Dear Readers, Welcome to Oracle 10g Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Oracle 10g. These Oracle 10g Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT companies.
Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files.
There are tablespaces and database's schema objects.
A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.
Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.
Each databases logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace.
A schema is collection of database objects of a user.
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.
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.
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.)
A Partial Backup is any operating system backup short of a full backup, taken while the database is open or shut down.
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.
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.
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.
Views do not contain or store data.
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.
There are two types of Synonyms Private and Public.
The set of Redo Log files YSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.
Each Index has an Index segment that stores all of its data.
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
Data Segment, Index Segment, Rollback Segment and Temporary Segment.
Clusters are groups of one or more tables physically stores together to share common columns and are often used together.
An integrity constraint is a declarative way to define a business rule for a column of a table.
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.
An Extent is a specific number of contiguous data blocks, obtained in a single allocation, and used to store a specific type of information.
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.)
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.
- Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
- Hide data complexity.
- Simplify commands for the user.
- Present the data in a different perspective from that of the base table.
- Store complex queries.
A sequence generates a serial list of unique numbers for numerical columns of a database's tables.
A synonym is an alias for a table, view, sequence or program unit.
There are two types of synonyms private and public.
Only its owner can access a private synonym.
Any database user can access a public synonym.
- Mask the real name and owner of an object.
- Provide public access to an object
- Provide location transparency for tables, views or program units of a remote database.
- Simplify the SQL statements for database users.
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.
Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes.
A database is divided into Logical Storage Unit called tablespace. A tablespace is used to grouped related logical structures together
A Database contains one or more Rollback Segments to temporarily store "undo" information.
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.
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 data blocks. Block size is specified in INIT.ORA file and can’t be changed latter.
A Control file records the physical structure of the database. It contains the following information.
Names and locations of a database's files and redolog files.
Time stamp of database creation.
A column defined as UNIQUE can contain Nulls while a column defined as PRIMARY KEY can't contain Nulls.
A Cluster with an index on the Cluster Key
When it is committed or Rollbacked.
Answer The OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the statement.
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.
One can create and modify Oracle databases using the Oracle "dbca" (Database Configuration Assistant) utility. The dbca utility is located in the $ORACLE_HOME/bin directory. The Oracle Universal Installer (oui) normally starts it after installing the database server software.
One can also create databases manually using scripts. This option, however, is falling out of fashion, as it is quite involved and error prone. Look at this example for creating and Oracle 9i database:
CONNECT SYS AS SYSDBA
ALTER SYSTEM SET DB_CREATE_FILE_DEST='/u01/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_1='/u02/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_2='/u03/oradata/';
Oracle recommends that your database block size match, or be multiples of your operating system block size. One can use smaller block sizes, but the performance cost is significant. Your choice should depend on the type of application you are running. If you have many small transactions as with OLTP, use a smaller block size. With fewer but larger transactions, as with a DSS application, use a larger block size. If you are using a volume manager, consider your "operating system block size" to be 8K. This is because volume manager products use 8K blocks (and this is not configurable).
Rule-based and Cost-based.
ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.
SMON coalesces free space (extents) into larger, contiguous extents every 2 hours and even then, only for a short period of time.
SMON will not coalesce free space if a tablespace's default storage parameter "pctincrease" is set to 0. With Oracle 7.3 one can manually coalesce a tablespace using the ALTER TABLESPACE ... COALESCE; command, until then use:
SQL> alter session set events 'immediate trace name coalesce level n';
Where 'n' is the tablespace number you get from SELECT TS#, NAME FROM SYS.TS$;
You can get status information about this process by selecting from the SYS.DBA_FREE_SPACE_COALESCED dictionary view.
Considering available access paths and determining the most efficient execution plan based on statistics in the data dictionary for the tables accessed by the statement and their associated clusters and indexes.
COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.
Oracle8 and above rounds off extents to a multiple of 5 blocks when more than 5 blocks are requested. If one requests 16K or 2 blocks (assuming a 8K block size), Oracle doesn't round it up to 5 blocks, but it allocates 2 blocks or 16K as requested. If one asks for 8 blocks, Oracle will round it up to 10 blocks.
Space allocation also depends upon the size of contiguous free space available. If one asks for 8 blocks and Oracle finds a contiguous free space that is exactly 8 blocks, it would give it you. If it were 9 blocks, Oracle would also give it to you. Clearly Oracle doesn't always round extents to a multiple of 5 blocks.
