Database Management systems phần 10 pdf

func-The transaction processing capabilities of database systems are improving continually.For example, many vendors offer distributed DBMS products today in which a transac-tion can exe

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developing transactions with the ACID properties In addition to providing a uniforminterface to the services of different resource managers, a TP monitor also routestransactions to the appropriate resource managers Finally, a TP monitor ensures that

an application behaves as a transaction by implementing concurrency control, logging,and recovery functions, and by exploiting the transaction processing capabilities of theunderlying resource managers

TP monitors are used in environments where applications require advanced featuressuch as access to multiple resource managers; sophisticated request routing (also called

workflow management); assigning priorities to transactions and doing

priority-based load-balancing across servers; and so on A DBMS provides many of the tions supported by a TP monitor in addition to processing queries and database up-dates efficiently A DBMS is appropriate for environments where the wealth of trans-action management capabilities provided by a TP monitor is not necessary and, inparticular, where very high scalability (with respect to transaction processing activ-ity) and interoperability are not essential

func-The transaction processing capabilities of database systems are improving continually.For example, many vendors offer distributed DBMS products today in which a transac-tion can execute across several resource managers, each of which is a DBMS Currently,all the DBMSs must be from the same vendor; however, as transaction-oriented servicesfrom different vendors become more standardized, distributed, heterogeneous DBMSsshould become available Eventually, perhaps, the functions of current TP monitorswill also be available in many DBMSs; for now, TP monitors provide essential infras-tructure for high-end transaction processing environments

28.1.2 New Transaction Models

Consider an application such as computer-aided design, in which users retrieve largedesign objects from a database and interactively analyze and modify them Eachtransaction takes a long time—minutes or even hours, whereas the TPC benchmarktransactions take under a millisecond—and holding locks this long affects performance.Further, if a crash occurs, undoing an active transaction completely is unsatisfactory,since considerable user effort may be lost Ideally we want to be able to restore most

of the actions of an active transaction and resume execution Finally, if several usersare concurrently developing a design, they may want to see changes being made byothers without waiting until the end of the transaction that changes the data

To address the needs of long-duration activities, several refinements of the transactionconcept have been proposed The basic idea is to treat each transaction as a collection

of related subtransactions Subtransactions can acquire locks, and the changes made

by a subtransaction become visible to other transactions after the subtransaction ends

(and before the main transaction of which it is a part commits) In multilevel

trans-824 Chapter 28

actions, locks held by a subtransaction are released when the subtransaction ends.

In nested transactions, locks held by a subtransaction are assigned to the parent

(sub)transaction when the subtransaction ends These refinements to the transactionconcept have a significant effect on concurrency control and recovery algorithms

be placed A soft deadline means the value of the transaction decreases after the

deadline, eventually going to zero For example, in a DBMS designed to monitor someactivity (e.g., a complex reactor), a transaction that looks up the current reading of asensor must be executed within a short time, say, one second The longer it takes toexecute the transaction, the less useful the reading becomes In a real-time DBMS, thegoal is to maximize the value of executed transactions, and the DBMS must prioritizetransactions, taking their deadlines into account

28.2 INTEGRATED ACCESS TO MULTIPLE DATA SOURCES

As databases proliferate, users want to access data from more than one source Forexample, if several travel agents market their travel packages through the Web, cus-tomers would like to look at packages from different agents and compare them Amore traditional example is that large organizations typically have several databases,created (and maintained) by different divisions such as Sales, Production, and Pur-chasing While these databases contain much common information, determining theexact relationship between tables in different databases can be a complicated prob-lem For example, prices in one database might be in dollars per dozen items, whileprices in another database might be in dollars per item The development of XML

DTDs (see Section 22.3.3) offers the promise that such semantic mismatches can be

avoided if all parties conform to a single standard DTD However, there are manylegacy databases and most domains still do not have agreed-upon DTDs; the problem

of semantic mismatches will be frequently encountered for the foreseeable future.Semantic mismatches can be resolved and hidden from users by defining relationalviews over the tables from the two databases Defining a collection of views to give

a group of users a uniform presentation of relevant data from multiple databases is

called semantic integration Creating views that mask semantic mismatches in a

natural manner is a difficult task and has been widely studied In practice, the task

is made harder by the fact that the schemas of existing databases are often poorly

in Chapter 5 Alternatively, the integrating views can be materialized and stored in

a data warehouse, as discussed in Chapter 23 Queries can then be executed over thewarehoused data without accessing the source DBMSs at run-time

