Collection of Most Common DBA Interview Questions

Hello Friends,

I know how tough it is to face the interviews inspite of being prepared well. Trust me even after facing so many interviews the day the interview is scheduled I go totally numb and cold feet. I really don't know what to revise and what to look into. I am sure you might be facing the same things.

Well the other night I had an interview call from one of the most renowned company in the market but due to hectic and busy schedule I was not able to revise the topics that needs to be done before an interview. After the interview I thought of maintaining the most basic questions asked during a DBA interview ready for the next time so that even if I dont get enough time a glance at the questions from my DATABANK would give me enough confidence to face the interview. I am sure you too would be feeling the same. So here goes most common DBA interview questions and there probable answers. Any suggestions and corrections are most welcomed.

Here is the Collection and Answer to some Interesting ORACLE DBA Interview Questions

1. How many memory layers are in the shared pool? 

Ans:  The shared pool portion of the SGA contains three major areas: library cache(contains parsed sql statements,cursor information,execution plans), dictionary cache (contains cache -user account information,priveleges information,datafile,segment and extent information), buffers for parallel execution messages, and control structure.

2. How do you find out from the RMAN catalog if a particular archive log has been backed-up? 

Ans:  list archivelog all;

3. How can you tell how much space is left on a given file system and how much space each of the file system’s subdirectories take-up? 

Ans: df -kh and du-sh

4. Define the SGA and:
i)  How you would configure SGA for a mid-sized OLTP environment?

ii) What is involved in tuning the SGA? 

Ans: SGA: The System Global Area (SGA) is a group of shared memory areas that are dedicated to an Oracle “instance” (an instance is your database programs and RAM). All Oracle processes use the SGA to hold information. The SGA is used to store incoming data (the data buffers as defined by the db_cache_size parameter), and internal control information that is needed by the database. You control the amount of memory to be allocated to the SGA by setting some of the Oracle “initialization parameters”.  These might include db_cache_size, shared_pool_size and log_buffer. 

i) 40% of RAM can be used for sizing SGA rest is reserved for OS and others in 64 bit machine and in 32 bit machine max SGA configured can be 1.5GB only.

ii) Check the statspack report. Check hit ratio of Data buffer. If it is less than 90%, then we need to increase the Data buffer. Check hit ratio of Shared pool. If it is less than 95%, then we need to increase the Shared pool. Check log buffer. If redo buffer allocation retries/redo entries is greater than 1%, then we need to increase log_buffer.

5. What is the cache hit ratio, what impact does it have on performance of an Oracle database and what is involved in tuning it? 

Ans:

Buffer cache hit ratio: It calculates how often a requested block has been found in the buffer cache without requiring disk access. This ratio is computed using data selected from the dynamic performance view V$SYSSTAT. The buffer cache hit ratio can be used to verify the physical I/O as predicted by V$DB_CACHE_ADVICE.

sql> select name, value From v$sysstat Where name in (‘db block gets’, ‘consistent gets’, ‘physical reads’); 

The cache-hit ratio can be calculated as follows: Hit ratio = 1 – (physical reads / (db block gets + consistent gets)) If the cache-hit ratio goes below 90% then:  increase the initialisation parameter DB_CACHE_SIZE.

Library cache hit ratio: It calculates how often the parsed representation of the statement can be reused. It also known as soft parse.

sql>  select namespace, pins, pinhits, reloads, invalidations from v$librarycache order by namespace;

Library Cache Hit Ratio = sum(pinhits) / sum(pins)

Dictionary cache hit ratio:It is a measure of the proportion of requests for information from the data dictionary, the collection of database tables and views containing reference information about the database, its structures, and its users. On instance startup, the data dictionary cache contains no data, so any SQL statement issued is likely to result in cache misses. As more data is read into the cache, the likelihood of cache misses should decrease. Eventually the database should reach a "steady state" in which the most frequently used dictionary data is in the cache.

6. Other than making use of the statspack utility, what would you check when you are monitoring or running a health check on an Oracle 8i or 9i database? 

