physical database design transparencies. ©pearson education 2009 chapter 11 - objectives purpose of...

Physical Database DesignTransparencies

©Pearson Education 2009

Chapter 11 - ObjectivesPurpose of physical database design.How to map the logical database design to a

physical database design.How to design base tables for target DBMS.How to design representation of derived

data.How to design integrity constraints for the

target DBMS.How to analyze the users’ transactions to

identify characteristics that may impact on performance.

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Chapter 11 - ObjectivesThat a database represents an essential

corporate resources that must be made secure.

The meaning of denormalization.About the importance of monitoring and

tuning the operational system.How to measure efficiency.

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Logical vs Physical Database Design Logical db design independent of

implementation details, such as functionality of target DBMS.

Logical db design concerned with the what, physical database design is concerned with the how.

Sources of information for physical design includes logical data model and data dictionary.

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Physical Database DesignProcess of producing a description of implementation of the database on secondary storage.It describes base tables, file organizations, and indexes used to achieve efficient access to the data, and any associated integrity constraints and security restrictions.

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Overview of Physical Database Design Methodology

Step 3 Translate logical database design for target DBMS

Step 4 Choose file organizations and indexes

Step 5 Design user viewsStep 6 Design security mechanismsStep 7 Consider introduction of controlled

redundancyStep 8 Monitor and tune operational system

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Step 3 Translate logical database design for target DBMS

To produce a basic working relational database from the logical data model.

Consists of the following steps:Step 3.1 Design base tablesStep 3.2 Design representation of

derived data Step 3.3 Design remaining integrity

constraints

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Step 3 Translate logical database design for target DBMS

Need to know functionality of target DBMS such as how to create base tables and whether DBMS supports the definition of:PKs, FKs, and AKs;required data – i.e. whether system

supports NOT NULL;domains;relational integrity rules;other integrity constraints.

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Step 3.1 Design base tables

To decide how to represent base tables identified in logical model in target DBMS.

Need to collate and assimilate the information about tables produced during logical database design (from data dictionary and tables defined in DBDL).

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Step 3.1 Design base tablesFor each table, need to define:

name of the table;list of simple columns in brackets;PK and, where appropriate, AKs and FKs.referential integrity constraints for any FKs

identified.For each column, need to define:

its domain, consisting of a data type, length, and any constraints on the domain;

optional default value for the column;whether column can hold nulls;whether column is derived.

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DBDL for the Branch table

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Step 3.2 Design representation of derived data

To design the representation of derived data in the database.

Produce list of all derived columns from logical data model and data dictionary.

Derived column can be stored in database or calculated every time it is needed.

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Step 3.2 Design representation of derived dataOption selected is based on:

additional cost to store the derived data and keep it consistent with data from which it is derived;

cost to calculate it each time it’s required.

Less expensive option is chosen subject to performance constraints.

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Step 3.3 Design remaining integrity constraintsTo design the remaining integrity constraints for the target DBMS.

Some DBMS provide more facilities than others for defining integrity constraints. Example:

CONSTRAINT member_not_renting_too_manyCHECK (NOT EXISTS (SELECT memberNo

FROM RentalDelivery r. DVDRental dWHERE r.deliveryNo=d.deliveryNo AND

dateReturn IS NULL GROUP BY memberNoHAVING COUNT(*) > 5))

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Step 4 Choose file organizations/indexes

Determine optimal file organizations to store the base tables, and the indexes required to achieve acceptable performance.

Consists of the following steps:Step 4.1 Analyze transactionsStep 4.2 Choose file organizations Step 4.3 Choose indexes

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Step 4.1 Analyze transactions To analyze the functionality of the transactions that will run on the database to identify those that may impact performance.

Identify performance criteria, such as:transactions that run frequently and will

have a significant impact on performance;transactions that are critical to the business;times during the day/week when there will

be a high demand made on the database (called the peak load).

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Step 4.1 Analyze transactionsUse this information to identify the parts of the

database that may cause performance problems.

To select appropriate file organizations and indexes, also need to know high-level functionality of the transactions, such as:columns that are updated in an update

transaction; criteria used to restrict records that are

retrieved in a query.One way to do this is to use the transaction

pathway diagram created in Step 1.8.

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Step 4.1 Analyze transactionsTo focus on areas that may be

problematic:

(1) Map all transaction paths to tables.

(2) Determine which tables are most frequently accessed by transactions.

(3) Analyze the data usage of selected transactions that involve these tables.

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Transaction pathway diagram (expected daily occurrences added)

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Step 4.1 Analyze transactionsFor each transaction determine:

(a) Tables and columns accessed and type of access.

(b) Columns used in any search conditions.

(c) For query, columns involved in joins.(d) Expected frequency of transaction.(e) Performance goals of transaction.

