A database in MySQL is a schema in OracleMySQL has a single
daemon that runs the database server. Within the server you can
create any number of databases - these databases have no direct
mapping to users.Oracle has a single database. When you create a
user in an Oracle database, it also creates a Schema with the same
name as the user that created it. This is equivalent to a database
in MySQL.
You really need to forget about mysql while you learn Oracle.
DDL and RDBMS concepts are completely different.Start with the
Oracle Server Concepts guide (link). The Oracle documentation home
is here: http://www.oracle.com/pls/db112/homepage
In Oracle, schema contains all the tables, constraints, etc.
When in mysql you create create database , in Oracle you create
schema. In Oracle You don'tuse db1, butalter session set
CURRENT_SCHEMA = db1.Each user has it's own schema, so you get list
of schemas byselect USERNAME from SYS.ALL_USERS
The Oracle definition of a database is the files on disk. These
can be shared between instances (Real Application Clusters) or only
used by a single instance
Background: The concept of "database" is different between
database vendors. As an Oracle DBA I'm careful when talking to
someone who is from an MySQL, DB2, SQL Server background, what they
call a "database" in Oracle is a user/schema (difference between
user and schema being a schema contains tables and a user is only a
login). Whenever someone, developer especially, uses the word
"database" question in what context.
Difference between Oracle Instance, database, schema and user
account
A database is the set of files where data is stored
An instance is the set of processes that manipulate the
database. For most folks, there is one instance per database. You
can get multiple instances per database when you're using RAC
though.
A user account is just a login to the database
A schema is the set of objects owned by a particular user
account. Most user accounts should not own any objects, so they are
often not associated with a schema.-- --
An Oracle instance is in essense a set of Oracle server
processes and a shared memory segment called the SGA. Think of it
as the Database Server Engine.
The Oracle database is a physical database - and is not the same
as a SQL-Server or mySQL database. A single physical Oracle
database is capable of containing 1000's of logical databases.
An Oracle schema is in fact a logical database. This equates to
the databases you get in SQL-Server and mySQL.
The norm in Oracle is to have a single Oracle Instance and a
single Oracle Physical Database per platform. This scales a lot
better than having multiple Instances and Physical Databases in
order to emulate how SQL-Server for example looks like.
A single Oracle Instance is capable of servicing 10's of 1000's
of users. And as I mentioned, a single Physical Database can have
1000's of logical databases in it. Oracle even extends the logical
database, allowing you to create Virtual Private Databases. This
allows you to use a single logical database (same tables and same
code) for multiple customers/users, without any one customer/user
being able to see the data of another.
As for scalability - that is RAC (Real Application Clusters)
that Justin mentioned.
A RAC is a single Physical Database. Usually residing on a
shared storage medium like a SAN. It has Multiple Oracle Instances
- one per platform.
So you wind up with a number of database engines, one per
platform, for the same physical database. This address issues such
as high availability (one platform fails and there are still other
db engines for the database running). And scalability - more users
can be supported and one Database Instance can enlist the
assistance of other Instances to run complex queries using massive
parallel processing.
-- --
A database is a collection of physical operating system files
(parameter files, data files, redo log files, control files, temp
files, password files). A database is identified by its SID (system
identifier - a name chosen on creation - usually describing its
purpose e.g. 'LIVE', 'QA', 'DEVEL').
An instance is a set of Oracle processes (on unix / linux this
will be pmon, smon, lgwr, dbwr etc - on windows these will be
threads in oracle.exe) and an SGA (a shared memory structure
containing the Fixed SGA, the shared pool, redo buffer, block
buffers etc).
More than one instance can mount and open the same database at
the same time (Oracle RAC for example, or old style Oracle Parallel
Server). An instance can only mount and open one database at a
time.
Most oracle users tend to think of a Database and an Instance as
interchangeable terms - fair enough for normal conversation as long
as you remember the real definitions - especially if working in a
clustered environment or talking to a DBA.
A Schema is all the database objects created by a database user
(e.g. tables, indexes, database packages). A database can contain
many schemas.