Ans: Daily Monitoring activities and check different logs for any sort of errors.

7. How do you tell what your machine name is and what is its IP address?
Ans: hostname, uname -n and ifconfig

8. How would you go about verifying the network name that the local_listener is currently using?
Ans: lsnrctl stat or ps-eaf|grep tns

9. You have 4 instances running on the same UNIX box. How can you determine which shared memory and semaphores are associated with which instance? 

Ans: 

SQL> oradebug setmypid
SQL> oradebug ipc
SQL>oradebug tracfile_name

Also you can check the spfile. The parameters will start with instance_name. parameter_name naming.

10. What view(s) do you use to associate a user’s SQLPLUS session with his o/s process? 

Ans: v$process and v$session
sql> select a.spid from v$process a, v$session b where a.addr = b.addr and b.audsid=userenv(‘sessionid’); 

11. What is the recommended interval at which to run statspack snapshots, and why? 

Ans: Should be in minutes (15-20 mins approx)  because where the time between the two snapshots is measured in hours, the events that caused serious performance issues for 20 minutes during peak processing don’t look so bad when they’re spread out over an 8-hour window. It’s also true with STATSPACK that measuring things over too long of a period tends to level them off over time. Nothing will stand out and strike you as being wrong.   

12. What spfile/init.ora file parameter exists to force the CBO to make the execution path of a given statement use an index, even if the index scan may appear to be calculated as more costly? 

Ans: OPTIMIZER_INDEX_COST_ADJ= FORCE

13. Assuming today is Monday, how would you use the DBMS_JOB package to schedule the execution of a given procedure owned by SCOTT to start Wednesday at 9AM and to run subsequently every other day at 2AM. 

Ans: dbms_job.submit(:jobno,'statspack.snap;',trunc(sysdate)+9/24,'trunc(SYSDATE+1/24,''HH'')', TRUE, :instno);

14. How would you edit your CRONTAB to schedule the running of /test/test.sh to run every other day at 2PM? 

Ans: 00 02 * * * /test/test.sh

15. In which dictionary table or view would you look to determine at which time a snapshot or MVIEW last successfully refreshed?

Ans: SQL> SELECT MVIEW_NAME,LAST_REFRESH_DATE from USER_MVIEWS;

16. How would you best determine why your MVIEW couldn’t FAST REFRESH? 

Ans: Possibly by checking the MVIEW LOG for errors.

20. How would you begin to troubleshoot an ORA-3113 error? 

Ans: End of File Communication Error. Check Alert Logfile. CheckNetwrok Latency. Check sqlnet.ora file has expire_time = 0, delete unwanted files and check the swap and temp spaces.

21. Which dictionary tables and/or views would you look at to diagnose a locking issue?

Ans:  v$lock, v$session, v$process

22. An automatic job running via DBMS_JOB has failed. Knowing only that “it’s failed”, how do you approach troubleshooting this issue? 

Ans:Check the log and possible reason for the JOB failed.

23. How would you extract DDL of a table without using a GUI tool? 

Ans: select dbms_metadata.get_ddl('OBJECT','OBJECT_NAME') from dual;

24. You’re getting high “busy buffer waits” - how can you find what’s causing it? 

Ans:  Buffer busy wait means that the queries are waiting for the blocks to be read into the db cache.There could be the reason when the block may be busy in the cache and session is waiting for it. It could be undo, data block or segment header wait.


Run the following query to find out the p1,p2 and p3 of a session causing buffer busy wait


sql> select p1 "File #",p2 "Block #",p3 "Reason Code" from v$session_wait where event = 'buffer busy waits';


After that running the following query to find the  segment causing buffer busy wait:-


sql> select owner,segment_name,segment_type from dba_extents where file_id = &P1 and &P2 between block_id and block_id + blocks -1;

25. What query tells you how much space a tablespace named “test” is taking up, and how much space is remaining? 

Ans: 

