Oracle DBA Interview Questions/FAQs Part1
1. What is an instance?
SGA + background processes.
2. What is SGA?
System/Shared Global Area.
3. What is PGA (or) what is pga_aggregate_target?
Programmable Global Area.
4. What are new memory parameters in Oracle 10g?
SGA_TARGET
PGA_TARGET
5. What are new memory parameters in Oracle 11g?
MEMORY_TARGET
6. What are the mandatory background processes?
DBWR LGWR SMON PMON CKPT RECO.
7. What are the optional background processes?
ARCH, MMAN, MMNL, MMON, CTWR, ASMB, RBAL, ARBx etc.
8. What are the new background processes in Oracle 10g?
MMAN MMON MMNL CTWR ASMB RBAL ARBx
10. What are the new features in Oracle 10g?
11. What are the new features in Oracle 11g?
12. What are the new features in Oracle 11g R2?
13. What are the new features in Oracle 12c?
14. What process will get data from datafiles to DB cache?
Server process
15. What background process will writes data to datafiles?
DBWR
16. What background process will write undo data?
DBWR
17. What are physical components of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two or more redo log files, and one or more control files. Password file and parameter file also come under physical components.
18. What are logical components of Oracle database?
Blocks, Extents, Segments, Tablespaces.
19. What is segment space management?
LMTS and DMTS.
20. What is extent management?
Auto and Manual.
21. What are the differences between LMTS and DMTS?
Tablespaces that record extent allocation in the dictionary are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header are called locally managed tablespaces.
21. What is a datafile?
Every Oracle database has one or more physical datafiles. Datafiles contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the datafiles allocated for a database.
22. What are the contents of control file?
Database name, SCN, LSN, datafile locations, redolog locations, archive mode, DB Creation Time, RMAN Backup & Recovery Details, Flashback mode.
23. What is the use of redo log files?
24. What are the uses of undo tablespace or redo segments?
25. How undo tablespace can guarantee retain of required undo data?
Alter tablespace undo_ts retention guarantee;
26. What is ORA-01555 - snapshot too old error and how do you avoid it?
27. What is the use/size of temporary tablespace?
28. What is the use of password file?
29. How to create password file?
$ orapwd file=orapwSID password=sys_password force=y nosysdba=y
30. How many types of indexes are there?
Clustered and Non-Clustered
1.B-Tree index
2.Bitmap index
3.Unique index
4.Function based index
Implicit index and explicit index.
Explicit indexes are again of many types like simple index, unique index, Bitmap index, Functional index, Organisational index, cluster index.
31. What is bitmap index & when it’ll be used?
Bitmap indexes are preferred in Data warehousing environment.
Preferred when cardinality is low.
32. What is B-tree index & when it’ll be used?
B-tree indexes are preferred in OLTP environment.
Preferred when cardinality is high.
33. How you will find out fragmentation of index?
AUTO_SPACE_ADVISOR_JOB will run in daily maintenance window and report fragmented indexes/Tables.
analyze index validate structure;
This populates the table ‘index_stats’. It should be noted that this table contains only one row and therefore only one index can be analysed at a time.
An index should be considered for rebuilding under any of the following conditions:
* The percentage of deleted rows exceeds 30% of the total, i.e. if del_lf_rows / lf_rows > 0.3.
* If the ‘HEIGHT’ is greater than 4.
* If the number of rows in the index (‘LF_ROWS’) is significantly smaller than ‘LF_BLKS’ this can indicate a large number of deletes, indicating that the index should be rebuilt.
34. What is the difference between delete and truncate?
Truncate will release the space. Delete won’t.
Delete can be used to delete some records. Truncate can’t.
Delete can be rollbacked.
Delete will generate undo (Delete command will log the data changes in the log file where as the truncate will simply remove the data without it. Hence data removed by Delete command can be rolled back but not the data removed by TRUNCATE).
Truncate is a DDL statement whereas DELETE is a DML statement.
Truncate is faster than delete.
