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Wednesday, 25 April 2012

DBA Interview Questions with Answers Part2

A user is getting an ORA-00942 error yet you know you have granted them permission on the table, what else should you check?
You need to check that the user has specified the full name of the object (SELECT empid FROM scott.emp; instead of SELECT empid FROM emp;) or has a synonym that points to that object (CREATE SYNONYM emp FOR scott.emp;)
A developer is trying to create a view and the database won’t let him. He has the "DEVELOPER" role which has the "CREATE VIEW" system privilege and SELECT grants on the tables he is using, what is the problem?
You need to verify the developer has direct grants on all tables used in the view. You can't create a stored object with grants given through a role.
If you have an example table, what is the best way to get sizing data for the production table implementation?
The best way is to analyze the table and then use the data provided in the DBA_TABLES view to get the average row length and other pertinent data for the calculation. The quick and dirty way is to look at the number of blocks the table is actually using and ratio the number of rows in the table to its number of blocks against the number of expected rows.
How can you find out how many users are currently logged into the database? How can you find their operating system id?
To look at the v$session or v$process views and check the current_logins parameter in the v$sysstat view. If you are on UNIX is to do a ps -ef|grep oracle|wc -l? Command, but this only works against a single instance installation.
How can you determine if an index needs to be dropped and rebuilt?
Run the ANALYZE INDEX command on the index to validate its structure and then calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn’t near 1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt or if the ratio BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3. It is not so easy to decide so I personally suggest contact to the expert before going to rebuild.
What is tkprof and how is it used?
The tkprof tool is a tuning tool used to determine CPU and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
What is Explain plan and how is it used?
The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.
How do you prevent output from coming to the screen?
The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.
How do you prevent Oracle from giving you informational messages during and after a SQL statement execution?
The SET options FEEDBACK and VERIFY can be set to OFF.
How do you generate file output from SQL?
By use of the SPOOL command
A tablespace has a table with 30 extents in it. Is this bad? Why or why not.
Multiple extents in and of themselves aren’t bad. However if you also have chained rows this can hurt performance.
How do you set up tablespaces during an Oracle installation?
You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.
You see multiple fragments in the SYSTEM tablespace, what should you check first?
Ensure that users don’t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.
What are some indications that you need to increase the SHARED_POOL_SIZE parameter?
Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.
Guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?
Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.
When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort parameter is the SORT_AREA_SIZE parameter.
When should you increase copy latches? What parameters control copy latches?
When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.
Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed?
You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.
Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning?
The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
Discuss row chaining, how does it happen? How can you reduce it? How do you correct it?
Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and would not fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
You are getting busy buffer waits. Is this bad? How can you find what is causing it?
Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the "count" column tells where the problem is, the "class" column tells you with what. UNDO is rollback segments, DATA is data base buffers.
If you see contention for library caches how you can fix it?
Increase the size of the shared pool.
If you see statistics that deal with "undo" what are they really talking about?
Rollback segments and associated structures.
If a tablespace has a default pctincrease of zero what will this cause (in relationship to the SMON process)?
The SMON process would not automatically coalesce its free space fragments.
If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)
In Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace name coalesce level ts#'; command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the alter tablespace coalesce; is best. If the free space is not contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.
How can you tell if a tablespace has excessive fragmentation?
If a select against the dba_free_space table shows that the count of tablespaces extents is greater than the count of its data files, then it is fragmented.
You see the following on a status report: redo log space requests 23 redo log space wait time 0 Is this something to worry about? What if redo log space wait time is high? How can you fix this?
Since the wait time is zero, no problem. If the wait time was high it might indicate a need for more or larger redo logs.
If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it?
This indicates that the shared pool may be too small. Increase the shared pool size.
If you see the value for reloads is high in the estat library cache report is this a matter for concern?
Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.
You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem?
A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the size of the extents and adjust optimal accordingly.
You look at the dba_rollback_segs view and see that you have a large number of wraps is this a problem?
A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average transaction size in the same view to get the information on transaction size.
You see multiple extents in the Temporary Tablespace. Is this a problem?
As long as they are all the same size this is not a problem. In fact, it can even improve performance since Oracle would not have to create a new extent when a user needs one.
How do you set up your Tablespace on installation Level: Low?
The answer here should show an understanding of separation of redo and rollback, data and indexes and isolation of SYSTEM tables from other tables. An example would be to specify that at least 7 disks should be used for an Oracle installation.
Disk Configuration:
SYSTEM tablespace on 1, Redo logs on 2 (mirrored redo logs), TEMPORARY tablespace on 3, ROLLBACK tablespace on 4, DATA and INDEXES 5,6
They should indicate how they will handle archive logs and exports as well as long as they have a logical plan for combining or further separation more or less disks can be specified.