--- ---
Instance, Database and Schema: The distinction between these
three is probably one of the most confusing concepts across
different RDBMS since different RDBMS have different architectures
and different terminologies. To put it simply in Oracle lingo, a
database is a set of files (data files, redo log files, control
files and so on). An instance is a set of processes (SMON, PMON,
MMON, DBWR, etc) and a shared memory segment (SGA). A database may
be mounted and opened by many instances (RAC installation)
concurrently (this is to do with the shared everything architecture
of Oracle RAC). An instance may mount and open any database however
it may only open a single database at any time. So, in ERD terms:
Database:Instance :: 1:N (a one to many relationship) in Oracle. A
schema is the container of the schema objects so all the tables,
views, procedures, packages, views etc. reside in the schema. There
can be multiple schemas per database.In the case of DB2 and SQL
Server, an instance can have many databases which internally can
have many schemas. In the case of Informix, an instance can have
many databases and there is no concept of schemas. For the sake of
simplicity, we refer a database as the container for the different
schemas and a schema to be the one that contains all the schema
objects (tables, views, procedures, packages, sequences etc.).
--- ---- ----- ----- ------ ------ ----- - - - -- --- -- - -- -
- --- -- - - - -- - - - - -- -- --- - - -- - - ---- - -- ----
SID = unique name of the INSTANCE (eg the oracle process running
on the machine). Oracle considers the "Database" to the be
files.Service Name = alias to an INSTANCE (or many instances). The
main purpose of this is if you are running a cluster, the client
can say "connect me to SALES.acme.com", the DBA can on the fly
change the number of instances which are available to
SALES.acme.com requests, or even move SALES.acme.com to a
completely different database without the client needing to change
any settings--- ---Oracle SID is the unique name that uniquely
identifies your instance/database where as Service name is the TNS
alias that you give when you remotely connect to your database and
this Service name is recorded in Tnsnames.ora file on your clients
and it can be the same as SID and you can also give it any other
name you want.SERVICE_NAME is the new feature from oracle 8i
onwards in which database can register itself with listener. If
database is registered with listener in this way then you can use
SERVICE_NAME parameter in tnsnames.ora otherwise - use SID in
tnsnames.ora.Also if you have OPS (RAC) you will have different
SERVICE_NAME for each instance.SERVICE_NAMES specifies one or more
names for the database service to which this instance connects. You
can specify multiple services names in order to distinguish among
different uses of the same database. For example:SERVICE_NAMES =
sales.acme.com, widgetsales.acme.comYou can also use service names
to identify a single service that is available from two different
databases through the use of replication.
--- ---
SID menas System Identification Number so for each instance
there is a unique SID in the server. Service_Name is nothing but a
name alias for the the database name in a non RAC system. But in a
RAC system it has some other additional functionalities.
Yes you can create a single listener for a 9i,10g and 11g
database but make sure when you are starting the listener you
should start it from Higher version , here in your case you should
start the listener from 11g binaries.
--- ---
The sid identifies the instance, the service_name the
database..
ORACLE INSTANCEA databaseinstanceis a set of memory structures
that manage database files. Adatabaseis a set of physical files on
disk created by theCREATE DATABASEstatement. The instance manages
its associated data and serves the users of the database.Every
running Oracle database is associated with at least one Oracle
database instance. Because an instance exists in memory and a
database exists on disk, an instance can exist without a database
and a database can exist without an instance.