35. What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where unique key creates a nonclustered index by default. Primary key doesn't allow NULLs, but unique key allows one NULL only.
36. What is the difference between schema and user?
Schema is collection of user’s objects.
37. What is the difference between SYSDBA, SYSOPER and SYSASM?
SYSOPER can’t create and drop database.
SYSOPER can’t do incomplete recovery.
SYSOPER can’t change character set.
SYSOPER can’t CREATE DISKGROUP, ADD/DROP/RESIZE DISK
SYSASM can do anything SYSDBA can do.
38. What is the difference between SYS and SYSTEM?
SYSTEM can’t shutdown the database.
SYSTEM can’t create another SYSTEM, but SYS can create another SYS or SYSTEM.
39. How to improve sqlldr (SQL*Loader) performance?
40. What is the difference between view and materialized view?
View is logical, will store only the query, and will always gets latest data.
Mview is physical, will store the data, and may not get latest data.
41. What are materialized view refresh types and which is default?
Complete, fast, force(default)
42. How fast refresh happens?
43. How to find out when was a materialized view refreshed?
Query dba_mviews or dba_mview_analysis or dba_mview_refresh_times
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mviews;
(or)
SQL> select NAME, to_char(LAST_REFRESH,'YYYY-MM-DD HH24:MI:SS') from dba_mview_refresh_times;
(or)
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mview_analysis;
44. What is materialized view log (type)?
45. What is atomic refresh in mviews?
From Oracle 10g, complete refresh of single materialized view can do delete instead of truncate. To force the refresh to do truncate instead of delete, parameter ATOMIC_REFRESH must be set to false.
ATOMIC_REFRESH = FALSE, mview will be truncated and whole data will be inserted. The refresh will go faster, and no undo will be generated.
ATOMIC_REFRESH = TRUE (default), mview will be deleted and whole data will be inserted. Undo will be generated. We will have access at all times even while it is being refreshed.
SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C', atomic_refresh=FALSE);
46. How to find out whether database/tablespace/datafile is in backup mode or not?
Query V$BACKUP view.
47. What is row chaining?
If the row is too large to fit into an empty data block in this case the oracle stores the data for the row in a chain of one or more data blocks. Can occur when the row is inserted.
48. What is row migration?
An update statement increases the amount of data in a row so that the row no longer fits in its data blocks. Now the oracle tries to find another free block with enough space to hold the entire row if such a block is available oracle moves entire row to new block.
49. What are different types of partitions?
With Oracle8, Range partitioning (on single column) was introduced.
With Oracle8i, Hash and Composite(Range-Hash) partitioning was introduced.
With Oracle9i, List partitioning and Composite(Range-List) partitioning was introduced.
With Oracle 11g, Interval partitioning, REFerence partitioning, Virtual column based partitioning, System partitioning and Composite partitioning [Range-Range, List-List, List-Range, List-Hash, Interval-Range, Interval-List, Interval-Interval] was introduced.
50. What is local partitioned index and global partitioned index?
A local index is an index on a partitioned table which is partitioned in the exact same manner as the underlying partitioned table. Each partition of a local index corresponds to one and only one partition of the underlying table.
A global partitioned index is an index on a partitioned or non partitioned tables which are partitioned using a different partitioning key from the table and can have different number of partitions. Global partitioned indexes can only be partitioned using range partitioning.
51. How you will recover if you lost one/all control file(s)?
52. Why more archivelogs are generated, when database is begin backup mode?
During begin backup mode datafile headers get freezed and as result row information cannot be retrieved as a result the entire block is copied to redo logs as a result more redo generated and more log switch and in turn more archive logs. Normally only deltas (change vectors) are logged to the redo logs. When in backup mode, Oracle will write complete changed blocks to the redo log files.
