Oracle :CONSTRAINTS

CONSTRAINTS
12. What is an Integrity Constraint ?
Integrity constraint is a rule that restricts values to a column in a table.
13. What is Referential Integrity ?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
14. What are the usage of SAVEPOINTS ?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
15. What is ON DELETE CASCADE ?
When ON DELETE CASCADE is specified ORACLE maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
16. What are the data types allowed in a table ?
CHAR,VARCHAR2,NUMBER,DATE,RAW,LONG and LONG RAW.
17. What is difference between CHAR and VARCHAR2 ? What is the maximum SIZE allowed for each type ?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces. For CHAR it is 255 and 2000 for VARCHAR2.
18. How many LONG columns are allowed in a table ? Is it possible to use LONG columns in WHERE clause or ORDER BY ?
Only one LONG columns is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
19. What are the pre requisites ?
To modify datatype of a column. To add a column with NOT NULL constraint. To Modify the datatype of a column the column must be empty. to add a column with NOT NULL constrain, the table must be empty.
20. Where the integrity constrints are stored in Data Dictionary ?
The integrity constraints are stored in USER_CONSTRAINTS.
21. How will you a activate/deactivate integrity constraints ?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE constraint/DISABLE constraint.
22. If an unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE ?
It won't, Because SYSDATE format contains time attached with it.
23. What is a database link ?
Database Link is a named path through which a remote database can be accessed.
24. How to access the current value and next value from a sequence ? Is it possible to access the current value in a session before accessing next value ?
Sequence name CURRVAL, Sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed.
25. What is CYCLE/NO CYCLE in a Sequence ?
CYCLE specifies that the sequence continues to generate values after reaching either maximum or minimum value. After pan ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum. NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.
26. What are the advantages of VIEW ?
To protect some of the columns of a table from other users. To hide complexity of a query. To hide complexity of calculations.
27. Can a view be updated/inserted/deleted? If Yes under what conditions ?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.
28.If a View on a single base table is manipulated will the changes be reflected on the base table ?
If changes are made to the tables which are base tables of a view will the changes be reference on the view.

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Oracle SQL PLUS STATEMENTS

SQL PLUS STATEMENTS
1. What are the types of SQL Statement ?
Data Definition Language : CREATE,ALTER,DROP,TRUNCATE,REVOKE,NO AUDIT & COMMIT.
Data Manipulation Language : INSERT,UPDATE,DELETE,LOCK TABLE,EXPLAIN PLAN & SELECT.
Transactional Control : COMMIT & ROLLBACK. Session Control : ALTERSESSION & SET ROLE. System Control : ALTER SYSTEM.
2. What is a transaction ?
Transaction is logical unit between two commits and commit and rollback.
3. What is difference between TRUNCATE & DELETE ?
TRUNCATE commits after deleting entire table i.e., can not be rolled back. Database triggers do not fire on TRUNCATE, DELETE allows the filtered deletion. Deleted records can be rolled back or committed. Database triggers fire on DELETE.
4. What is a join ? Explain the different types of joins ?
Join is a query which retrieves related columns or rows from multiple tables. Self Join - Joining the table with itself. Equi Join - Joining two tables by equating two common columns. Non-Equi Join - Joining two tables by equating two common columns. Outer Join - Joining two tables in such a way that query can also retrive rows that do not have corresponding join value in the other table.
5. What is the Subquery ?
Subquery is a query whose return values are used in filtering conditions of the main query.
6. What is correlated sub-query ?
Correlated sub_query is a sub_query which has reference to the main query.
7. Explain Connect by Prior ?
Retrives rows in hierarchical order.
e.g. select empno, ename from emp where.
8. Difference between SUBSTR and INSTR ?
INSTR (String1,String2(n,(m)), INSTR returns the position of the mth occurrence of the string 2 in
string1. The search begins from nth position of string1. SUBSTR (String1 n,m) SUBSTR returns a character string of size m in string1, starting from nth postion of string1.
9. Explain UNION,MINUS,UNION ALL, INTERSECT ?
INTERSECT returns all distinct rows selected by both queries. MINUS - returns all distinct rows selected by the first query but not by the second. UNION - returns all distinct rows selected by either query. UNION ALL - returns all rows selected by either query,including all duplicates.
10. What is ROWID ?
ROWID is a pseudo column attached to each row of a table. It is 18 character long, blockno, rownumber are the components of ROWID.
11. What is the fastest way of accessing a row in a table ?
Using ROWID.

