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Oracle
Database
Architecture
Processing
Chapter 1
The Oracle Architecture
Oracle Architecture
Vocabulary/Terminology
• Let’s review some terms and concepts
from DB in general...
• http://www.hp.isc.usouthal.edu/isc563/ISC563.htm
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Review from 561
Oracle Architecture
• Transaction Processing Schema
Customer
Sales Rep
Document
of
Transaction
Product
Line Item
Service
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Southern Floors
Oracle Architecture
• Fill in the blanks...
Customer
Renter
Sales
Clerk
Rep
Document
Rental
of
Agreement
Transaction
Product
Sander
Equipment
Line Item
List
Service
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Review from 561
Oracle Architecture
• Transaction Processing Schema
Clerk
------------ClerkID (PK)
Other stuff
Customer
------------CustomerID (PK)
Other stuff
Recall, the
ERD won’t
necessarily
map to the
table
structure
as it does
in this case.
Rental
Agreement
------------Rental_Num (PK)
CustomerID (FK)
ClerkID (FK)
DateTime
Copyright © 2001 Harold Pardue, University of South Alabama
Equipment
------------EquipID (PK)
Other stuff
Equipment
List
------------Rental_Num (PK)(FK)
EquipID (PK)(FK)
Other stuff
Chapter 1 The Oracle Architecture
Time to look under the hood... Oracle Architecture
hmm... PGA, SGA, PMON,
SMON, LGWR, CKPT,
DBWR, ARCH, DataFiles,
Data Buffers
image source: http://askanexpert.net/howto.html
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Storage Hierarchy
Database
Tablespace
(Logical)
Oracle Architecture
Let’s start at
the begining...
Data file
(Physical)
Segment
Extent
OracleBlock
OS Block
Page
12
Oracle Corporation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Database
Oracle Architecture
(logical)
Why multiple files?
(physical)
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Storage Hierarchy
Database
Tablespace
(Logical)
Oracle Architecture
A database is
physically stored as
data files.
Data file
(Physical)
Segment
Extent
OracleBlock
OS Block
Page
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Oracle Corporation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Datafiles
Oracle Architecture
Name of database
At a minimum there is a
SYSTEM Tablespace
Physical data files
DBA_DATA_FILES is a data dictionary view
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Datafiles
Oracle Architecture
In this case, the tablespaces match a physical file 1:1, but a
Tablespace can be distributed across multiple data files.
Tablespace = logical
Datafile = physical
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Datafiles
Oracle Architecture
USER_DATA
I can add a datafile to the
USER_DATA tablespace.
USER1ORCL.ORA
USER1ORCL2.ORA
SQL> ALTER TABLESPACE USER_DATA
ADD DATAFILE ‘C:\ORACLE\ORA563\DATABASE\USER1ORCL2' SIZE 1M;
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Datafiles
Oracle Architecture
Once you add a
datafile, you can’t
remove it from the
tablespace.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Storage Hierarchy
Database
Tablespace
(Logical)
Oracle Architecture
Data files are logically
grouped by tablespaces.
Data file
(Physical)
Segment
Extent
OracleBlock
OS Block
Page
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Oracle Corporation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tablespaces
Oracle Architecture
View tablespaces by querying the data dictionary.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tablespaces
Oracle Architecture
• You can query across tablespaces
• Why organize database datafiles into
these logical storage units?
– Control disk space allocation
– Assign space quotas for users
– Control availability of data: individual
tablespaces can be brought offline and online
separately
– Partial backup and recovery
– Tuning by allocating data across multiple
storage devices
– Manage I/O contention
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tablespaces
Oracle Architecture
• Tablespaces can contain:
–
–
–
–
–
–
Tables
Views
Sequences
Synonyms
Indexes
Clusters
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tablespaces
Oracle Architecture
• Oracle vs. SQL Server

Page 12http://www.microsoft.com/sql/techinfo/deployment/2000/MigrateOracle.asp (109 pages)
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Schemas
Oracle Architecture
• All objects in a Tablespace must have an
owner.
• Objects owned by the same user are
members of that user’s schema
– For example: SCOTT.DEPT indicates that the
table DEPT is a member of the SCOTT schema
• Therefore, ownership (by schema) is
another means of logically organizing the
objects in a tablespace.
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Schemas
Oracle Architecture
The tables in
the SCOTT
schema
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tables
Oracle Architecture
Query the data
dictionary for
tables in the
USER_DATA
tablespace
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Tables
Oracle Architecture
• No discussion on tables per se...
• An administrative issue is when tables
get very large (teraflation)
– Very large database (VLDB)
• What is large?
– giga, tera, peta?
– Numerical threshold or maintenance
overhead? Recovery time?
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Partitions
Oracle Architecture
• Oracle supports the partitioning of tables
– Dividing a table into smaller sub-tables
(partitions) based on a range value.
