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1 Prentice Hall, 2002

Chapter 6:Physical Database Design

and Performance

M odern Database M anagement

6 th Edition

Jeff rey A. H off er , M ary B. Prescott, Fred R.M cFadden


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Chapter 6 3 Prentice Hall, 2002

Physical Database Design

Purpose - translate the logical descriptionof data into the technical specifications forstoring and retrieving dataGoal - create a design for storing data thatwill provide adequate performance and

insure database integrity , security andrecoverability


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Physical Design Process

Normalized relations

Volume estimates

Attribute definitionsResponse time expectations

Data security needs

Backup/recovery needs

Integrity expectations

DBMS technology used

Inputs

Attribute data types

Physical record descriptions(doesnt always match logicaldesign)

File organizations

Indexes and databasearchitectures

Query optimization

Leads to

Decisions


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Figure 6.1 - Composite usage map(Pine Valley Furniture Company)


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Data volumes


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Access Frequencies(per hour)


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Usage analysis:200 purchased parts accessed per hour 80 quotations accessed fromthese 200 purchased partaccesses

70 suppliers accessed fromthese 80 quotation accesses


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Usage analysis:75 suppliers accessed perhour 40 quotations accessed fromthese 75 supplier accesses 40 purchased parts accessedfrom these 40 quotationaccesses


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Designing Fields

Field: smallest unit of data in

databaseField design Choosing data type Coding, compression, encryption Controlling data integrity


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Choosing Data Types

CHAR fixed-length characterVARCHAR2 variable-length character

(memo)LONG large number

NUMBER positive/negative number

DATE actual dateBLOB binary large object (good forgraphics, sound clips, etc.)


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Figure 6.2Example code-look-up table (Pine Valley Furniture Company)

Code saves space, but costsan additional lookup toobtain actual value.


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Field Data Integrity

Default value - assumed value if no explicitvalue

Range control allowable value limitations(constraints or validation rules) Null value control allowing or prohibitingempty fields

Referential integrity range control (and nullvalue allowances) for foreign-key to primary-key match-ups


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Handling Missing Data

Substitute an estimate of the missing value(e.g. using a formula)

Construct a report listing missing valuesIn programs, ignore missing data unless thevalue is significant

Triggers can be used to perform these operations


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Physical RecordsPhysical Record: A group of fields stored inadjacent memory locations and retrievedtogether as a unitPage: The amount of data read or written inone I/O operationBlocking Factor: The number of physicalrecords per page


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PartitioningHorizontal Partitioning: Distributing the rows of a tableinto several separate files Useful for situations where different users need access to different

rows Three types: Key Range Partitioning, Hash Partitioning, or

Composite Partitioning

Vertical Partitioning: Distributing the columns of a tableinto several separate files Useful for situations where different users need access to different

columns

The primary key must be repeated in each fileCombinations of Horizontal and Vertical

Partitions often correspond with User Schemas (user views)


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Partitioning

Advantages of Partitioning: Records used together are grouped together

Each partition can be optimized for performance Security, recovery Partitions stored on different disks: contention Take advantage of parallel processing capability

Disadvantages of Partitioning: Slow retrievals across partitions Complexity


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Data Replication

Purposely storing the same data in multiplelocations of the database

Improves performance by allowing multipleusers to access the same data at the sametime with minimum contentionSacrifices data integrity due to dataduplicationBest for data that is not updated often


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Designing Physical Files

Physical File: A named portion of secondary memory allocated for

the purpose of storing physical records

Constructs to link two pieces of data: Sequential storage. Pointers.

File Organization: How the files are arranged on the disk.

Access Method: How the data can be retrieved based on the file

organization.


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Figure 6-7 (a)Sequential fileorganization

If not sorted

Average time to finddesired record = n/2.

1

2

n

Records of thefile are stored insequence by the

primary keyfield values.

If sorted every insert ordelete requiresresort


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Indexed File OrganizationsIndex a separate table that contains organizationof records for quick retrievalPrimary keys are automatically indexedOracle has a CREATE INDEX operation, and MSACCESS allows indexes to be created for mostfield typesIndexing approaches: B-tree index, Fig. 6-7b Bitmap index, Fig. 6-8 Hash Index, Fig. 6-7c Join Index, Fig 6-9


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Fig. 6-7b B-tree index

uses a t r e e s e a r c hAverage time to find desiredrecord = depth of the tree

Leaves of the treeare all at samelevel

consistent accesstime


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Fig 6-7cHashed file orindex

organization

Hash algorithm Usually uses division-remainder to determinerecord position. Recordswith same position aregrouped in lists.


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Fig 6-8Bitmap indexindex

organization

Bitmap saves on space requirements Rows - possible values of the attributeColumns - table rowsBit indicates whether the attribute of a row has the values


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Fig 6-9 Join Index speeds up join operations


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Clustering Files

In some relational DBMSs, related records fromdifferent tables can be stored together in the samedisk areaUseful for improving performance of joinoperationsPrimary key records of the main table are stored

adjacent to associated foreign key records of thedependent tablee.g. Oracle has a CREATE CLUSTER command


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Rules for Using Indexes

1. Use on larger tables2. Index the primary key of each table

3. Index search fields (fields frequently inWHERE clause)

4. Fields in SQL ORDER BY and GROUP

BY commands5. When there are >100 values but not when

there are


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Rules for Using Indexes

6. DBMS may have limit on number ofindexes per table and number of bytes perindexed field(s)

7. Null values will not be referenced from anindex8. Use indexes heavily for non-volatile

databases; limit the use of indexes forvolatile databases

Why? Because modifications (e.g. inserts,deletes) require updates to occur in index files


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RAID

Redundant Array of Inexpensive DisksA set of disk drives that appear to the userto be a single disk driveAllows parallel access to data (improvesaccess speed)Pages are arranged in stripes


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Figure 6-10 RAID with four

disks and

striping

Here, pages 1-4can beread/writtensimultaneously


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Raid Types (Figure 6-11)

Raid 0 Maximized parallelism No redundancy No error correction no fault-tolerance

Raid 1 Redundant data fault tolerant Most common form

Raid 2 No redundancy

One record spans across datadisks Error correction in multiple

disks reconstruct damaged data

Raid 3 Error correction in one disk Record spans multiple data disks (more

than RAID2) Not good for multi-user environments,

Raid 4 Error correction in one disk Multiple records per stripe Parallelism, but slow updates due to

error correction contention

Raid 5 Rotating parity array Error correction takes place in same disks as

data storage Parallelism, better performance than Raid4

s


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D a

t a b a s e

A r c

h i t e c

t u r e s

( f i g u r e

6 - 1

2

LegacySystems

Current

Technology

DataWarehouses


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Query Optimization

Parallel Query ProcessingOverride Automatic Query OptimizationData Block Size -- Performance tradeoffs: Block contention Random vs. sequential row access speed

Row size Overhead

Balancing I/O Across Disk Controllers


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Ch 6 36

Query OptimizationWise use of indexesCompatible data typesSimple queriesAvoid query nestingTemporary tables for query groups

Select only needed columns No sort without index

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