SET SERVEROUTPUT ON
SET LINESIZE 1000
SET FEEDBACK OFF

rem column dummy noprintcolumn  pct_used format 999.9       heading "%|Used"
column  name    format a25      heading "Tablespace Name"
column  Kbytes   format 999,999,999    heading "MBytes"
column  used    format 999,999,999   heading "Used(MB)"
column  free    format 999,999,999  heading "Free(MB)"
column  largest    format 999,999,999  heading "Largest"
break   on report
compute sum of kbytes on report
compute sum of free on report
compute sum of used on report
set pagesize 100
select nvl(b.tablespace_name,
nvl(a.tablespace_name,'UNKOWN')) name,(kbytes_alloc/1024) kbytes,
((kbytes_alloc-nvl(kbytes_free,0))/1024) used,(nvl(kbytes_free,0)/1024) free,
((kbytes_alloc-nvl(kbytes_free,0))/kbytes_alloc)*100 "%used",
nvl(largest,0)/1024  largest
from ( select sum(bytes)/1024 Kbytes_free,
              max(bytes)/1024 largest,              tablespace_name
       from  sys.dba_free_space       group by tablespace_name ) a,
     ( select sum(bytes)/1024 Kbytes_alloc,              tablespace_name
       from sys.dba_data_files       group by tablespace_name )b
where a.tablespace_name (+) = b.tablespace_name
order by 1
/

26. Database is hung. Old and new user connections alike hang on impact. What do you do? Your SYS SQLPLUS session is able to connect. 

Ans: Log into the system and find whether there are any deadlocks in the system using the following query.

select    'SID ' || l1.sid ||' is blocking  ' || l2.sid blocking
from    v$lock l1, v$lock l2
where    l1.block =1 and l2.request > 0
and    l1.id1=l2.id1
and    l1.id2=l2.id2
/

If so kill the processes caught in deadlock

alter system kill session 'SID,SERIAL#' immediate;

Also find out which wait events exist in the system using following commands and go in detail as to what events are causing these waits and take appropriate actions.

select event,count(*) from v$session group by event

/

select u.sid,u.serial#, u.username,p.spid,to_char(u.logon_time,'DD-MON-YYYY:HH24:MI:SS')  from v$session u, v$session w,v$process p where u.sid = w.sid and w.event like '%&a%' and u.paddr = p.addr

/

27. Database crashes. Corruption is found scattered among the file system neither of your doing nor of Oracle’s. What database recovery options are available? Database is in archive log mode.
Ans: First of all secure all the archives and all the backups you have on the tape or other system. Then run fschk to check the filesystem. If the corruption is detected at the filesystem level and is not recoverable by fschk format the file system and restore the database through  RMAN.

28. Illustrate how to determine the amount of physical CPUs a Unix Box possesses (LINUX and/or Solaris).
Ans:

29. How do you increase the OS limitation for open files (LINUX and/or Solaris)?
Ans:  Set the file-max parameter is /etc/sysctl.conf to the number you want.Save the file and execute it by using command /etc/sysctl.conf-p

30. Provide an example of a shell script which logs into SQLPLUS as SYS, determines the current date, changes the date format to include minutes & seconds, issues a drop table command, displays the date again, and finally exits.

Ans: 

export ORACLE_BASE=/oracle

export ORACLE_HOME=/oracle/ora10g

export ORACLE_SID=ora10g

export path=$ORACLE_HOME/lib

sqlplus sys as sysdba << EOF

@/oracle/date.sql

exit;

Now the contents of /oracle/date.sql

select SYSDATE from dual;

select to_char(SYSDATE,'dd-mon-yyyy hh24:mi:ss') from dual;
drop table tablename cascade constraints;
select to_char(SYSDATE,'dd-mon-yyyy hh24:mi:ss') from dual;

/

       

31. Explain how you would restore a database using RMAN to Point in Time?

Ans:  

restore database

until time "to_date('Aug 27 2001 02:00:00','Mon DD YYYY HH24:MI:SS')";
recover database

32. How does Oracle guarantee data integrity of data changes?

Ans: Oracle enables you to define and enforce data integrity constraints like PRIMARY KEY CONSTRAINTS, FOREIGN KEY CONSTRAINTS and UNIQUE CONSTRAINTS.