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Oracle: MANAGING BACKUP & RECOVERY

MANAGING BACKUP & RECOVERY
73. What are the different methods of backing up oracle database ?
- Logical Backups - Cold Backups - Hot Backups (Archive log)
74. What is a logical backup ?
Logical backup involves reading a set of databse records and writing them into a file. Export utility is used for taking backup and Import utility is used to recover from backup.
75. What is cold backup ? What are the elements of it ?
Cold backup is taking backup of all physical files after normal shutdown of database. We need to take.
- All Data files. - All Control files. - All on-line redo log files. - The init.ora file (Optional)
76. What are the different kind of export backups ?
Full back - Complete database. Incremental - Only affected tables from last incremental date/full backup date. Cumulative backup - Only affected table from the last cumulative date/full backup date.
77. What is hot backup and how it can be taken ?
Taking backup of archive log files when database is open. For this the ARCHIVELOG mode should be enabled. The following files need to be backed up. All data files. All Archive log, redo log files. All control files.
78. What is the use of FILE option in EXP command ?
To give the export file name.
79. What is the use of COMPRESS option in EXP command ?
Flag to indicate whether export should compress fragmented segments into single extents.
80. What is the use of GRANT option in EXP command ?
A flag to indicate whether grants on databse objects will be exported or not. Value is 'Y' or 'N'.
81. What is the use of INDEXES option in EXP command ?
A flag to indicate whether indexes on tables will be exported.
82. What is the use of ROWS option in EXP command ?
Flag to indicate whether table rows should be exported. If 'N' only DDL statements for the databse objects will be created.
83. What is the use of CONSTRAINTS option in EXP command ?
A flag to indicate whether constraints on table need to be exported.

84. What is the use of FULL option in EXP command ?
A flag to indicate whether full databse export should be performed.
85. What is the use of OWNER option in EXP command ?
List of table accounts should be exported.
86. What is the use of TABLES option in EXP command ?
List of tables should be exported.
87. What is the use of RECORD LENGTH option in EXP command ?
Record length in bytes.
88. What is the use of INCTYPE option in EXP command ?
Type export should be performed COMPLETE,CUMULATIVE,INCREMENTAL.
89. What is the use of RECORD option in EXP command ?
For Incremental exports, the flag indirects whether a record will be stores data dictionary tables recording the export.
90. What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
91. What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
92. What is the use of ANALYSE ( Ver 7) option in EXP command ?
A flag to indicate whether statistical information about the exported objects should be written to export dump file.
93. What is the use of CONSISTENT (Ver 7) option in EXP command ?
A flag to indicate whether a read consistent version of all the exported objects should be maintained.
94. What is use of LOG (Ver 7) option in EXP command ?
The name of the file which log of the export will be written.
95.What is the use of FILE option in IMP command ?
The name of the file from which import should be performed.
96. What is the use of SHOW option in IMP command ?
A flag to indicate whether file content should be displayed or not.
97. What is the use of IGNORE option in IMP command ?
A flag to indicate whether the import should ignore errors encounter when issuing CREATE commands.
98. What is the use of GRANT option in IMP command ?
A flag to indicate whether grants on database objects will be imported.
99. What is the use of INDEXES option in IMP command ?
A flag to indicate whether import should import index on tables or not.
100. What is the use of ROWS option in IMP command ?
A flag to indicate whether rows should be imported. If this is set to 'N' then only DDL for database objects will be exectued.

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Oracle: MANAGING DISTRIBUTED DATABASES.

