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Physical Database Design

40 6706

Database Storage Structures


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Learning Outcomes

Database Architecture

Role of the Database Administrator

General Performance Issues

Examine Oracles StorageStructures


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Physical Design Activities

Designing data structures to improveperformance

Defining recovery policies Designing and implementing

Concurrency Control policies

Tuning applications (SQL statement

tuning) Enroling users & granting them

necessary privileges


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Administrator vs.

Developer The DBA is the sole authority for

allocating system-wide resources

(disk space usage, auditing, etc) However, there are many areas of

overlap in responsibility between

the DBA and Developers


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Activity/User TypeDeveloper DBA

Database Design

Data Structure Definition

Recovery Policy

Design and Implementation of Concurrency

Control Policies

Application Tuning

Enrolment of Users

Who has Primary Responsibility foreach Activity?


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Database Architecture

QueryOptimiser

Data Dictionary

Security Manager

Concurrency ControllerRecovery Manager

b


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Interactions betweenComponents

Lets look at a typical scenario1. Query is parsed

2. Table and column names sent by QO tothe DD

3. DD validates names, sends a message tothe SM

4. SM validates access rights

5. DD now replies to QO6. QO generates a plan for execution andsends messages to CC for obtaining locks

7. As transaction executes it writes

information in the rollback segmentsmaintained b the Recover Mana er


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Performance Issues

Two main factors:1. processing time at the CPU

2. Data retrieval time at the disk Data retrieval time >> Processing

time

Thus major objective is tominimise data retrieval (I/O)costs


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Performance Issues -

Compression Data retrieval costs minimised by

clustering and/or compressingdata

Compression can be achieved byreplacing large data fields by a codingscheme that takes up less space

This will typically be used for a table

with large-sized text fields These fields can be replaced by codes

which reference the original textvalues in a lookup table


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Clustering Oracles

Solution Oracle clusters data at the physical

level into data blocks

Q) What happens when data (table)size > data block size?

A)

This is precisely what Oracle does!


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Oracles StorageStructures-Extents

Oracle organises data blocks that areaccessed together into a singlecontiguous unit called an extent

Thus when creating a table you specifythe size of the extent

When the table size exceeds the size ofthe extent, Oracle obtains another

extent However there is no guarantee that the

next extent is contiguous with theprevious one


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Extents

Oracle allows the designer tospecify the size of the first (initial)

and subsequent (next) extents For a table that would be expected

to grow linearly with time, the size

of the initialand nextextentswould be the same


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Relationships between Data Blocks,Extents and Segments


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Oracles Storage Structures-Segments

Segments are collections of extentsthat belong to one logical unit e.g.a table

In general there four types ofsegments:

1. Table

2. Index3. Rollback (for Recovery)

4. Temporary (to store intermediateresults, e.g. during sorting)


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Oracles Storage Structures-

Tablespaces Tablespace: a database is divided into

logical storage units called tablespaces.

Each tablespace in an ORACLE databaseis comprised of one or more operatingsystem files called data files.

A tablespace's data files physically storethe associated database data on disk


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Tablespaces and Datafiles


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Logical Structure Versus

Physical Structure Logical structures are composed of orderly

groupings of information that allow you to

manipulate and access related data. Theycannot be viewed outside the database.

Physical structures are composed of operating

system components and have a physical nameand location.


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Logical Structure

The logical structure of an Oracledatabase include

Schema objects,

Data blocks,

Extents,

Segments and

Tablespaces


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Physical Database Structure

Every database has one or morephysical datafiles. The Physical

Structure of an Oracle databaseinclude:

Data Files

Redo log FilesControl Files


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Data Files: the datafiles contain all thedatabase data, and can be associated

with only one database. Redo Log files: primary function is to

record all changes made to data. Theinformation in a redo log file is used

only to recover the database from asystem or media failure.


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Control File: contains entries thatspecify the physical structure of

the database, such as:Database name

Names and locations of datafiles and

redo log files andTime stamp of database creation.


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Database

Logical Physica l

Tablespace Data file

OS blockOracle

b lock

Segment

Extent

Database Storage Hierarchy


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Using Tablespaces

Oracle uses a SYSTEM tablespaceto store internal information suchas the Data Dictionary

User tables can also be stored inSYSTEM but this is notrecommended

Mutiple tablespaces have theseadvantages: gives users betterperformance and flexibility


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Advantages of MultipleTablespaces

1. Control over space quotas for users

2. Better control over data availability bytaking individual tablespaces online or

offline3. Perform partial database backup or

recovery

4. Parallelise Queries by partitioning

tables and allocating them to separatetablespaces

5. Reduce contention between objectsby allocating them to separate

tablespaces


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Controlling Database Size

Tablespaces can be used tocontrol database size in three

ways:1. Adding a new tablespace

2. Adding a new datafile to an existingtablespace

3. Allowing datafiles to growdynamically

Choice made by DBA depending

on circumstances


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Oracles Space Management

Oracle maintains a list of free

blocks for each tablespace When a new extent (N blocks) is

required, Oracle uses the following

algorithm:1. Search for N+1 contiguous blocks (N, if N N

If M < N+5, then ALL blocks are allocated to new extent

If M > N+5, allocate N to new extent and return remainder to

free list

3. If neither 1 or 2 applies, Oracle coalesces adjacent blocks

(used) to release space and obtain more free blocks


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References

Modern Database Management fifth edition (Hoffer, Prescott and

McFadden) Oracle 11g Database Concepts

Chapters 2, 3 & 7

Oracle 11g Administrators GuideChapter 8


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Review Questions

1. List the main activities involved inPhysical Database Design

2. Distinguish between clusteringand compression

3. How does Oracle ensure that datain tables are clustered?

4. Identify four advantages of usingtablespaces

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