28.3 MOBILE DATABASES

The availability of portable computers and wireless communications has created a newbreed of nomadic database users At one level these users are simply accessing adatabase through a network, which is similar to distributed DBMSs At another levelthe network as well as data and user characteristics now have several novel properties,which affect basic assumptions in many components of a DBMS, including the queryengine, transaction manager, and recovery manager;

Users are connected through a wireless link whose bandwidth is ten times lessthan Ethernet and 100 times less than ATM networks Communication costs aretherefore significantly higher in proportion to I/O and CPU costs

Users’ locations are constantly changing, and mobile computers have a limitedbattery life Therefore, the true communication costs reflect connection time andbattery usage in addition to bytes transferred, and change constantly depending

on location Data is frequently replicated to minimize the cost of accessing it fromdifferent locations

As a user moves around, data could be accessed from multiple database serverswithin a single transaction The likelihood of losing connections is also muchgreater than in a traditional network Centralized transaction management maytherefore be impractical, especially if some data is resident at the mobile comput-ers We may in fact have to give up on ACID transactions and develop alternativenotions of consistency for user programs

The price of main memory is now low enough that we can buy enough main memory

to hold the entire database for many applications; with 64-bit addressing, modernCPUs also have very large address spaces Some commercial systems now have several

gigabytes of main memory This shift prompts a reexamination of some basic DBMS

a bottleneck To minimize this problem, rather than commit each transaction as

it completes, we can collect completed transactions and commit them in batches;

this is called group commit Recovery algorithms can also be optimized since

pages rarely have to be written out to make room for other pages

The implementation of in-memory operations has to be optimized carefully sincedisk accesses are no longer the limiting factor for performance

A new criterion must be considered while optimizing queries, namely the amount

of space required to execute a plan It is important to minimize the space overheadbecause exceeding available physical memory would lead to swapping pages to disk(through the operating system’s virtual memory mechanisms), greatly slowingdown execution

Page-oriented data structures become less important (since pages are no longer theunit of data retrieval), and clustering is not important (since the cost of accessingany region of main memory is uniform)

In an object-relational DBMS, users can define ADTs with appropriate methods, which

is an improvement over an RDBMS Nonetheless, supporting just ADTs falls short of

what is required to deal with very large collections of multimedia objects, including

audio, images, free text, text marked up in HTML or variants, sequence data, andvideos Illustrative applications include NASA’s EOS project, which aims to create arepository of satellite imagery, the Human Genome project, which is creating databases

of genetic information such as GenBank, and NSF/DARPA’s Digital Libraries project,which aims to put entire libraries into database systems and then make them accessiblethrough computer networks Industrial applications such as collaborative development

of engineering designs also require multimedia database management, and are beingaddressed by several vendors

We outline some applications and challenges in this area:

Content-based retrieval: Users must be able to specify selection conditions

based on the contents of multimedia objects For example, users may search forimages using queries such as: “Find all images that are similar to this image” and

“Find all images that contain at least three airplanes.” As images are inserted into

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the database, the DBMS must analyze them and automatically extract features

that will help answer such content-based queries This information can then beused to search for images that satisfy a given query, as discussed in Chapter 26

As another example, users would like to search for documents of interest usinginformation retrieval techniques and keyword searches Vendors are moving to-wards incorporating such techniques into DBMS products It is still not clear howthese domain-specific retrieval and search techniques can be combined effectivelywith traditional DBMS queries Research into abstract data types and ORDBMSquery processing has provided a starting point, but more work is needed

Managing repositories of large objects: Traditionally, DBMSs have

concen-trated on tables that contain a large number of tuples, each of which is relativelysmall Once multimedia objects such as images, sound clips, and videos are stored

in a database, individual objects of very large size have to be handled efficiently.For example, compression techniques must be carefully integrated into the DBMSenvironment As another example, distributed DBMSs must develop techniques

to efficiently retrieve such objects Retrieval of multimedia objects in a distributedsystem has been addressed in limited contexts, such as client-server systems, but

in general remains a difficult problem

Video-on-demand: Many companies want to provide video-on-demand services

that enable users to dial into a server and request a particular video The videomust then be delivered to the user’s computer in real time, reliably and inex-pensively Ideally, users must be able to perform familiar VCR functions such asfast-forward and reverse From a database perspective, the server has to contendwith specialized real-time constraints; video delivery rates must be synchronized

at the server and at the client, taking into account the characteristics of the munication network

