generated:
Let me tell you what exactly what this concept is,when we issue the command ALTER TABLESPACE TS_NAME BEGIN BACKUP,Users are doing the normal transaction activities in the form of insert/update/delete so when the OS command CP is issued to copy the file to backup destination at that time if there is ANY UPDATE is issued or any change happen to the block than the ENTIRE IMAGE OF THE BLOCK IS COPIED TO REDOLOG FILES.This happen frequently whenever there is change happening to the block,so each time any change happening to the block the ENTIRE IMAGE OF THE BLOCK IS COPIED TO THE REDOLOG FILE FOR RECOVERY PURPOSE so we say in HOT BACKUP MORE REDOS ARE GENERATED.
There is one more interesting thing happening as CP is a dummy command it doesn't know the change happened in the block when there is some data written by DBWR so the block in such a state during the hot backup is known as "SPLIT BLOCK" or "FRUCTURED BLOCK".Thats why we say the hot backup is inconsistent backup.

When we issue the command:

SQL>ALTER TABLESPACE/DATABASE TBS_NAME end backup;
The datafile headers get open and the normal database operations can go on as it is.

Hope it helps .........


2)Describe the steps for upgrading the Database?(SONY,ORACLE)
Refer my previous posts.

3)How can you determine the CPU usage,IO usage ?
Ans: IN Operating system:
top -c can give up the %CPU usage.
iostat can give up the blocks read/write per second detail information along with the iowait.

IN Database:
A Join query of v$session and v$process can help in getting the CPU and process related details as below:

select s.username “Oracle User”,s.osuser “OS User”,i.consistent_gets “Consistent Gets”,i.physical_reads “Physical Reads”,s.status “Status”,s.sid “SID”,s.serial# “Serial#”,
s.machine “Machine”,s.program “Program”,to_char(logon_time, ‘DD/MM/YYYY HH24:MI:SS’) “Logon Time”,w.seconds_in_wait “Idle Time”, P.SPID “PROC”,
name “Stat CPU”, value
from v$session s, v$sess_io i, v$session_wait w, V$PROCESS P, v$statname n, v$sesstat t
where s.sid = i.sid
and s.sid = w.sid (+)
and ‘SQL*Net message from client’ = w.event(+)
and s.osuser is not null
and s.username is not null
and s.paddr=p.addr
and n.statistic# = t.statistic#
and n.name like ‘%cpu%’
and t.SID = s.sid
order by 6 asc, 3 desc, 4 desc;

4)Describe the apps file system?(ROBERT BOSCH)
Ans: Refer the below link for ORACLE 11i/12 filesystem:

http://becomeappsdba.blogspot.com/2007/01/oracle-apps-r12-file-system-changes.html

5)How you tune your query?(COGNIZANT,ORACLE,SONY)
Ans:Refer my previous post.

In general:

1)Generate the execution plan with the help of tool like EXPLAIN PLAN,SQL_TRACE,DBMS_PROFILER
2)Gather the statistics
3)Check the joins and joins order in the query.
4)Check the where clause conditions
5)Check the index usage details
6)Use or guide developers for using various hints to alter the execution plans.

6) Where do we see concurrent manager log files?
Ans: $cd $APPLCSF/$APPLLOG

7)How is clonning/Migration done?(YODLEE,SONY)
Ans The below details by Burleson is quite helpful:
database cloning procedure is especially useful for the DBA who wants to give his developers a full-sized TEST and DEV instance by cloning the PROD instance into the development server areas.

This Oracle clone procedure can be use to quickly migrate a system from one UNIX server to another. It clones the Oracle database and this Oracle cloning procedures is often the fastest way to copy a Oracle database.

STEP 1: On the old system, go into SQL*Plus, sign on as SYSDBA and issue: “alter database backup controlfile to trace”. This will put the create database syntax in the trace file directory. The trace keyword tells oracle to generate a script containing a create controlfile command and store it in the trace directory identified in the user_dump_dest parameter of the init.ora file. It will look something like this:

STARTUP NOMOUNT
CREATE CONTROLFILE REUSE DATABASE "OLDLSQ" NORESETLOGS
NOARCHIVELOG
MAXLOGFILES 16
MAXLOGMEMBERS 2
MAXDATAFILES 240
MAXINSTANCES 1
MAXLOGHISTORY 113
LOGFILE
GROUP 1 ('/u03/oradata/oldlsq/log1a.dbf',
'/u03/oradata/olslsq/log1b.dbf') SIZE 30M,
GROUP 2 ('/u04/oradata/oldlsq/log2a.dbf',
'/u04/oradata/oldlsq/log2b.dbf') SIZE 30M
DATAFILE
'/u01/oradata/oldlsq/system01.dbf',
'/u01/oradata/oldlsq/mydatabase.dbf'
;
# Recovery is required if any of the datafiles are restored
# backups, or if the last shutdown was not normal or immediate.
RECOVER DATABASE
# Database can now be opened normally.
ALTER DATABASE OPEN;


STEP 2: Shutdown the old database

STEP 3: Copy all data files into the new directories on the new server. You may change the file names if you want, but you must edit the controlfile to reflect the new data files names on the new server.

rcp /u01/oradata/oldlsq/* newhost:/u01/oradata/newlsq
rcp /u01/oradata/oldlsq/* newhost:/u01/oradata/newlsq
rcp /u03/oradata/oldlsq/* newhost:/u03/oradata/newlsq
rcp /u04/oradata/oldlsq/* newhost:/u04/oradata/newlsq


STEP 4: Copy and Edit the Control file – Using the output syntax from STEP 1, modify the controlfile creation script by changing the following:

Old:

CREATE CONTROLFILE REUSE DATABASE "OLDLSQ" NORESETLOGS

New:

CREATE CONTROLFILE SET DATABASE "NEWLSQ" RESETLOGS

STEP 5: Remove the “recover database” and “alter database open” syntax

# Recovery is required if any of the datafiles are restored
# backups, or if the last shutdown was not normal or immediate.
RECOVER DATABASE
# Database can now be opened normally.
ALTER DATABASE OPEN;


STEP 6: Re-names of the data files names that have changed.

Save as db_create_controlfile.sql.

Old:

DATAFILE
'/u01/oradata/oldlsq/system01.dbf',
'/u01/oradata/oldlsq/mydatabase.dbf'

New:

DATAFILE
'/u01/oradata/newlsq/system01.dbf',
'/u01/oradata/newlsq/mydatabase.dbf'

STEP 7: Create the bdump, udump and cdump directories

cd $DBA/admin
mkdir newlsq
cd newlsq
mkdir bdump
mkdir udump
mkdir cdump
mkdir pfile


STEP 8: Copy-over the old init.ora file

rcp $DBA/admin/olslsq/pfile/*.ora newhost:/u01/oracle/admin/newlsq/pfile

STEP 9: Start the new database

@db_create_controlfile.sql

STEP 10: Place the new database in archivelog mode