Mainly to overcome fractured blocks. Most of the cases Oracle block size is equal to or a multiple of the operating system block size.
e.g. Consider Oracle blocksize is 2k and OSBlocksize is 4k. so each OS Block is comprised of 2 Oracle Blocks. Now you are doing an update when your db is in backup mode. An Oracle Block is updating and at the same time backup is happening on the OS block which is having this particular DB block. Backup will not be consistent since the one part of the block is being updated and at the same time it is copied to the backup location. In this case we will have a fractured block, so as to avoid this Oracle will copy the whole OS block to redo logfile which can be used for recovery. Because of this redo generation is more.
53. What UNIX parameters you will set while Oracle installation?
shmmax, shmmni, shmall, sem,
54. What is the use of inittrans and maxtrans in table definition?
55. What are differences between dbms_job and dbms_schedular?
Through dbms_schedular we can schedule OS level jobs also.
56. What are differences between dbms_schedular and cron jobs?
Through dbms_schedular we can schedule database jobs, through cron we can’t set.
57. Difference between CPU & PSU patches?
CPU - Critical Patch Update - includes only Security related patches.
PSU - Patch Set Update - includes CPU + other patches deemed important enough to be released prior to a minor (or major) version release.
58. What you will do if (local) inventory corrupted [or] opatch lsinventory is giving error?
59. What are the entries/location of oraInst.loc?
/etc/oraInst.loc is pointer to central/local Oracle Inventory.
60. What is the difference between central/global inventory and local inventory?
61. How you will connect to database when the database is in hung state (Not even sysdba could not connect)
A. Use sqlplus "/ as sysdba" -prelim
oradebug set myospid
oradebug hanganalyze 3
62. What happens when undo datafile is lost when database is running?
Instance Crashes.. need to restore and recover undo datafile from backup. Current going transactions are lost.
63. What happens when temporary datafile is lost.
64. Explain PITR when a whole tablespace is lost.
65. How you'll find whether a table and Index are fragmented?
66. What is SCAN IP and SCAN Listener? What is remote listener?
67. What is the shutdown process of 11g RAC?
1. What is an instance?
SGA + background processes.
2. What is SGA?
System/Shared Global Area.
3. What is PGA (or) what is pga_aggregate_target?
Programmable Global Area.
4. What are new memory parameters in Oracle 10g?
SGA_TARGET
PGA_TARGET
5. What are new memory parameters in Oracle 11g?
MEMORY_TARGET
6. What are the mandatory background processes?
DBWR LGWR SMON PMON CKPT RECO.
7. What are the optional background processes?
ARCH, MMAN, MMNL, MMON, CTWR, ASMB, RBAL, ARBx etc.
8. What are the new background processes in Oracle 10g?
MMAN MMON MMNL CTWR ASMB RBAL ARBx
10. What are the new features in Oracle 10g?
11. What are the new features in Oracle 11g?
12. What are the new features in Oracle 11g R2?
13. What are the new features in Oracle 12c?
14. What process will get data from datafiles to DB cache?
Server process
15. What background process will writes data to datafiles?
DBWR
16. What background process will write undo data?
DBWR
17. What are physical components of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two or more redo log files, and one or more control files. Password file and parameter file also come under physical components.
18. What are logical components of Oracle database?
Blocks, Extents, Segments, Tablespaces.
19. What is segment space management?
LMTS and DMTS.
20. What is extent management?
Auto and Manual.
21. What are the differences between LMTS and DMTS?
Tablespaces that record extent allocation in the dictionary are called dictionary managed tablespaces, and tablespaces that record extent allocation in the tablespace header are called locally managed tablespaces.
21. What is a datafile?
Every Oracle database has one or more physical datafiles. Datafiles contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the datafiles allocated for a database.
22. What are the contents of control file?
Database name, SCN, LSN, datafile locations, redolog locations, archive mode, DB Creation Time, RMAN Backup & Recovery Details, Flashback mode.