MANAGING DISTRIBUTED DATABASES.
62. How can we reduce the network traffic ?
Replictaion of data in distributed environment. Using snapshots to replicate data. Using remote procedure calls.
63. What is snapshots ?
Snapshot is an object used to dynamically replicate data between distribute database at specified time intervals. In ver 7.0 they are read only.
64. What are the various type of snapshots ?
Simple and Complex.
65. Differentiate simple and complex, snapshots ?
A simple snapshot is based on a query that does not contains GROUP BY clauses, CONNECT BY clauses, JOINs, sub-query or snashot of operations. A complex snapshots contain atleast any one of the above.
66. What dynamic data replication ?
Updating or Inserting records in remote database through database triggers. It may fail if remote database is having any problem.
67. How can you Enforce Refrencial Integrity in snapshots ?
Time the references to occur when master tables are not in use. Peform the reference the manually immdiately locking the master tables. We can join tables in snopshots by creating a complex snapshots that will based on the master tables.
68. What are the options available to refresh snapshots ?
COMPLETE - Tables are completly regenerated using the snapshot's query and the master tables every time the snapshot referenced. FAST - If simple snapshot used then a snapshot log can be used to send the changes to the snapshot tables. FORCE - Default value. If possible it performs a FAST refresh; Otherwise it will perform a complete refresh.
69. what is snapshot log ?
It is a table that maintains a record of modifications to the master table in a snapshot. It is stored in the same database as master table and is only available for simple snapshots. It should be created before creating snapshots.
70. When will the data in the snapshot log be used ?
We must be able to create a after row trigger on table (i.e., it should be not be already available )
After giving table privileges. We cannot specify snapshot log name because oracle uses the name of the master table in the name of the database objects that support its snapshot log. The master table name should be less than or equal to 23 characters. (The table name created will be MLOGS_tablename, and trigger name will be TLOGS name).
72. What are the benefits of distributed options in databases ?
Database on other servers can be updated and those transactions can be grouped together with others in a logical unit. Database uses a two phase commit.

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Oracle: DATABASE SECURITY & ADMINISTRATION

DATABASE SECURITY & ADMINISTRATION
48. What is user Account in Oracle database ?
An user account is not a physical structure in Database but it is having important relationship to the objects in the database and will be having certain privileges.
49. How will you enforce security using stored procedures ?
Don't grant user access directly to tables within the application. Instead grant the ability to access the procedures that access the tables. When procedure executed it will execute the privilege of procedures owner. Users cannot access tables except via the procedure.
50. What are the dictionary tables used to monitor a database spaces ?
DBA_FREE_SPACE, DBA_SEGMENTS, DBA_DATA_FILES.
51. What are the responsibilities of a Database Administrator ?
Installing and upgrading the Oracle Server and application tools. Allocating system storage and planning future storage requirements for the database system. Managing primary database structures (tablespaces). Managing primary objects (table,views,indexes). Enrolling users and maintaining system security. Ensuring compliance with Oralce license agreement. Controlling and monitoring user access to the database. Monitoring and optimising the performance of the database. Planning for backup and recovery of database information. Maintain archived data on tape. Backing up and restoring the database. Contacting Oracle Corporation for technical support.
52. What are the roles and user accounts created automatically with the database ?
DBA - role Contains all database system privileges. SYS user account - The DBA role will be assigned to this account. All of the basetables and views for the database's dictionary are store in this schema and are manipulated only by ORACLE. SYSTEM user account - It has all the system privileges for the database and additional tables and views that display administrative information and internal tables and views used by oracle tools are created using this username.
54. What are the database administrators utilities avaliable ?
SQL * DBA - This allows DBA to monitor and control an ORACLE database. SQL * Loader - It loads data from standard operating system files (Flat files) into ORACLE database tables. Export (EXP) and Import (imp) utilities allow you to move existing data in ORACLE format to and from ORACLE database.
55. What are the minimum parameters should exist in the parameter file (init.ora) ?
DB NAME - Must set to a text string of no more than 8 characters and it will be stored inside the datafiles, redo log files and control files and control file while database creation. DB_DOMAIN - It is string that specifies the network domain where the database is created. The global database name is identified by setting these parameters (DB_NAME & DB_DOMAIN)
CONTORL FILES - List of control filenames of the database. If name is not mentioned then default name will be used. DB_BLOCK_BUFFERS - To determine the no of buffers in the buffer cache in SGA. PROCESSES - To determine number of operating system processes that can be connected to ORACLE concurrently. The value should be 5 (background process) and additional 1 for each user. ROLLBACK_SEGMENTS - List of rollback segments an ORACLE instance acquires at database startup. Also optionally LICENSE_MAX_SESSIONS,LICENSE_SESSION_WARNING and LICENSE_MAX_USERS.
56. What is a trace file and how is it created ?
Each server and background process can write an associated trace file. When an internal error is detected by a process or user process, it dumps information about the error to its trace. This can be used for tuning the database.
57. What are roles ? How can we implement roles ?
Roles are the easiest way to grant and manage common privileges needed by different groups of database users. Creating roles and assigning provies to roles. Assign each role to group of users. This will simplify the job of assigning privileges to individual users.
58. What are the steps to switch a database's archiving mode between NO ARCHIVELOG and ARCHIVELOG mode ?
1. Shutdown the database instance. 2. Backup the databse. 3. Perform any operating system specific steps (optional). 4. Start up a new instance and mount but do not open the databse. 5. Switch the databse's archiving mode.
59. How can you enable automatic archiving ?
Shut the database. Backup the database. Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file. Start up the databse.
60. How can we specify the Archived log file name format and destination ?
By setting the following values in init.ora file. LOG_ARCHIVE_FORMAT = arch %S/s/T/tarc (%S - Log sequence number and is zero left paded, %s - Log sequence number not padded. %T - Thread number lef-zero-paded and %t - Thread number not padded). The file name created is arch 0001 are if %S is used. LOG_ARCHIVE_DEST = path.
61. What is the use of ANALYZE command ?
To perform one of these function on an index,table, or cluster: - to collect statisties about object used by the optimizer and store them in the data dictionary. - to delete statistics about the object used by object from the data dictionary. - to validate the structure of the object. - to identify migrated and chained rows of the table or cluster.