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Step 4.2 Choose file organizations To determine an efficient file organization for each base table.

File organizations include Heap, Hash, Indexed Sequential Access Method (ISAM), B+-Tree, and Clusters.

Some DBMSs (particularly PC-based DBMS) have fixed file organization that you cannot alter.

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Step 4.3 Choose indexes Determine whether adding indexes will improve the performance of the system.

One approach is to keep records unordered and create as many secondary indexes as necessary.

Have to balance overhead in maintenance and use of secondary indexes against performance improvement gained when retrieving data.

This includes: adding an index record to every secondary index whenever

record is inserted; updating a secondary index when corresponding record is

updated; increase in disk space needed to store the secondary index; possible performance degradation during query optimization

to consider all secondary indexes.

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Step 4.3 Choose indexes – Guidelines for choosing ‘wish-list’

(1) Do not index small tables. (2) Index PK of a table if it is not a key of the file

organization. (3) Add secondary index to any column that is

heavily used as a secondary key. (4) Add secondary index to a FK if it is frequently

accessed.(5) Add secondary index on columns that are

involved in: selection or join criteria; ORDER BY; GROUP BY; and other operations involving sorting (such as UNION or DISTINCT).

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Step 4.3 Choose indexes – Guidelines for choosing ‘wish-list’

(6) Add secondary index on columns involved in built-in functions.

(7) Add secondary index on columns that could result in an index-only plan.

(8) Avoid indexing an column or table that is frequently updated.

(9) Avoid indexing an column if the query will retrieve a significant proportion of the records in the table.

(10) Avoid indexing columns that consist of long character strings.

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Step 5 Design User Views Design user views identified during Requirements Collection and Analysis stage of the database system development lifecycle.

Normally views created using SQL or a QBE-like facility. For example:

CREATE VIEW Staff1ViewAS SELECT staffNo, name, position,

eMailFROM StaffWHERE dCenterNo = ‘D001’;

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Step 6 Design Security Measures Design security measures for the database as specified by the users.

RDBMSs generally provide two types of database security:system security: access and use of database

at system level (such as username/password);

data security: access and use of database objects (such as tables and views).

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Step 6 Design Security Measures - SQLEach database user assigned an

authorization identifier by DBA (usually has an associated password).

Each object created has an owner.Privileges are the actions that a user is

permitted to carry out on a given base table or view (such as SELECT, UPDATE).

GRANT statement allows owner to give privileges to other users.

REVOKE statement takes privileges away.

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Step 7 Consider Introduction of Controlled Redundancy

Determine whether introducing redundancy in a controlled manner by relaxing the normalization rules will improve system performance.

Normalization results in a logical database design that is structurally consistent and has minimal redundancy.

However, sometimes a normalized database design does not provide maximum processing efficiency.

May be necessary to accept loss of some of the benefits of a fully normalized design in favor of performance.

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Denormalization Refinement to relational schema such that

the degree of normalization for a modified table is less than the degree of at least one of the original tables.

Also use term more loosely to refer to situations where two tables are combined into one new table, which is still normalized but contains more nulls than original tables.

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Step 7 Consider Introduction of Controlled Redundancy

Also consider that denormalization:makes implementation more complex;often sacrifices flexibility;may speed up retrievals but it slows down

updates.

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Step 8 Monitor and tune the operational system

To monitor operational system and improve performance of system to correct inappropriate design decisions or reflect changing requirements.

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Step 8 Monitor and tune the operational system

Number of factors that may be used to measure efficiency:

- Transaction throughput: number of transactions processed in given time interval.

- Response time: elapsed time for completion of a single transaction.

- Disk storage: amount of disk space required to store database files.

No one factor is always correct. Typically, have to trade one factor off against another to achieve a reasonable balance.

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Step 8 Monitor and tune the operational system

Must be aware how 4 main hardware components interact and affect performance:main memoryCPUdisk I/Onetwork.

Note that an improvement in one resource may effect an improvement in other resources.

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Step 8 Monitor and tune the operational system

Main memory:Significantly faster than secondary storage.More main memory, faster applications will

run.Should always have between 5%-10% main

memory available.Need to understand how target DBMS uses

main memory.

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Step 8 Monitor and tune the operational system

CPU:Controls tasks of other system resources and

executes user processes.Most costly resource, so needs to be correctly

utilized.Want to prevent CPU contention (processes

waiting for CPU).Need to understand typical 24-hour workload

and ensure sufficient resources available for both normal and peak workload.

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Step 8 Monitor and tune the operational system

Disk I/O:With any large DBMS, significant amount of

disk I/O.Need to carefully design how data is

organized on disk.

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Step 8 Monitor and tune the operational system

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