The exception to this rule is locally managed tablespaces. If a tablespace is created with local extent management and the extent size is 64K, then Oracle allocates 64K or 8 blocks assuming 8K-block size. Oracle doesn't round it up to the multiple of 5 when a tablespace is locally managed.
A Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.
A Read-Only transaction ensures that the results of each query executed in the transaction are consistant with respect to the same point in time.
Two processes wating to update the rows of a table which are locked by the other process then deadlock arises. In a database environment this will often happen because of not issuing proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.
The set of objects owned by user account is called the schema.
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.
Multiple instances accessing the same database (Only In Multi-CPU environments)
What are the basic element of Base configuration of an oracle Database ?
It consists of
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONito)
PMON (Process MONitor)
LGWR (LoG Write)
DBWR (Data Base Write)
CKPT (Check Point)
User Process with associated PGS
Group of tables physically stored together because they share common columns and are often used together is called Cluster.
An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table comman (Ver 7.0)
A database instance (Server) is a set of memory structure and background processes that access a set of database files.
The process can be shared by all users. The memory structure that are used to store most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.
To perform one of these function on an index, table, or cluster:
- To collect statistics 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.
The Tablespace to contain schema objects created without specifying a tablespace name.
The number of concurrent sessions the user can establish the CPU processing time available to the user's session the CPU processing time available to a single call to ORACLE made by a SQL statement the amount of logical I/O available to the user's session the amout of logical I/O available to a single call to ORACLE made by a SQL statement the allowed amount of idle time for the user's session the allowed amount of connect time for the user's session.
The collective amount of disk space available to the objects in a schema on a particular tablespace.
Statement Auditing, Privilege Auditing and Object Auditing.
Statement auditing is the auditing of the powerful system privileges without regard to specifically named objects.
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.
Shut the database
Backup the database
Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
Start up the database.
Roles are the easiest way to grant and manage common privileges needed by different groups of database users. Creating roles and assigning provides to roles. Assign each role to group of users. This will simplify the job of assigning privileges to individual users.
Roles are named groups of related privileges that are granted to users or other roles.
What are the use of Roles ?
REDUCED GRANTING OF PRIVILEGES - Rather than explicitly granting the same set of privileges to many users a database administrator can grant the privileges for a group of related users granted to a role and then grant only the role to each member of the group.
DYNAMIC PRIVILEGE MANAGEMENT - When the privileges of a group must change, only the privileges of the role need to be modified. The security domains of all users granted the group's role automatically reflect the changes made to the role.
SELECTIVE AVAILABILITY OF PRIVILEGES - The roles granted to a user can be selectively enable (available for use) or disabled (not available for use). This allows specific control of a user's privileges in any given situation.
APPLICATION AWARENESS - A database application can be designed to automatically enable and disable selective roles when a user attempts to use the application.
Privilege auditing is the auditing of the use of powerful system privileges without regard to specifically named objects.
Object auditing is the auditing of accesses to specific schema objects without regard to user.
Monitoring of user access to aid in the investigation of database use.
Query v$fixed_view_definition. Example: SELECT * FROM v$fixed_view_definition WHERE view_name='V$SESSION';
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.
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.
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).
Updating or Inserting records in remote database through database triggers. It may fail if remote database is having any problem.
Two-phase commit is mechanism that guarantees a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database. It has two phase, a Prepare Phase and a Commit Phase.
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.
SQL *NET is ORACLE's mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed databases. It is used in Clint-Server and Server-Server communications.
Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes made to the master table.
Snapshots and SNAPSHOT LOGs
Snapshot is an object used to dynamically replicate data between distribute database at specified time intervals. In ver 7.0 they are read only.
Simple and Complex.
Prepare phase - The global coordinator (initiating node) ask a participants to prepare (to promise to commit or rollback the transaction, even if there is a failure) Commit - Phase - If all participants respond to the coordinator that they are prepared, the coordinator asks all nodes to commit the transaction, if all participants cannot prepare, the coordinator asks all nodes to roll back the transaction.
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.
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.
COMPLETE - Tables are completely regenerated using the snapshots 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.
A snapshot log is a table in the master database that is associated with the master table. ORACLE uses a snapshot log to track the rows that have been updated in the master table. Snapshot logs are used in updating the snapshots based on the master table.
A distributed database is a network of databases managed by multiple database servers that appears to a user as single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified.
- Replication of data in distributed environment.
- Using snapshots to replicate data.
- Using remote procedure calls.
- 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.