Tag Archives: Oracle Interview Questions

Oralce Interview Questions – Year 2009 Part – II

1. Give one method for transferring a table from one schema to another: There are several possible methods, export-import, CREATE TABLE… AS SELECT, or COPY

 

2. What is the purpose of the IMPORT option IGNORE? What is it?s default setting The IMPORT IGNORE option tells import to ignore “already exists” errors. If it is not specified the tables that already exist will be skipped. If it is specified, the error is ignored and the tables data will be inserted. The default value is N.

 

3. You have a rollback segment in a version 7.2 database that has expanded beyond optimal, how can it be restored to optimal

Use the ALTER TABLESPACE ….. SHRINK command.

 

4. If the DEFAULT and TEMPORARY tablespace clauses are left out of a CREATE USER command what happens? Is this bad or good? Why

       The user is assigned the SYSTEM tablespace as a default and temporary tablespace. This is bad because it causes user objects and temporary segments to be placed into the SYSTEM tablespace resulting in fragmentation and improper table placement (only data dictionary objects and the system rollback segment should be in SYSTEM).

 

 

5. What are some of the Oracle provided packages that DBAs should be aware of

Oracle provides a number of packages in the form of the DBMS_ packages owned by the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra credit but aren?t part of the answer

 

6. What happens if the constraint name is left out of a constraint clause?

The Oracle system will use the default name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it makes tracking which table the constraint belongs to or what the constraint does harder.

 

7. What happens if a tablespace clause is left off of a primary key constraint clause?

This result in the index that is automatically generated being placed in then users default tablespace. Since this will usually be the same tablespace as the table is being created in, this can cause serious performance problems.

 

8. What is the proper method for disabling and re-enabling a primary key constraint?

You use the ALTER TABLE command for both. However, for the enable clause you must specify the USING INDEX and TABLESPACE clause for primary keys.

 

9. What happens if a primary key constraint is disabled and then enabled without fully specifying the index clause

The index is created in the user’s default tablespace and all sizing information is lost. Oracle doesn’t store this information as a part of the constraint definition, but only as part of the index definition, when the constraint was disabled the index was dropped and the information is gone.

 

10.(On UNIX) When should more than one DB writer process be used? How many should be used

If the UNIX system being used is capable of asynchronous IO then only one is required, if the system is not capable of asynchronous IO then up to twice the number of disks used by Oracle number of DB writers should be specified by use of the db_writers initialization parameter.

 

11. You are using hot backup without being in archivelog mode, can you recover in the event of a failure? Why or why not

You can’t use hot backup without being in archivelog mode. So no, you couldn?t recover.

 

12. What causes the “snapshot too old” error? How can this be prevented or mitigated

This is caused by large or long running transactions that have either wrapped onto their own rollback space or have had another transaction write on part of their rollback space. This can be prevented or mitigated by breaking the transaction into a set of smaller transactions or increasing the size of the rollback segments and their extents.

 

13. How can you tell if a database object is invalid?

By checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending upon whether you own or only have permission on the view or are using a DBA account.

 

14. 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 balls to the object (create synonym emp for scott.emp;)

 

15. 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 views.

 

16. 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.

 

17. How can you find out how many users are currently logged into the database? How can you find their operating system id

      There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another 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.

 

      18. A user selects from a sequence and gets back two values, his select is:

SELECT pk_seq.nextval FROM dual;What is the problem

Somehow two values have been inserted into the dual table. This table is a single row, single column table that should only have one value in it.

 

19. 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 are nearing 0.3.

 

Oracle Interview Questions – Year 2009 – Part I

1. Explain the difference between a hot backup and a cold backup and the benefits associated with each.
A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any ball in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery

In addition, since you are taking cold backups the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk.

2. You have just had to restore from backup and do not have any control files. How would you go about bringing up this database?

I would create a text based backup control file, stipulating where on disk all the data files where and then issue the recover command with the using backup control file clause.

3. How do you switch from an init.ora file to a spfile?
Issue the create spfile from pfile command.

4. Explain the difference between a data block, an extent and a segment?
A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.

5. Give two examples of how you might determine the structure of the table DEPT?
Use the describe command or use the dbms_metadata.get_ddl package.

6. Where would you look for errors from the database engine?
In the alert log

7. Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete.

8. Give the reasoning behind using an index.
Faster access to data blocks in a table.

9. Give the two types of tables involved in producing a star schema and the type of data they hold.
Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.

10. What type of index should you use on a fact table?
A Bitmap index

11. Give two examples of referential integrity constraints.
A primary key and a foreign key are the example of refe

12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables?
Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.

13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.
ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any ball in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any ball in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.

14. What command would you use to create a backup control file?
Alter database backup control file to trace.

15. Give the stages of instance startup to a usable state where normal users may access it.
STARTUP NOMOUNT – Instance startup
STARTUP MOUNT – The database is mounted
STARTUP OPEN – The database is opened

16. What column differentiates the V$ views to the GV$ views and how?
The INST_ID column which indicates the instance in a RAC environment the information came from.