Geographic Information Systems (GIS) contain spatial information about cities,

states, countries, streets, highways, lakes, rivers, and other geographical features, andsupport applications to combine such spatial information with non-spatial data Asdiscussed in Chapter 26, spatial data is stored in either raster or vector formats Inaddition, there is often a temporal dimension, as when we measure rainfall at severallocations over time An important issue with spatial data sets is how to integrate datafrom multiple sources, since each source may record data using a different coordinatesystem to identify locations

Now let us consider how spatial data in a GIS is analyzed Spatial information is mostnaturally thought of as being overlaid on maps Typical queries include “What citieslie on I-94 between Madison and Chicago?” and “What is the shortest route fromMadison to St Louis?” These kinds of queries can be addressed using the techniques

In addition, many applications involve interpolating measurements at certain locations

across an entire region to obtain a model, and combining overlapping models For

ex-ample, if we have measured rainfall at certain locations, we can use the TIN approach

to triangulate the region with the locations at which we have measurements being thevertices of the triangles Then, we use some form of interpolation to estimate therainfall at points within triangles Interpolation, triangulation, map overlays, visual-izations of spatial data, and many other domain-specific operations are supported inGIS products such as ESRI Systems’ ARC-Info Thus, while spatial query processingtechniques as discussed in Chapter 26 are an important part of a GIS product, con-siderable additional functionality must be incorporated as well How best to extendORDBMS systems with this additional functionality is an important problem yet to

be resolved Agreeing upon standards for data representation formats and coordinatesystems is another major challenge facing the field

Currently available DBMSs provide little support for queries over ordered collections

of records, or sequences, and over temporal data Typical sequence queries include

“Find the weekly moving average of the Dow Jones Industrial Average,” and “Find thefirst five consecutively increasing temperature readings” (from a trace of temperatureobservations) Such queries can be easily expressed and often efficiently executed bysystems that support query languages designed for sequences Some commercial SQLsystems now support such SQL extensions

The first example is also a temporal query However, temporal queries involve morethan just record ordering For example, consider the following query: “Find the longestinterval in which the same person managed two different departments.” If the period

during which a given person managed a department is indicated by two fields from and

to, we have to reason about a collection of intervals, rather than a sequence of records.

Further, temporal queries require the DBMS to be aware of the anomalies associatedwith calendars (such as leap years) Temporal extensions are likely to be incorporated

in future versions of the SQL standard

A distinct and important class of sequence data consists of DNA sequences, which arebeing generated at a rapid pace by the biological community These are in fact closer

to sequences of characters in text than to time sequences as in the above examples.The field of biological information management and analysis has become very popular

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in recent years, and is called bioinformatics Biological data, such as DNA sequence

data, is characterized by complex structure and numerous relationships among dataelements, many overlapping and incomplete or erroneous data fragments (because ex-perimentally collected data from several groups, often working on related problems,

is stored in the databases), a need to frequently change the database schema itself as

new kinds of relationships in the data are discovered, and the need to maintain severalversions of data for archival and reference

28.8 INFORMATION VISUALIZATION

As computers become faster and main memory becomes cheaper, it becomes ingly feasible to create visual presentations of data, rather than just text-based reports.Data visualization makes it easier for users to understand the information in largecomplex datasets The challenge here is to make it easy for users to develop visualpresentation of their data and to interactively query such presentations Although anumber of data visualization tools are available, efficient visualization of large datasetspresents many challenges

increas-The need for visualization is especially important in the context of decision support;when confronted with large quantities of high-dimensional data and various kinds ofdata summaries produced by using analysis tools such as SQL, OLAP, and data miningalgorithms, the information can be overwhelming Visualizing the data, together withthe generated summaries, can be a powerful way to sift through this information andspot interesting trends or patterns The human eye, after all, is very good at findingpatterns A good framework for data mining must combine analytic tools to processdata, and bring out latent anomalies or trends, with a visualization environment inwhich a user can notice these patterns and interactively drill down to the original datafor further analysis