• What types of tables should be considered
for partitioning?
– Historical data
– Tables that are static except for regularly
appended data.
– Tables with a logical partition column
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Partitions
Oracle Architecture
• Why not define separate tables?
– Eg. the Sales table becomes:
•
•
•
•
SalesQ1y01
SalesQ2y01
SalesQ3y01
SalesQ4O1 and so on...
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Partitions
Oracle Architecture
A sample partitioning
In this case, each partition is a separate tablespace.
image source: Oracle documentation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Views
Oracle Architecture
• We briefly reviewed this.
• Any questions?
Query DBA_VIEWS
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Sequences
Oracle Architecture
• Oracle doesn’t have an autonumber or
identity option for the integer
datatype
• Instead, you define a sequence object
to generate unique sequential
numbers (e.g., for PKs)
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Sequences
Oracle Architecture
• From an application design standpoint,
what is a problem with autonumber
and identity?
– How do you enforce uniqueness across
tables?
– A recent example, Senior project:
wanted to name jpg files same as PK of
tables (Staff, Athletes, and
Journalists). Problem, not unique across
tables.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Sequences
Oracle Architecture
• Why not generate sequences at the
application level?
– processing bottleneck
– Inefficient in large TPS, multi-user
environment
• A sequence object can be used to generate
unique values for a table, across tables, or
for an application.
• What problem does concurrency cause?
• What is the solution?
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Sequences
Copyright © 2001 Harold Pardue, University of South Alabama
Oracle Architecture
Chapter 1 The Oracle Architecture
Synonym
Oracle Architecture
• Essentially an alias to avoid having to
spell out entire qualifier
• Scope is either public or schema
CREATE SYNONYM market
FOR scott.market_research;
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
• What’s the larger issue?
– Data access method
– Reduces size of scan and therefore I/O
– Does not impact formulation of SQL
expressions
• An index is an ordered list of all the
values that reside in a group of one or
more columns at a given time.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
• The index contains two entries:
– The key value (e.g., empno = 7369)
– The ROWID
• The ROWID is a unique address that
specifies the row’s position in the
datafile.
– Object, Block, Row, Datafile
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
ROWID is a
pseudocolumn
Why not use
ROWID as PK?
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Storage Hierarchy
Oracle Architecture
Database
Tablespace
(Logical)
Segment
Extent
OracleBlock
Data file
ROWID
bridges this
gap
(Physical)
An object in
a tablespace
to a row in a
datafile
OS Block
Page
12
Oracle Corporation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
• Oracle automatically creates an index
on the PK of each table
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
• The most common index structure
used by Oracle is a B-tree
– A B-tree is an optimally height-balanced,
multi-way search tree
• What does it mean that Oracle
maintains a shallow tree?
– The maximum number of levels is 4
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
Root node
What order?
Branch node
Leaf node
Oracle corporation
Doubly linked
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Indexes
Oracle Architecture
• Why use a B-tree?
– Relatively uniform access time to
ROWID. Why?
– Automatically stays balanced
– Efficient for both equivalence and range
searches.
• Where emp = 7360
• Where emp > 700 and emp < 1000
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Clusters
Oracle Architecture
• Another access method is clustering
• I/O is reduced by storing data in
clusters that are physically close to
each other.
– For example, Invoice and Invoice Details
– Physical proximity enhances performance
of SQL join statements
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Oracle Architecture
Clusters
image source: Oracle documentation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Storage Hierarchy
Database
Tablespace
(Logical)
Oracle Architecture
The space within a
tablespace is allocated
as segments.
Data file
(Physical)
Segment
Extent
OracleBlock
OS Block
Page
12
Oracle Corporation
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Segments
Oracle Architecture
• Tablespaces are large logical areas
for the creation of objects (e.g.,
tables, views, indexes, etc...
• The space for an object is allocated
in terms of segments.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Segments
Copyright © 2001 Harold Pardue, University of South Alabama
Oracle Architecture
Chapter 1 The Oracle Architecture
Extents
Oracle Architecture
• A segment is comprised of multiple
extents
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Extents
Oracle Architecture
• An extent is a
set of contiguous
data blocks used
to store a
particular type
of information.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Extents
Oracle Architecture
• An initial extent is allocated.
• When an extent becomes full, another
extent from the freespace in the
tablespace is allocated to the
segment.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Data Blocks
Oracle Architecture
• Data blocks are the smallest unit of
I/O for Oracle
• When creating a database, you should
define data blocks to be a multiple of
the OS data blocks.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Database
Oracle Architecture
• So... that’s the basics of the Oracle
architecture from a data storage
perspective.
• But in of itself, there’s not much you
can do with the data.
• Major distinction
– Database versus Instance
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Instance
Oracle Architecture
Why multiple
instances?
