Introduction to DBA

1. What is a database instance ? Explain
A database instance (server) is a set of memory structures and background processes that access a set of database files.

The process can be shared by all users.

The memory structures that are used to store most queried data from database. This helps us 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 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 or primary key constraint clause is specified in create table command (Ver 7.0)

5. What is clustres ?
Group of tables physically stored together because they share common columns and are often used together is called Clusters.

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 stores only once for multiple tables in the cluster.

7. 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
Multiple users/schemas
one or more rollback segments
one or more tablespaces
Data dictionary tables

User objects (tables,indexes,views etc)

The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, redo log buffers,Shared SQL pool)
SMON
PMON
LGWR
DBWR
ARCH
CKPT
RECO
Dispatcher
User process with associated PGA

8. What is 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.