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Oracle: LOGICAL & PHYSICAL ARCHITECTURE OF DATABASE.

LOGICAL & PHYSICAL ARCHITECTURE OF DATABASE.
14. What is Database Buffers ?
Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
15. What is dictionary cache ?
Dictionary cache is information about the databse objects stored in a data dictionary table.
16. What is meant by recursive hints ?
Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache.
17. What is meant by redo log buffer ?
Change made to entries are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size.
18. How will you swap objects into a different table space for an existing database ?
Export the user, Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql. Drop necessary objects.
Run the script newfile.sql after altering the tablespaces. Import from the backup for the necessary objects.
19. List the Optional Flexible Architecture (OFA) of Oracle database ? or How can we organise the tablespaces in Oracle database to have maximum performance ?
SYSTEM - Data dictionary tables. DATA - Standard operational tables. DATA2- Static tables used for standard operations INDEXES - Indexes for Standard operational tables. INDEXES1 - Indexes of static tables used for standard operations. TOOLS - Tools table. TOOLS1 - Indexes for tools table. RBS - Standard Operations Rollback Segments, RBS1,RBS2 - Additional/Special Rollback segments. TEMP - Temporary purpose tablespace TEMP_USER - Temporary tablespace for users. USERS - User tablespace.
20. How will you force database to use particular rollback segment ?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.
21. What is meant by free extent ?
A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
22. How free extents are managed in Ver 6.0 and Ver 7.0 ?
Free extents cannot be merged together in Ver 6.0. Free extents are periodically coalesces with the neighboring free extent in Ver 7.0
23.Which parameter in Storage clause will reduce no. of rows per block?
PCTFREE parameter Row size also reduces no of rows per block.
24. What is the significance of having storage clause ?
We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updations etc.,
25. How does Space allocation table place within a block ?
Each block contains entries as follows, Fixied block header, Variable block header, Row Header,row date (multiple rows may exists), PCTEREE (% of free space for row updation in future)
26. What is the role of PCTFREE parameter is Storage clause ?
This is used to reserve certain amount of space in a block for expansion of rows.
27. What is the OPTIMAL parameter ?
It is used to set the optimal length of a rollback segment.
28. What is the functionality of SYSTEM table space ?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.
29. How will you create multiple rollback segments in a database ?
Create a database which implicitly creates a SYSTEM Rollback Segment in a SYSTEM tablespace.
Create a Second Rollback Segment name R0 in the SYSTEM tablespace. Make new rollback segment available (After shutdown, modify init.ora file and Start database) Create other tablespaces (RBS) for rollback segments. Deactivate Rollback Segment R0 and activate the newly created rollback segments.
30. How the space utilisation takes place within rollback segments ?
It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (No. of extents is based on the optimal size)
31. Why query fails sometimes ?
Rollback segment dynamically extent to handle larger transactions entry loads.
A single transaction may wipeout all avaliable free space in the Rollback Segment Tablespace. This prevents other user using Rollback segments.
32. How will you monitor the space allocation ?
By quering DBA_SEGMENT table/view.
33. How will you monitor rollback segment status ?
Querying the DBA_ROLLBACK_SEGS view, IN USE - Rollback Segment is on-line. AVAILABLE - Rollback Segment available but not on-line. OFF-LINE - Rollback Segment off-line, INVALID - Rollback Segment Dropped. NEEDS RECOVERY - Contains data but need recovery or corupted. PARTLY AVAILABLE - Contains data from an unresolved transaction involving a distributed database.
34. List the sequence of events when a large transaction that exceeds beyond its optimal value when an entry wraps and causes the rollback segment to expand into another extend.