The database area continues to grow vigorously, both in terms of technology and interms of applications The fundamental reason for this growth is that the amount ofinformation stored and processed using computers is growing rapidly Regardless ofthe nature of the data and its intended applications, users need database managementsystems and their services (concurrent access, crash recovery, easy and efficient query-ing, etc.) as the volume of data increases As the range of applications is broadened,however, some shortcomings of current DBMSs become serious limitations Theseproblems are being actively studied in the database research community

The coverage in this book provides a good introduction, but is not intended to coverall aspects of database systems Ample material is available for further study, as this

intro-Determining which entities are the same across different databases is a difficult problem;

it is an example of a semantic mismatch Resolving such mismatches has been addressed

in many papers, including [362, 412, 558, 576] [329] is an overview of theoretical work inthis area Also see the bibliographic notes for Chapter 21 for references to related work onmultidatabases, and see the notes for Chapter 2 for references to work on view integration.[260] is an early paper on main memory databases [345, 89] describe the Dali main memorystorage manager [359] surveys visualization idioms designed for large databases, and [291]discusses visualization for data mining

Visualization systems for databases include DataSpace [515], DEVise [424], IVEE [23], theMineset suite from SGI, Tioga [27], and VisDB [358] In addition, a number of general toolsare available for data visualization

Querying text repositories has been studied extensively in information retrieval; see [545] for

a recent survey This topic has generated considerable interest in the database communityrecently because of the widespread use of the Web, which contains many text sources Inparticular, HTML documents have some structure if we interpret links as edges in a graph.Such documents are examples of semistructured data; see [2] for a good overview Recentpapers on queries over the Web include [2, 384, 457, 493]

See [501] for a survey of multimedia issues in database management There has been muchrecent interest in database issues in a mobile computing environment, for example, [327, 337].See [334] for a collection of articles on this subject [639] contains several articles that coverall aspects of temporal databases The use of constraints in databases has been activelyinvestigated in recent years; [356] is a good overview Geographic Information Systems havealso been studied extensively; [511] describes the Paradise system, which is notable for itsscalability

The book [695] contains detailed discussions of temporal databases (including the TSQL2language, which is influencing the SQL standard), spatial and multimedia databases, anduncertainty in databases Another SQL extension to query sequence data, called SRQL, isproposed in [532]

A DATABASE DESIGN CASE STUDY: THE INTERNET SHOP

Advice for software developers and horse racing enthusiasts: Avoid hacks

—Anonymous

We now present an illustrative, ‘cradle-to-grave’ design example DBDudes Inc., awell-known database consulting firm, has been called in to help Barns and Nobble(B&N) with their database design and implementation B&N is a large bookstorespecializing in books on horse racing, and they’ve decided to go online DBDudes firstverify that B&N is willing and able to pay their steep fees and then schedule a lunchmeeting—billed to B&N, naturally—to do requirements analysis

The owner of B&N has thought about what he wants and offers a concise summary:

“I would like my customers to be able to browse my catalog of books and to place ordersover the Internet Currently, I take orders over the phone I have mostly corporatecustomers who call me and give me the ISBN number of a book and a quantity Ithen prepare a shipment that contains the books they have ordered If I don’t haveenough copies in stock, I order additional copies and delay the shipment until the newcopies arrive; I want to ship a customer’s entire order together My catalog includesall the books that I sell For each book, the catalog contains its ISBN number, title,author, purchase price, sales price, and the year the book was published Most of mycustomers are regulars, and I have records with their name, address, and credit cardnumber New customers have to call me first and establish an account before they canuse my Web site

On my new Web site, customers should first identify themselves by their unique tomer identification number Then they should be able to browse my catalog and toplace orders online.”

cus-DBDudes’s consultants are a little surprised by how quickly the requirements phasewas completed—it usually takes them weeks of discussions (and many lunches anddinners) to get this done—but return to their offices to analyze this information

831

832 Appendix A

In the conceptual design step, DBDudes develop a high level description of the data

in terms of the ER model Their initial design is shown in Figure A.1 Books andcustomers are modeled as entities and are related through orders that customers place.Orders is a relationship set connecting the Books and Customers entity sets For eachorder, the following attributes are stored: quantity, order date, and ship date As soon

as an order is shipped, the ship date is set; until then the ship date is set to null,

indicating that this order has not been shipped yet

DBDudes has an internal design review at this point, and several questions are raised

To protect their identities, we will refer to the design team leader as Dude 1 and thedesign reviewer as Dude 2:

Dude 2: What if a customer places two orders for the same book on the same day? Dude 1: The first order is handled by creating a new Orders relationship and the second

order is handled by updating the value of the quantity attribute in this relationship

Dude 2: What if a customer places two orders for different books on the same day? Dude 1: No problem Each instance of the Orders relationship set relates the customer

to a different book

Dude 2: Ah, but what if a customer places two orders for the same book on different

days?