Page 12
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
OPS
Oracle Architecture
Oracle Parallel Server
(OPS) enables one
database to be mounted
and opened concurrently
by multiple instances.
High availability
Better scalability
Load balancing
Each OPS instance is
like any standalone
Oracle instance and runs
on a separate node
having its own CPU and
memory.
The database resides on
a disk subsystem shared
by all nodes.
source:
Page
12http://www.oreilly.com/catalog/oraclepp/chapter/ch01.html
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Interfaces
Two interface points
Copyright © 2001 Harold Pardue, University of South Alabama
Oracle Architecture
Most tuning activity
will focus on these
two points.
Chapter 1 The Oracle Architecture
The big picture...
Oracle Architecture
Now let’s look
at the SGA
We’ve looked at this so far...
image source: http://www.windowsitlibrary.com/Content/277/02/1.html
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
• When an instance is started, Oracle
allocates space in memory for shared
processes
• An area in computer memory used to
hold information for a particular
instance
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
System Global Area
Database
buffer
cache
Redo log
buffer
Shared Pool
Library
Cache
Dictionary
Cache
Copyright © 2001 Harold Pardue, University of South Alabama
Cursors
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
• What is the purpose of the SGA?
• Why create all these buffers and
caches?
– Avoid disk I/O
– Users can access, share, and modify data
in memory without the necessity of
continuously reading and writing to disk
storage
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
• Database buffer cache
– Collection of buffers the size of the
database blocks
– Free space managed with LRU algorithm
– CRUD performed in memory, deferred
I/O
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
• Shared Pool
– Library cache stores parsed SQL and execution
plan (reuse)
• text of SQL expression
• parse tree: compiled version
• execution plan
– Dictionary cache stores recently access
dictionary values
• Shared by multiple users
– improves performance in multi-user
environments
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
SGA
Oracle Architecture
• Redo log buffer
– Huh? why a redo?
– Stuff happens...
• The redo buffer stores the
information needed to reconstruct a
transaction in the event of a failure
– This information will eventually be
written to the redo log file(s)
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
PGA
Oracle Architecture
• Program Global Area
• Private storage area for a user’s
process
– Although, e.g., a parsed SQL statement
might be common to multiple users, the
parameters and qualifiers might not.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Processes
Oracle Architecture
Work gets
accomplished
through the user,
server, and
background
processes
image source: http://www.windowsitlibrary.com/Content/277/02/1.html
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
User Processes
Oracle Architecture
• Any program or system designed to
access the Oracle database
– Application program written to access
Oracle database
– An Oracle Tool such as SQL*Plus
• Creates connection
• A specific connection is a session,
e.g., session for SCOTT/TIGER
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Server Processes
Oracle Architecture
• Server processes manage connections
and perform the I/O against the
datafiles
– Parse and execute SQL statements
– Retrieve data blocks from datafiles into
SGA if needed
– Returns results of SQL statements
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Background processes handle all the oddsand-ends (specific tasks) of interacting
with the database
–
–
–
–
–
–
–
–
–
DBWR
LGWR
CKPT
SMON
PMON
ARCH
RECO
Dnnn
LCKn
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Database Write (DBWR)
• Writes modified data from buffers in
cache to datafiles on disk
• When immediately? Deferred?
• When should DBWR write to disk?
– At the conclusion of transaction?
• Checkpoint
• Dirty list if full
• Timeout
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Log Write (LGWR)
• Writes redo information from the
redo buffer to the redo log file
• Because DBWR defers I/O
operations to the datafiles, the
LGWR needs to write to the redo log
file when transaction is committed
– option: group committ or mulitple LGWR
slaves
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Check Point (CKPT)
• The point at which the DBWR writes
all modified database buffers
(buffers on the dirty list) to the data
files
• The dirty list is a list of all dirty
buffers, i.e., buffers that contain
modified data
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• A “Check Point” as an event is when
the data is written. The LGWR
causes a check point
• The CKPT process forces a check
point to occur
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• System Monitor (SMON)
– Checks to see whether recovery is
needed
– Removes temporary or unneeded objects
from the database
– House-cleaning function.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Process Monitor (PMON)
• Watches over the user processes
– Releases locked resources
– cleans up behind failed user processes
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Archiver (ARCH)
• Redo log files will eventually be overwritten
• The archiver archives logs such that
a complete history of all transactions
for the database are maintained.
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
Background Processes
Oracle Architecture
• Recoverer (RECO)
– Attempts to resolve transaction failures
in distributed databases (i.e., SQL
statements issued against tables located
on different machines)
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture
External Structures
Oracle Architecture
• Redo logs
• Control files
• Trace files and alert files
Copyright © 2001 Harold Pardue, University of South Alabama
Chapter 1 The Oracle Architecture