17. How would you go about generating an EXPLAIN plan?
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = ‘tst1’ into plan_table for a SQL statement
Look at the explain plan with utlxplp.sql or utlxpls.sql

18. How would you go about increasing the buffer cache hit ratio?
Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.

19. Explain an ORA-01555
you get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.

20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.

Oracle Databse Questions and Answers

  1. What is Log Switch? – The point at which ORACLE ends writing to one online redo log file and begins writing to another is called a log switch.
  2. What is On-line Redo Log? – The On-line Redo Log is a set of tow or more on-line redo files that record all committed changes made to the database. Whenever a transaction is committed, the corresponding redo entries temporarily stores in redo log buffers of the SGA are written to an on-line redo log file by the background process LGWR. The on-line redo log files are used in cyclical fashion.
  3. Which parameter specified in the DEFAULT STORAGE clause of CREATE TABLESPACE cannot be altered after creating the tablespace? – All the default storage parameters defined for the tablespace can be changed using the ALTER TABLESPACE command. When objects are created their INITIAL and MINEXTENS values cannot be changed.
  4. What are the steps involved in Database Startup? – Start an instance, Mount the Database and Open the Database.
  5. What are the steps involved in Instance Recovery? – Rolling forward to recover data that has not been recorded in data files, yet has been recorded in the on-line redo log, including the contents of rollback segments. Rolling back transactions that have been explicitly rolled back or have not been committed as indicated by the rollback segments regenerated in step a. Releasing any resources (locks) held by transactions in process at the time of the failure. Resolving any pending distributed transactions undergoing a two-phase commit at the time of the instance failure.
  6. Can Full Backup be performed when the database is open? – No.
  7. What are the different modes of mounting a Database with the Parallel Server? – Exclusive Mode If the first instance that mounts a database does so in exclusive mode, only that Instance can mount the database. Parallel Mode If the first instance that mounts a database is started in parallel mode, other instances that are started in parallel mode can also mount the database.
  8. What are the advantages of operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG mode? – Complete database recovery from disk failure is possible only in ARCHIVELOG mode. Online database backup is possible only in ARCHIVELOG mode.
  9. What are the steps involved in Database Shutdown? – Close the Database, Dismount the Database and Shutdown the Instance.
  10. What is Archived Redo Log? – Archived Redo Log consists of Redo Log files that have archived before being reused.
  11. What is Restricted Mode of Instance Startup? – An instance can be started in (or later altered to be in) restricted mode so that when the database is open connections are limited only to those whose user accounts have been granted the RESTRICTED SESSION system privilege.
  12. 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.
  13. 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.
  14. 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.
  15. Can a View based on another View? – Yes.
  16. Can a Tablespace hold objects from different Schemes? – Yes.
  17. Can objects of the same Schema reside in different tablespaces? – Yes.
  18. 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.
  19. Do View contain Data? – Views do not contain or store data.
  20. 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.
  21. What are the type of Synonyms? – There are two types of Synonyms Private and Public
  22. 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.
  23. What is an Index Segment? – Each Index has an Index segment that stores all of its data.
  24. 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
  25. What are the different type of Segments? – Data Segment, Index Segment, Rollback Segment and Temporary Segment.
  26. What are Clusters? – Clusters are groups of one or more tables physically stores together to share common columns and are often used together.
  27. What is an Integrity Constrains? – An integrity constraint is a declarative way to define a business rule for a column of a table.
  28. 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.
  29. 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.
  30. 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.)
  31. 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.
  32. What is a Synonym? – A synonym is an alias for a table, view, sequence or program unit.
  33. What is a Sequence? – A sequence generates a serial list of unique numbers for numerical columns of a database’s tables.
  34. What is a Segment? – A segment is a set of extents allocated for a certain logical structure.
  35. What is schema? – A schema is collection of database objects of a User.
  36. Describe Referential Integrity? – A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data.
  37. What is Hash Cluster? – A row is stored in a hash cluster based on the result of applying a hash function to the row’s cluster key value. All rows with the same hash key value are stores together on disk.
  38. What is a Private Synonyms? – A Private Synonyms can be accessed only by the owner.
  39. What is Database Link? – A database link is a named object that describes a “path” from one database to another.
  40. What is a Tablespace? – A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together
  41. What is Rollback Segment? – A Database contains one or more Rollback Segments to temporarily store “undo” information.
  42. 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.
  43. 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 datablocks. Block size is specified in INIT.ORA file and can’t be changed latter.
  44. 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.
  45. 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. 47.What is Index Cluster? – A Cluster with an index on the Cluster Key 48.When does a Transaction end? – When it is committed or Rollbacked.
  46. 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.
  47. 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.
  48. What is the function of Optimizer? – The goal of the optimizer is to choose the most efficient way to execute a SQL statement.
  49. What is Execution Plan? – The combinations of the steps the optimizer chooses to execute a statement is called an execution plan.
  50. What are the different approaches used by Optimizer in choosing an execution plan? – Rule-based and Cost-based.