Transaction Begins. An entry is made in the RES header for new transactions entry, Transaction acquires blocks in an extent of RBS The entry attempts to wrap into second extent. None is available, so that the RBS must extent. The RBS checks to see if it is part of its OPTIMAL size. RBS chooses its oldest inactive segment. Oldest inactive segment is eliminated. RBS extents. The Data dictionary table for space management are updated. Transaction Completes. 35. How can we plan storage for very large tables ? Limit the number of extents in the table Separate Table from its indexes. Allocate Sufficient temporary storage.
36. How will you estimate the space required by a non-clustered tables?
Calculate the total header size, Calculate the available dataspace per data block Calculate the combined column lengths of the average row. Calculate the total average row size. Calculate the average number rows that can fit in a block. Calculate the number of blocks and bytes required for the table. After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table.
37. It is possible to use raw devices as data files and what is the advantages over file. system files ?
Yes. The advantages over file system files. I/O will be improved because Oracle is bye-passing the kernnel which writing into disk. Disk Corruption will be very less.
38. What is a Control file ?
Database's overall physical architecture is maintained in a file called control file. It will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable.
39. How to implement the multiple control files for an existing database ?
Shutdown the databse. Copy one of the existing control file to new location. Edit Config ora file by adding new control file.name. Restart the database.
40. What is meant by Redo Log file mirrorring ? How it can be achieved?
Process of having a copy of redo log files is called mirroring. This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the members of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance.
41. What is advantage of having disk shadowing/ Mirroring ?
Shadow set of disks save as a backup in the event of disk failure. In most Operating System if any disk failure occurs it automatically switchover to place of failed disk. Improved performance because most OS support volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks.
42. What is use of Rollback Segments In Database ?
They allow the database to maintain read consistency between multiple transactions.
43. What is a Rollback segment entry ?
It is the set of before image data blocks that contain rows that are modified by a transaction.
Each Rollback Segment entry must be completed within one rollback segment.
A single rollback segment can have multiple rollback segment entries.
44. What is hit ratio ?
It is a measure of well the data cache buffer is handling requests for data. Hit Ratio = (Logical Reads - Physical Reads - Hits Misses)/ Logical Reads.
45. When will be a segment released ?
When Segment is dropped. When Shrink (RBS only). When truncated (TRUNCATE used with drop storage option).
46. What are disadvanteges of having raw devices ?
We should depend on export/import utility for backup/recovery (fully reliable). The tar command cannot be used for physical file backup, instead we can use dd command which is less flexible and has limited recoveries.
47. List the factors that can affect the accuracy of the estimations ?
The space used transaction entries and deleted records does not become free immediately after completion due to delayed cleanout. Trailling nulls and length bytes are not stored. Inserts of, updates to and deletes of rows as well as columns larger than a single datablock, can cause fragmentation an chained row pieces.

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Oracle: MEMORY MANAGEMENT

MEMORY MANAGEMENT
9. What is SGA ? How it is different from Ver 6.0 and Ver 7.0 ?
The System Global Area in a Oracle database is the area in memory to facilitates the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is Database buffers, Dictionary cache, Redo Log Buffer and Shared SQL pool (ver 7.0 only) area.
10. What is a Shared SQL pool ?
The data dictionary cache is stored in an area in SGA called the Shared SQL Pool. This will allow sharing of parsed SQL statements among concurrent users.
11. What is mean by Program Global Area (PGA) ?
It is area in memory that is used by a Single Oracle User Process.
12. What is a data segment ?
Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored.
13. What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient Shared SQL pool size.
Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE.

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