You have installed Oracle and you are now setting up the actual instance. You have been waiting an hour for the initialization script to finish, what should you check first to determine if there is a problem?
Check to make sure that the archiver is not stuck. If archive logging is turned on during install a large number of logs will be created. This can fill up your archive log destination causing Oracle to stop to wait for more space.
When configuring SQLNET on the server what files must be set up?
When configuring SQLNET on the client what files need to be set up?
You have just started a new instance with a large SGA on a busy existing server. Performance is terrible, what should you check for?
The first thing to check with a large SGA is that it is not being swapped out.
What OS user should be used for the first part of an Oracle installation (on UNIX)?
You must use root first.
When should the default values for Oracle initialization parameters be used as is?
How many control files should you have? Where should they be located?
At least 2 on separate disk spindles (Mirrored by Oracle).
How many redo logs should you have and how should they be configured for maximum recoverability?
You should have at least 3 groups of two redo logs with the two logs each on a separate disk spindle (mirrored by Oracle). The redo logs should not be on raw devices on UNIX if it can be avoided.
Why are recursive relationships bad? How do you resolve them?
A recursive relationship defines when or where a table relates to itself.  It is considered as bad when it is a hard relationship (i.e. neither side is a "may" both are "must") as this can result in it not being possible to put in a top or perhaps a bottom of the table. For example in the EMPLOYEE table you could not put in the PRESIDENT of the company because he has no boss, or the junior janitor because he has no subordinates. These type of relationships are usually resolved by adding a small intersection entity.
What does a hard one-to-one relationship mean (one where the relationship on both ends is "must")?
This means the two entities should probably be made into one entity.
How should a many-to-many relationship be handled?
By adding an intersection entity table
What is an artificial (derived) primary key? When should an artificial (or derived) primary key be used?
A derived key comes from a sequence. Usually it is used when a concatenated key becomes too cumbersome to use as a foreign key.
When should you consider de-normalization?
Whenever performance analysis indicates it would be beneficial to do so without compromising data integrity.
How can you determine the space left in a file system?
There are several commands to do this: du, df, or bdf
How can you determine the number of SQLNET users logged in to the UNIX system?
SQLNET users will show up with a process unique name that begins with oracle, if you do a ps -ef|grep oracle|wc -l you can get a count of the number of users.
What command is used to type files to the screen?
cat, more, pg
Can you remove an open file under UNIX?
What is the purpose of the grep command?
grep is a string search command that parses the specified string from the specified file or files
The system has a program that always includes the word nocomp in its name, how can you determine the number of processes that are using this program?
ps -ef|grep *nocomp*|wc -l
The system administrator tells you that the system has not been rebooted in 6 months, should he be proud of this?
Most UNIX systems should have a scheduled periodic reboot so file systems can be checked and cleaned and dead or zombie processes cleared out. May be, Some UNIX systems do not clean up well after themselves. Inode problems and dead user processes can accumulate causing possible performance and corruption problems.
How can you find dead processes?
ps -ef|grep zombie -- or -- who -d depending on the system.
How can you find all the processes on your system?
Use the ps command
How can you find your id on a system?
Use the "who am i" command.
What is the finger command?
The finger command uses data in the passwd file to give information on system users.
What is the easiest method to create a file on UNIX?
Use the touch command
What does >> do?
The ">>" redirection symbol appends the output from the command specified into the file specified. The file must already have been created.
If you are not sure what command does a particular UNIX function what is the best way to determine the command?
The UNIX man -k command will search the man pages for the value specified. Review the results from the command to find the command of interest.
How can you determine if an Oracle instance is up from the operating system level?
There are several base Oracle processes that will be running on multi-user operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has them using their operating system process showing feature to check for these is acceptable. For example, on UNIX ps -ef|grep pmon will show what instances are up.
Users from the PC clients are getting messages indicating : ORA-06114: NETTCP: SID lookup failure. What could the problem be?
The instance name is probably incorrect in their connection string.
Users from the PC clients are getting the following error stack:
ERROR: ORA-01034: ORACLE not available ORA-07318: smsget: open error when opening sgadef.dbf file. HP-UX Error: 2: No such file or directory What is the probable cause?
The Oracle instance is shutdown that they are trying to access, restart the instance.
How can you determine if the SQLNET process is running for SQLNET V1? How about V2?
For SQLNET V1 check for the existence of the orasrv process. You can use the command "tcpctl status" to get a full status of the V1 TCPIP server, other protocols have similar command formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you can issue the command "lsnrctl status".
What file will give you Oracle instance status information? Where is it located?
The alert.ora log. It is located in the directory specified by the background_dump_dest parameter in the v$parameter table.
Users are not being allowed on the system. The following message is received: ORA-00257 archiver is stuck. Connect internal only, until freed. What is the problem?
The archive destination is probably full, backup the archivelogs and remove them and the archiver will re-start.
Where would you look to find out if a redo log was corrupted assuming you are using Oracle mirrored redo logs?
There is no message that comes to the SQLDBA or SRVMGR programs during startup in this situation, you must check the alert. log file for this information.
You attempt to add a datafile and get: ORA-01118: cannot add anymore datafiles: limit of 40 exceeded. What is the problem and how can you fix it?
When the database was created the db_files parameter in the initialization file was set to 40. You can shutdown and reset this to a higher value, up to the value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES is set to low, you will have to rebuild the control file to increase it before proceeding.