Dude 1: We can use the attribute order date of the orders relationship to distinguish

the two orders

Dude 2: Oh no you can’t The attributes of Customers and Books must jointly contain

a key for Orders So this design does not allow a customer to place orders for the samebook on different days

Dude 1: Yikes, you’re right Oh well, B&N probably won’t care; we’ll see.

DBDudes decides to proceed with the next phase, logical database design

Using the standard approach discussed in Chapter 3, DBDudes maps the ER diagramshown in Figure A.1 to the relational model, generating the following tables:

CREATE TABLE Books ( isbn CHAR(10),

Design Case Study: An Internet Shop 833

isbn

title price

year_published

qty_in_stock author

cardnum cid

Customers

address cname

order_date ship_date qty

Orders Books

Figure A.1 ER Diagram of the Initial Design

CREATE TABLE Orders ( isbn CHAR(10),

order date DATE,ship date DATE,PRIMARY KEY (isbn,cid),FOREIGN KEY (isbn) REFERENCES Books,FOREIGN KEY (cid) REFERENCES Customers )

CREATE TABLE Customers ( cid INTEGER,

cname CHAR(80),address CHAR(200),cardnum CHAR(16),PRIMARY KEY (cid)UNIQUE (cardnum))

The design team leader, who is still brooding over the fact that the review exposed

a flaw in the design, now has an inspiration The Orders table contains the field

order date and the key for the table contains only the fields isbn and cid Because of

this, a customer cannot order the same book on different days, a restriction that was

not intended Why not add the order date attribute to the key for the Orders table?

This would eliminate the unwanted restriction:

CREATE TABLE Orders ( isbn CHAR(10),

PRIMARY KEY (isbn,cid,ship date), )

The reviewer, Dude 2, is not entirely happy with this solution, which he calls a ‘hack’

He points out that there is no natural ER diagram that reflects this design, and stresses

834 Appendix A

the importance of the ER diagram as a design document Dude 1 argues that whileDude 2 has a point, it is important to present B&N with a preliminary design and getfeedback; everyone agrees with this, and they go back to B&N

The owner of B&N now brings up some additional requirements that he did not mentionduring the initial discussions: “Customers should be able to purchase several differentbooks in a single order For example, if a customer wants to purchase three copies of

‘The English Teacher’ and two copies of ‘The Character of Physical Law,’ the customershould be able to place a single order for both books.”

The design team leader, Dude 1, asks how this affects the shippping policy Does B&Nstill want to ship all books in an order together? The owner of B&N explains theirshipping policy: “As soon as we have have enough copies of an ordered book we ship

it, even if an order contains several books So it could happen that the three copies

of ‘The English Teacher’ are shipped today because we have five copies in stock, butthat ‘The Character of Physical Law’ is shipped tomorrow, because we currently haveonly one copy in stock and another copy arrives tomorrow In addition, my customerscould place more than one order per day, and they want to be able to identify theorders they placed.”

The DBDudes team thinks this over and identifies two new requirements: first, itmust be possible to order several different books in a single order, and second, acustomer must be able to distinguish between several orders placed the same day Toaccomodate these requirements, they introduce a new attribute into the Orders table

called ordernum, which uniquely identifies an order and therefore the customer placing the order However, since several books could be purchased in a single order, ordernum and isbn are both needed to determine qty and ship date in the Orders table.

