Chapter 10
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Oracle Server
• An Oracle Server consists of an Oracle
database (stored data, control and log files.)
The Server will support SQL to define and
manipulate data and PL/SQL to control the
flow of SQL, allow the use of variable &
error handling procedures
• Oracle Instance which is made up of the
various processes (both system and user) to
support a specific instance.
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Oracle Database Structure
• One or more Data files
• Two or more Log files called redo files.
This files are used in recovery and trace the
source of specific transaction.
• One or more Control files which contain the
database name, related file names and
locations and the creation timestamp.
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Oracle Database Structure (con’t)
• Trace Files and alert Log
• A database is defined in terms of Schema
Objects and Tablespaces.
– Schema Objects contain definitions of tables,
views, sequences, stored procedures indexes,
clusters and database links.
– Tablespaces, segments and extents are the terms
used to describe the physical storage.
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Oracle Instance
• System Global Area (SGA): Real memory
area where database information is shared
by users. For performance it should be as
large as possible.
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System Global Area
• SGA is further divided into 3 areas.
– Database buffer cache: Most recently accessed
data blocks from the database.
– Redo log buffer
– Shared pool, contains shared memory
constructs; such as, shared SQL area which is
made up of parse trees of SQL queries &
execution plans for executing SQL statements.
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Oracle Instance Processes
• User processes: A user process is created
for the execution of some application.
• Program Global Area (PGA): Contains data
and control information for the server
processes.
• Oracle Processes: A Process or Thread of
control which is assigned a private memory
area in which it runs.
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Oracle Processes
• Oracle creates Server Processes to handle
requests. This is a multithreaded server
operation for efficiency rather than creating
a separate process for each request to the
server.
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Oracle Background Processes
• Database Writer (DBWR): Writes those
blocks that were modified from the cache to
the disk
• Log Writer (LGWR): Writes from the log
buffer to the online disk log file.
• Checkpoint (CKPT): Refers to an event
during which all the modified buffers in the
SGA since the last checkpoint are written to
the data files. It works with DBWR.
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Oracle Background Processes
• System Monitor(SMON): Performs instance
recovery, manages storage areas by making
the space contiguous & recovers
transactions.
• Process Monitor(PMON): Performs
process recovery when a user process fails.
It manages the cache and other resources
used by a user process.
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Oracle Background Processes
• Archiver (ARCH): Archives on-line log
files to archival storage location; such as,
disk or tape ( if configured to do so).
• Recoverer Process(RECO): Resolves
distributed transactions that are pending due
to a network or system failure in a
Distributed database.
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Oracle Background Processes
• Dispatchers (Dnnn): In a multi-threaded
server configurations, routes requests from
the connected user processes to available
shared server processes. There is 1
dispatcher per standard communication
protocol supported.
• Lock Processes(LCKn): Used for interinstance locking when Oracle in Parallel
Mode
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Oracle Startup & Shutdown
• Starting an Instance: This is where the SGA
is allocated, the background processes are
created and the instance is initialized.
• Mounting the Database: This associates a
database with an instance. This is one step
that is required to make the database
available to users.
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Oracle Startup & Shutdown
• Opening a database: This is the second
operation needed to make the database
available to users. It opens the on-line data
files and the log files.
• The reverse of the above procedure is used
to shutdown the database.
• These are ALL actions done by the DBA.
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Schema Objects
• TABLES: Tables are the basic unit that is
used in Oracle to store data. It conforms
with the relational model. Each column has
a column name, datatype and width.
• Views: View are virtual tables which on
defined either on other views or tables.
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Schema Objects
• Synonyms: Direct references to objects.
They are used to provide public access to an
object while masking its real name or
owner. Private synonyms may be created
that are only available to that user.
• Program Unit: Procedures, functions or
packages written in PL/SQL
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Schema Objects
• Sequence: A special provision of a data type
in Oracle for attribute value generation.
• Indexes: An index can be generated on one
or more columns of a table as requested
using SQL.
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Schema Objects
• Cluster: A group of records from one or
more tables physically stored in a mixed
file. This is to improve performance. Such
as using an Index Cluster.
• Database Links: Named objects in Oracle
that establish paths from one database to
another. (Used in distributed databases.)
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Oracle’s Data Dictionary
• This is made up of a read-only set of tables
that store the Metadata. Access to the
information in these tables is usually done
through views .
• These views begin with one of three special
prefixes: USER,DBA, or ALL.
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Item in the Data Dictionary
•
•
•
•
•
Names of users
Security Information: Privileges and roles.
Schema Object Information
Integrity constraints
Space Allocation & Utilization of the
database objects.
• Statistics on attributes, tables and predicates.
• Access audit trail information.
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Triggers
• There are some business rules that we wish
to actively enforce every time some action
is done; such as, insert, delete or update. In
Oracle this is done through the use of
triggers.
• Triggers either enforce additional
constraints or perform additional actions.
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Storage
• Each Database is divided into one or more
different tablespaces.
• As a minimum there is always a System and
Users tablespace.
• These tablespaces are stored in one or more
data files.
• These data files are associated with only
one database.
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Physical Storage
• Physical Storage is organized in terms of data
blocks, extents and segments.
• Data blocks are the finest (smallest) size of
storage. They are also called logical blocks,
page or Oracle blocks.
• An Extent is a specific number of contiguous
data blocks.
• A Segment is a set of extents for a data
structure.
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Data Blocks
• A Data Block has the following
components:
– Header: Contains the general block information
such as block address & type of segment.
– Table Directory: Contains information about
tables that have data in the data block.
– Row Directory: Contains information about the
actual rows.
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Data Blocks (con’t)
– Row Data: Uses the bulk of the space of the
Data Block. A row may span multiple blocks.
– Free Space: Space allocated for row updates
and new rows.
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Extents
• The amount of space initial allocated to an
extent is determine by the ‘Create’
command. Incremental extents are
allocated when the initial one becomes full
and their size is determined by ‘Create’
command.
• All extents allocated to index exist as long
as the index exists.
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Segments
• There are four types of Segments:
– Data segments: Each non-clustered table and
each cluster has a Single data segment.
– Index segments: Each index has a single index
segment.
– Temporary segments: Used by SQL as a
temporary work area.
– Rollback segments: Used for undoing
transactions.
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