. 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 point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. 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 were 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 few rollback data. The delete command, on the other hand, is a DML operation, which will produce rollback data 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 some examples of the types of database contraints you may find in Oracle and indicate their purpose.
A Primary or Unique Key can be used to enforce uniqueness on one or more columns.
A Referential Integrity Contraint can be used to enforce a Foreign Key relationship between two tables.
A Not Null constraint - to ensure a value is entered in a column
A Value Constraint - to check a column value against a specific set of values.
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 point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point 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 $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.
Oracle Interview Questions
1. How would you determine the time zone under which a database was operating?
SELECT dbtimezone FROM DUAL;
2. Explain the use of setting GLOBAL_NAMES equal to TRUE.
It ensure the use of consistent naming conventions for databases and links in a networked environment.
3. What command would you use to encrypt a PL/SQL application?
WRAP
4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.
They are all named PL/SQL blocks.
Function must return a value. Can be called inside a query.
Procedure may or may not return value.
Package is the collection of functions, procedures, variables which can be logically grouped together.
5. Explain the use of table functions.
6. Name three advisory statistics you can collect.
7. Where in the Oracle directory tree structure are audit traces placed?
8. Explain materialized views and how they are used.
9. When a user process fails, what background process cleans up after it?
PMON
10. What background process refreshes materialized views?
Job Queue Process (CJQ)
11. How would you determine what sessions are connected and what resources they are waiting for?
v$session,v$session_wait
12. Describe what redo logs are.
13. How would you force a log switch?
alter system switch logfile;
14. Give two methods you could use to determine what DDL changes have been made.
15. What does coalescing a tablespace do?
Coalesce simply takes contigous free extents and makes them into a single bigger free extent.
16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?
TEMP tablespace gets cleared once the transaction is done where as PERMANENT tablespace retails the data.
17. Name a tablespace automatically created when you create a database.
SYSTEM
18. When creating a user, what permissions must you grant to allow them to connect to the database?
Grant create session to username;
19. How do you add a data file to a tablespace?
alter tablespace USERS add datafile '/ora01/oradata/users02.dbf' size 50M;
20. How do you resize a data file?
alter database datafile '/ora01/oradata/users02.dbf' resize 100M;
21. What view would you use to look at the size of a data file?
dba_data_files
22. What view would you use to determine free space in a tablespace?
dba_free_space
23. How would you determine who has added a row to a table?
By implementing an INSERT trigger for logging details during each INSERT operation on the table
24. How can you rebuild an index?
ALTER INDEX index_name REBUILD;
25. Explain what partitioning is and what its benefit is.
A table partition is also a table segment, and by using partitioning technique we can enhance performance of table access.
26. You have just compiled a PL/SQL package but got errors, how would you view the errors?
show errors
27. How can you gather statistics on a table?
exec dbms_stats.gather_table_stats
Also, remember to analyze all associated indexes on that table using dbms_stats.gather_index_stats
28. How can you enable a trace for a session?
alter session set sql_trace='TRUE';
29. What is the difference between the SQL*Loader and IMPORT utilities?
SQL*LOADER loads external data which is in OS files to oracle database tables while IMPORT utility imports data only which is exported by EXPORT utility of oracle database.
30. Name two files used for network connection to a database.
TNSNAMES.ORA and SQLNET.ORA
Oracle Interview Questions
1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn't have to.
2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
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 point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. 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 were 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 few rollback data. The delete command, on the other hand, is a DML operation, which will produce rollback data 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 some examples of the types of database contraints you may find in Oracle and indicate their purpose.
A Primary or Unique Key can be used to enforce uniqueness on one or more columns.
A Referential Integrity Contraint can be used to enforce a Foreign Key relationship between two tables.
A Not Null constraint - to ensure a value is entered in a column
A Value Constraint - to check a column value against a specific set of values.
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 point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point 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 $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.
Oracle Interview Questions
1. How would you determine the time zone under which a database was operating?
SELECT dbtimezone FROM DUAL;
2. Explain the use of setting GLOBAL_NAMES equal to TRUE.
It ensure the use of consistent naming conventions for databases and links in a networked environment.
3. What command would you use to encrypt a PL/SQL application?
WRAP
4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.
They are all named PL/SQL blocks.
Function must return a value. Can be called inside a query.
Procedure may or may not return value.
Package is the collection of functions, procedures, variables which can be logically grouped together.
5. Explain the use of table functions.
6. Name three advisory statistics you can collect.
7. Where in the Oracle directory tree structure are audit traces placed?
8. Explain materialized views and how they are used.
9. When a user process fails, what background process cleans up after it?
PMON
10. What background process refreshes materialized views?
Job Queue Process (CJQ)
11. How would you determine what sessions are connected and what resources they are waiting for?
v$session,v$session_wait
12. Describe what redo logs are.
13. How would you force a log switch?
alter system switch logfile;
14. Give two methods you could use to determine what DDL changes have been made.
15. What does coalescing a tablespace do?
Coalesce simply takes contigous free extents and makes them into a single bigger free extent.
16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?
TEMP tablespace gets cleared once the transaction is done where as PERMANENT tablespace retails the data.
17. Name a tablespace automatically created when you create a database.
SYSTEM
18. When creating a user, what permissions must you grant to allow them to connect to the database?
Grant create session to username;
19. How do you add a data file to a tablespace?
alter tablespace USERS add datafile '/ora01/oradata/users02.dbf' size 50M;
20. How do you resize a data file?
alter database datafile '/ora01/oradata/users02.dbf' resize 100M;
21. What view would you use to look at the size of a data file?
dba_data_files
22. What view would you use to determine free space in a tablespace?
dba_free_space
23. How would you determine who has added a row to a table?
By implementing an INSERT trigger for logging details during each INSERT operation on the table
24. How can you rebuild an index?
ALTER INDEX index_name REBUILD;
25. Explain what partitioning is and what its benefit is.
A table partition is also a table segment, and by using partitioning technique we can enhance performance of table access.
26. You have just compiled a PL/SQL package but got errors, how would you view the errors?
show errors
27. How can you gather statistics on a table?
exec dbms_stats.gather_table_stats
Also, remember to analyze all associated indexes on that table using dbms_stats.gather_index_stats
28. How can you enable a trace for a session?
alter session set sql_trace='TRUE';
29. What is the difference between the SQL*Loader and IMPORT utilities?
SQL*LOADER loads external data which is in OS files to oracle database tables while IMPORT utility imports data only which is exported by EXPORT utility of oracle database.
30. Name two files used for network connection to a database.
TNSNAMES.ORA and SQLNET.ORA
Oracle Interview Questions
1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn't have to.
2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
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