http://docs.oracle.com/cd/E11882_01/server.112/e25789/toc.htmhttp://docs.oracle.com/cd/E11882_01/index.htm
Database and InstanceAn Oracle database server consists of
adatabaseand at least one databaseinstance(commonly referred to as
simply aninstance). Because an instance and a database are so
closely connected, the termOracle databaseis sometimes used to
refer to both instance and database. In the strictest sense the
terms have the following meanings: DatabaseA database is a set of
files, located on disk, that store data. These files can exist
independently of a database instance. Database instanceAn instance
is a set of memory structures that manage database files. The
instance consists of a shared memory area, called thesystem global
area (SGA), and a set ofbackground processes. An instance can exist
independently of database files.Figure 1-1shows a database and its
instance. For each user connection to the instance, the application
is run by aclient process. Each client process is associated with
its own server process. The server process has its own private
session memory, known as theprogram global area (PGA).Figure 1-1
Oracle Instance and Database
Description of "Figure 1-1 Oracle Instance and Database"A
database can be considered from both a physical and logical
perspective. Physical data is data viewable at the operating system
level. For example, operating system utilities such as the
Linuxlsandpscan list database files and processes. Logical data
such as a table is meaningful only for the database. A SQL
statement can list the tables in an Oracle database, but an
operating system utility cannot.The database hasphysical
structuresandlogical structures. Because the physical and logical
structures are separate, the physical storage of data can be
managed without affecting access to logical storage structures. For
example, renaming a physical database file does not rename the
tables whose data is stored in this file.See Also:Chapter 13,
"Oracle Database Instance"Database Storage StructuresAn essential
task of a relational database is data storage. This section briefly
describes the physical and logical storage structures used by
Oracle Database.Physical Storage StructuresThe physical database
structures are the files that store the data. When you execute the
SQL commandCREATE DATABASE, the following files are created: Data
filesEvery Oracle database has one or more physicaldata files,
which contain all the database data. The data of logical database
structures, such as tables and indexes, is physically stored in the
data files. Control filesEvery Oracle database has acontrol file. A
control file contains metadata specifying the physical structure of
the database, including the database name and the names and
locations of the database files. Online redo log filesEvery Oracle
Database has anonline redo log, which is a set of two or
moreonlineredo log files. An onlineredo logis made up of redo
entries (also calledredo records), which record all changes made to
data.Many other files are important for the functioning of an
Oracle database server. These files include parameter files and
diagnostic files. Backup files andarchived redo log filesare
offline files important for backup and recovery.See Also:Chapter
11, "Physical Storage Structures"Logical Storage StructuresThis
section discusses logical storage structures. The following logical
storage structures enable Oracle Database to have fine-grained
control of disk space use: Data blocksAt the finest level of
granularity, Oracle Database data is stored in data blocks. Onedata
blockcorresponds to a specific number of bytes on disk.
ExtentsAnextentis a specific number of logically contiguous data
blocks, obtained in a single allocation, used to store a specific
type of information. SegmentsAsegmentis a set of extents allocated
for a user object (for example, a table or index),undo data, or
temporary data. TablespacesA database is divided into logical
storage units calledtablespaces. A tablespace is the logical
container for a segment. Each tablespace contains at least one data
file.See Also:Chapter 12, "Logical Storage Structures"Database
Instance StructuresAn Oracle database uses memory structures and
processes to manage and access the database. All memory structures
exist in the main memory of the computers that constitute the
RDBMS.When applications connect to an Oracle database, they are
connected to a database instance. The instance services
applications by allocating other memory areas in addition to the
SGA, and starting other processes in addition to background
processes.Oracle Database ProcessesAprocessis a mechanism in an
operating system that can run a series of steps. Some operating
systems use the termsjob,task, orthread. For the purpose of this
discussion, a thread is equivalent to a process. An Oracle database
instance has the following types of processes: Client
processesThese processes are created and maintained to run the
software code of an application program or an Oracle tool. Most
environments have separate computers for client processes.
Background processesThese processes consolidate functions that
would otherwise be handled by multiple Oracle Database programs
running for each client process. Background processes
asynchronously perform I/O and monitor other Oracle Database
processes to provide increased parallelism for better performance
and reliability. Server processesThese processes communicate with
client processes and interact with Oracle Database to fulfill
requests.Oracle processesinclude server processes and background
processes. In most environments, Oracle processes and client
processes run on separate computers.See Also:Chapter 15, "Process
Architecture"Instance Memory StructuresOracle Database creates and
uses memory structures for purposes such as memory for program
code, data shared among users, and private data areas for each
connected user. The following memory structures are associated with
an instance: System Global Area (SGA)The SGA is a group of shared
memory structures that contain data and control information for one
database instance. Examples of SGA components include cached data
blocks and shared SQL areas. Program Global Areas (PGA)A PGA is a
memory region that contain data and control information for a
server or background process. Access to the PGA is exclusive to the
process. Each server process and background process has its own
PGA.See Also:Chapter 14, "Memory Architecture"