33. Which environment variables are absolutely critical in order to run the OUI?
Ans: ORACLE_BASE, ORACLE_HOME, ORACLE_SID,path and library path

34. What SQL query from v$session can you run to show how many sessions are logged in as a particular user account?
Ans: select count(1) from v$session where USERNAME='username';

35. Why does Oracle not permit the use of PCTUSED with indexes?
Ans: Imagine you've got an index on people's names. And, just for the sake of argument, only 4 entries can fit in a block.

So you have

Adam
Bob
Charles
David

and

Edward
Frank
Graham
Howard

You want to insert a new entry in the table (and hence into the index on that table) of Zebedee.

As things stand, both blocks of the index are full, so Zebedee has to go into a third block all on his own.

Now say you delete some existing rows:

delete from employees where name in ('Bob', 'Charles', 'David'). You're left in this state:

Block 1:

Adam

Block 2:

Edward
Frank
Graham
Howard

That means block 1 has 3 empty slots for new entries. So you still want to insert a record for someone called Zebedee: can it go into the first block, with all that empty space? No. Because if it did, you'd end up with an index that ran:

Adam
Zebedee
Edward
Frank
Graham
Howard

...and you might notice that the alphabetical order has been screwed to buggery at this point. So no, although the first block contains lots of empty space, it's still defined as an "A-ish" sort of block. It actually has to have entries between Adam and Edward, and it can't suddenly decide to accept entries from Wilma, Susan or Mary.

So when can block 1 be used to house those sorts of entries? When Adam gets deleted. Because when that row goes, you're in this position:

Block 1: completely empty
Block 2: values from E to G

Being completely empty, there's no "A or B'ness' about block 1 any more. So suddenly, it can accept any entry at all. Now, physically, block 1 might come "before" block 2... but we jig things around in the root and branch blocks so that doesn't matter. Therefore, you can now insert all those other records you wanted and get this situation:

Block 1:
Mary
Susan
Wilma
Zebedee

Block 2:
Edward
Frank
Graham
Howard

And we have a branch block (actually, in this case, the root node) which says:

A -> L: goto block 2
M -> Z: goto block 1

So, once we deleetd Adam, we were allowed to insert new items into all that empty space. But of course, once we deleted Adam, the first block was completely and utterly empty. And what does a completely empty block equate to in PCTUSED terms? Er, 0%.

It has to be 0% PCTUSED, in other words, because entries in an index have to be organised according to some order. Allowing you to reuse space 'out of order' would destroy the point of an index. And you can only say, "Order is irrelevant at this point" when no entries exist at all... which equates to PCTUSED of 0.

PCTFREE is fine to set, because that simply says when to stop filling one block and start filling another. In indexes, it reserves space that might come in handy for preventing block splits. So there's a use for PCTFREE, definitely. Imagine we'd set PCTFREE of 25%. Then the earlier index would be:

Block 1:

Adam
Bob
Charles

Block 2:
David
Edward
Frank

Block 3:
Graham
Howard

With a PCTFREE of 25% and only 4 entries allowed per block, we end up only storing 3 entries per block: the missing entry being reserved as 'free space'. Now insert a guy called 'Brian': is there room to fit him between Bob and Charles? In this new index, yes: block 1 is allowed to grow to holding 4 records, and there's only currently 3, so yup: there's room to accomodate the new guy.

But in the earlier example? No, the block was allowed to hold four records. It's holding four records. You want to insert a new one that HAS to go between two existing records... we have to perform a block split and reorganisation to allow that to happen. Block splits are costly affairs. PCTFREE would have saved us from having to do one.

So yes, PCTFREE in an index is useful. PCTUSED is meaningless. Once you impose an order on rows, they cannot just go anywhere. Space reserved for future inserts is fine. But a flag to say 'insert away!' when inserting would disrupt a carefully-achieved ordering is a complete no-no.