1. What is a Database instance?

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 is 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.

2. What is Parallel Server?

Multiple instances accessing the same database (Only In Multi-CPU environments)

3. What is a Schema?

The set of objects owned by user account is called the schema.

4. What is an Index? How it is implemented in Oracle Database?

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 command

5. What is a cluster?

Group of tables physically stored together because they share common columns and are often used together is called Cluster.

6. What is a cluster Key?

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.

7. What is the basic element of Base configuration of an oracle Database?

It consists of

1. one or more data files.
2. one or more control files.
3. two or more redo log files.

1. multiple users/schemas
2. one or more rollback segments
3. one or more tablespaces
4. Data dictionary tables
5. User objects (table,indexes,views etc.,)

1. SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
2. SMON (System MONito)
3. PMON (Process MONitor)
4. LGWR (LoG Write)
5. DBWR (Data Base Write)
6. ARCH (ARCHiver)
7. CKPT (Check Point)
8. RECO
9. Dispatcher
10. User Process with associated PGS

8. What is a deadlock? Explain.

Two processes waiting 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.

9. What is SGA?

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 databse.

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.

14. What is Distributed database?

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.

15. What is Two-Phase Commit?

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.

16. Describe two phases of Two-phase commit?

1. 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)
2. 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, and the coordinator asks all nodes to roll back the transaction.

17. What is the mechanism provided by ORACLE for table replication?

Snapshots and SNAPSHOT LOGs

18. What is a SNAPSHOT?

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.

19. What is a SNAPSHOT LOG?

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.

20. What is a SQL * NET?

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.