23. What is the use of redo log files?
24. What are the uses of undo tablespace or redo segments?
25. How undo tablespace can guarantee retain of required undo data?
Alter tablespace undo_ts retention guarantee;
26. What is ORA-01555 - snapshot too old error and how do you avoid it?
27. What is the use/size of temporary tablespace?
28. What is the use of password file?
29. How to create password file?
$ orapwd file=orapwSID password=sys_password force=y nosysdba=y
30. How many types of indexes are there?
Clustered and Non-Clustered
1.B-Tree index
2.Bitmap index
3.Unique index
4.Function based index
Implicit index and explicit index.
Explicit indexes are again of many types like simple index, unique index, Bitmap index, Functional index, Organisational index, cluster index.
31. What is bitmap index & when it’ll be used?
Bitmap indexes are preferred in Data warehousing environment.
Preferred when cardinality is low.
32. What is B-tree index & when it’ll be used?
B-tree indexes are preferred in OLTP environment.
Preferred when cardinality is high.
33. How you will find out fragmentation of index?
AUTO_SPACE_ADVISOR_JOB will run in daily maintenance window and report fragmented indexes/Tables.
analyze index validate structure;
This populates the table ‘index_stats’. It should be noted that this table contains only one row and therefore only one index can be analysed at a time.
An index should be considered for rebuilding under any of the following conditions:
* The percentage of deleted rows exceeds 30% of the total, i.e. if del_lf_rows / lf_rows > 0.3.
* If the ‘HEIGHT’ is greater than 4.
* If the number of rows in the index (‘LF_ROWS’) is significantly smaller than ‘LF_BLKS’ this can indicate a large number of deletes, indicating that the index should be rebuilt.
34. What is the difference between delete and truncate?
Truncate will release the space. Delete won’t.
Delete can be used to delete some records. Truncate can’t.
Delete can be rollbacked.
Delete will generate undo (Delete command will log the data changes in the log file where as the truncate will simply remove the data without it. Hence data removed by Delete command can be rolled back but not the data removed by TRUNCATE).
Truncate is a DDL statement whereas DELETE is a DML statement.
Truncate is faster than delete.
35. What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where unique key creates a nonclustered index by default. Primary key doesn't allow NULLs, but unique key allows one NULL only.
36. What is the difference between schema and user?
Schema is collection of user’s objects.
37. What is the difference between SYSDBA, SYSOPER and SYSASM?
SYSOPER can’t create and drop database.
SYSOPER can’t do incomplete recovery.
SYSOPER can’t change character set.
SYSOPER can’t CREATE DISKGROUP, ADD/DROP/RESIZE DISK
SYSASM can do anything SYSDBA can do.
38. What is the difference between SYS and SYSTEM?
SYSTEM can’t shutdown the database.
SYSTEM can’t create another SYSTEM, but SYS can create another SYS or SYSTEM.
39. How to improve sqlldr (SQL*Loader) performance?
40. What is the difference between view and materialized view?
View is logical, will store only the query, and will always gets latest data.
Mview is physical, will store the data, and may not get latest data.
41. What are materialized view refresh types and which is default?
Complete, fast, force(default)
42. How fast refresh happens?
43. How to find out when was a materialized view refreshed?
Query dba_mviews or dba_mview_analysis or dba_mview_refresh_times
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mviews;
(or)
SQL> select NAME, to_char(LAST_REFRESH,'YYYY-MM-DD HH24:MI:SS') from dba_mview_refresh_times;
(or)
SQL> select MVIEW_NAME, to_char(LAST_REFRESH_DATE,'YYYY-MM-DD HH24:MI:SS') from dba_mview_analysis;
44. What is materialized view log (type)?
45. What is atomic refresh in mviews?
From Oracle 10g, complete refresh of single materialized view can do delete instead of truncate. To force the refresh to do truncate instead of delete, parameter ATOMIC_REFRESH must be set to false.
ATOMIC_REFRESH = FALSE, mview will be truncated and whole data will be inserted. The refresh will go faster, and no undo will be generated.
ATOMIC_REFRESH = TRUE (default), mview will be deleted and whole data will be inserted. Undo will be generated. We will have access at all times even while it is being refreshed.
SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C', atomic_refresh=FALSE);
46. How to find out whether database/tablespace/datafile is in backup mode or not?
Query V$BACKUP view.
47. What is row chaining?
If the row is too large to fit into an empty data block in this case the oracle stores the data for the row in a chain of one or more data blocks. Can occur when the row is inserted.
48. What is row migration?
An update statement increases the amount of data in a row so that the row no longer fits in its data blocks. Now the oracle tries to find another free block with enough space to hold the entire row if such a block is available oracle moves entire row to new block.
49. What are different types of partitions?
With Oracle8, Range partitioning (on single column) was introduced.
With Oracle8i, Hash and Composite(Range-Hash) partitioning was introduced.
With Oracle9i, List partitioning and Composite(Range-List) partitioning was introduced.
With Oracle 11g, Interval partitioning, REFerence partitioning, Virtual column based partitioning, System partitioning and Composite partitioning [Range-Range, List-List, List-Range, List-Hash, Interval-Range, Interval-List, Interval-Interval] was introduced.
50. What is local partitioned index and global partitioned index?
A local index is an index on a partitioned table which is partitioned in the exact same manner as the underlying partitioned table. Each partition of a local index corresponds to one and only one partition of the underlying table.
A global partitioned index is an index on a partitioned or non partitioned tables which are partitioned using a different partitioning key from the table and can have different number of partitions. Global partitioned indexes can only be partitioned using range partitioning.
51. How you will recover if you lost one/all control file(s)?
52. Why more archivelogs are generated, when database is begin backup mode?
During begin backup mode datafile headers get freezed and as result row information cannot be retrieved as a result the entire block is copied to redo logs as a result more redo generated and more log switch and in turn more archive logs. Normally only deltas (change vectors) are logged to the redo logs. When in backup mode, Oracle will write complete changed blocks to the redo log files.
Mainly to overcome fractured blocks. Most of the cases Oracle block size is equal to or a multiple of the operating system block size.
e.g. Consider Oracle blocksize is 2k and OSBlocksize is 4k. so each OS Block is comprised of 2 Oracle Blocks. Now you are doing an update when your db is in backup mode. An Oracle Block is updating and at the same time backup is happening on the OS block which is having this particular DB block. Backup will not be consistent since the one part of the block is being updated and at the same time it is copied to the backup location. In this case we will have a fractured block, so as to avoid this Oracle will copy the whole OS block to redo logfile which can be used for recovery. Because of this redo generation is more.
53. What UNIX parameters you will set while Oracle installation?
shmmax, shmmni, shmall, sem,
54. What is the use of inittrans and maxtrans in table definition?
55. What are differences between dbms_job and dbms_schedular?
Through dbms_schedular we can schedule OS level jobs also.
56. What are differences between dbms_schedular and cron jobs?
Through dbms_schedular we can schedule database jobs, through cron we can’t set.
57. Difference between CPU & PSU patches?
CPU - Critical Patch Update - includes only Security related patches.
PSU - Patch Set Update - includes CPU + other patches deemed important enough to be released prior to a minor (or major) version release.
58. What you will do if (local) inventory corrupted [or] opatch lsinventory is giving error?
59. What are the entries/location of oraInst.loc?
/etc/oraInst.loc is pointer to central/local Oracle Inventory.
60. What is the difference between central/global inventory and local inventory?
61. How you will connect to database when the database is in hung state (Not even sysdba could not connect)
A. Use sqlplus "/ as sysdba" -prelim
oradebug set myospid
oradebug hanganalyze 3
62. What happens when undo datafile is lost when database is running?
Instance Crashes.. need to restore and recover undo datafile from backup. Current going transactions are lost.
63. What happens when temporary datafile is lost.
64. Explain PITR when a whole tablespace is lost.
65. How you'll find whether a table and Index are fragmented?
66. What is SCAN IP and SCAN Listener? What is remote listener?
67. What is the shutdown process of 11g RAC?
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