You look at your fragmentation report and see that smon has not coalesced any of you tablespaces, even though you know several have large chunks of contiguous free extents. What is the problem?
Check the dba_tablespaces view for the value of pct_increase for the tablespaces. If pct_increase is zero, smon will not coalesce their free space.
Your users get the following error: ORA-00055 maximum number of DML locks exceeded? What is the problem and how do you fix it?
The number of DML Locks is set by the initialization parameter DML_LOCKS. If this value is set to low (which it is by default) you will get this error. Increase the value of DML_LOCKS. If you are sure that this is just a temporary problem, you can have them wait and then try again later and the error should clear.
You get a call from you backup DBA while you are on vacation. He has corrupted all of the control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do?
As long as all datafiles are safe and he was successful with the BACKUP controlfile command you can do the following:
CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before next step
ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back online)
Shutdown and backup the system, then restart If they have a recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they can use that to recover as well.
If no backup of the control file is available then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE CONTROL FILE .....; However, they will need to know all of the datafiles, logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.
You have taken a manual backup of a datafile using OS. How RMAN will know about it?
Whenever we take any backup through RMAN in the repository information of the backup is recorded. The RMAN repository can be either controlfile or recovery catalog. However if you take a backup through OS command then RMAN does not aware of that and hence recorded are not reflected in the repository. This is also true whenever we create a new controlfile or a backup taken by RMAN is transferred to another place using OS command then controlfile/recovery catalog does not know about the prior backups of the database.
So in order to restore database with a new created controlfile we need to inform RMAN about the backups taken before so that it can pick one to restore.
This task can be done by catalog command in RMAN.
§         Add information of backup pieces and image copies in the repository that are on disk.
§         Record a datafile copy as a level 0 incremental backup in the RMAN repository.
§         Record of a datafile copy that was taken by OS.
But CATALOG command has some restrictions. It can't do the following.
§         Can't catalog a file that belong to different database.
§         Can't catalog a backup piece that exists on an sbt device.
Example: Catalog Archive log
RMAN>CATALOG ARCHIVELOG '/oracle/oradata/arju/arc001_223.arc' '/oracle/oradata/arju/arc001_224.arc';
Catalog Datafile
To catalog datafile copy '/oradata/backup/users01.dbf' as an incremental level 0 backup your command will be
RMAN>CATALOG DATAFILE COPY '/oradata/backup/users01.dbf' LEVEL 0;
Note that this datafile copy was taken backup either using the RMAN BACKUP AS COPY command or by using operating system utilities in conjunction with ALTER TABLESPACE BEGIN/END BACKUP.
Catalog multiple copies in a directory:
Catalog files in the flash recovery area:
To catalog all files in the currently enabled flash recovery area without prompting the user for each one issue
Catalog backup pieces:
RMAN>CATALOG BACKUPPIECE '/oradata2/o4jccf4';
How to Uncatalog Backup?
In many cases you need to uncatalog command. Suppose you do not want a specific backup or copy to be eligible to be restored but also do not want to delete it.
To uncatalog all archived logs issue:
To uncataog tablespace USERS issue:
To uncatalog a backuppiece name /oradata2/oft7qq issue:
How would you find total size of database in OS level
The size of the database is the total size of the datafiles that make up the tablespaces of the database. These details are found in the dba_extents view.
select sum(bytes)/(1024*1024) from   V$datafile;
select sum(bytes)/(1024*1024) from dba_data_files;
select sum(bytes)/(1024*1024) from   dba_extents; Can we take incremental Backup with out taking complete Backup?
No, First full backup is needed


  1. How can you tell if a tablespace has excessive fragmentation?
    If a select against the dba_free_space table shows that the count of tablespaces extents is greater than the count of its data files, then it is fragmented.

    Can you explain this answer ,i am unable to understand

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