Orders are assigned order numbers sequentially and orders that are placed later havehigher order numbers If several orders are placed by the same customer on a singleday, these orders have different order numbers and can thus be distinguished TheSQL DDL statement to create the modified Orders table is given below:

CREATE TABLE Orders ( ordernum INTEGER,

Design Case Study: An Internet Shop 835

Next, DBDudes analyzes the set of relations for possible redundancy The Books

rela-tion has only one key (isbn), and no other funcrela-tional dependencies hold over the table Thus, Books is in BCNF The Customers relation has the key (cid), and since a credit card number uniquely identifies its card holder, the functional dependency cardnum → cid also holds Since cid is a key, cardnum is also a key No other dependencies hold,

and so Customers is also in BCNF

DBDudes has already identified the pair hordernum, isbni as the key for the Orders

table In addition, since each order is placed by one customer on one specific date, thefollowing two functional dependencies hold:

ordernum → cid, and ordernum → order date

The experts at DBDudes conclude that Orders is not even in 3NF (Can you see why?)They decide to decompose Orders into the following two relations:

Orders(ordernum, cid, order date, and

Orderlists(ordernum, isbn, qty, ship date)

The resulting two relations, Orders and Orderlists, are both in BCNF, and the

decom-position is lossless-join since ordernum is a key for (the new) Orders The reader is

invited to check that this decomposition is also dependency-preserving For ness, we give the SQL DDL for the Orders and Orderlists relations below:

complete-CREATE TABLE Orders ( ordernum INTEGER,

order date DATE,PRIMARY KEY (ordernum),FOREIGN KEY (cid) REFERENCES Customers )

CREATE TABLE Orderlists ( ordernum INTEGER,

ship date DATE,PRIMARY KEY (ordernum, isbn),FOREIGN KEY (isbn) REFERENCES Books)

Figure A.2 shows an updated ER diagram that reflects the new design Note thatDBDudes could have arrived immediately at this diagram if they had made Orders anentity set instead of a relationship set right at the beginning But at that time they didnot understand the requirements completely, and it seemed natural to model Orders

cardnum cid

Customers

address cname

Orders Books

qty ship_date order_date Order_List Place_Order

ordernum

Figure A.2 ER Diagram Reflecting the Final Design

as a relationship set This iterative refinement process is typical of real-life databasedesign processes As DBDudes has learned over time, it is rare to achieve an initialdesign that is not changed as a project progresses

The DBDudes team celebrates the successful completion of logical database design andschema refinement by opening a bottle of champagne and charging it to B&N Afterrecovering from the celebration, they move on to the physical design phase

A.5 PHYSICAL DATABASE DESIGN

Next, DBDudes considers the expected workload The owner of the bookstore expectsmost of his customers to search for books by ISBN number before placing an order.Placing an order involves inserting one record into the Orders table and inserting one

or more records into the Orderlists relation If a sufficient number of books is available,

a shipment is prepared and a value for the ship date in the Orderlists relation is set In

addition, the available quantities of books in stocks changes all the time since ordersare placed that decrease the quantity available and new books arrive from suppliersand increase the quantity available

The DBDudes team begins by considering searches for books by ISBN Since isbn is

a key, an equality query on isbn returns at most one record Thus, in order to speed

up queries from customers who look for books with a given ISBN, DBDudes decides

to build an unclustered hash index on isbn.

Next, they consider updates to book quantities To update the qty in stock value for

a book, we must first search for the book by ISBN; the index on isbn speeds this

up Since the qty in stock value for a book is updated quite frequently, DBDudes also

considers partitioning the Books relation vertically into the following two relations:

Design Case Study: An Internet Shop 837

BooksQty(isbn, qty), and

BookRest(isbn, title, author, price, year published).

Unfortunately, this vertical partition would slow down another very popular query:Equality search on ISBN to retrieve full information about a book would require ajoin between BooksQty and BooksRest So DBDudes decide not to vertically partitionBooks

DBDudes thinks it is likely that customers will also want to search for books by title

and by author, and decides to add unclustered hash indexes on title and author—these

indexes are inexpensive to maintain because the set of books is rarely changed eventhough the quantity in stock for a book changes often

Next, they consider the Customers relation A customer is first identified by the uniquecustomer identifaction number Thus, the most common queries on Customers areequality queries involving the customer identification number, and DBDudes decides

to build a clustered hash index on cid to achieve maximum speedup for this query.

Moving on to the Orders relation, they see that it is involved in two queries: insertion

of new orders and retrieval of existing orders Both queries involve the ordernum

attribute as search key and so they decide to build an index on it What type ofindex should this be—a B+ tree or a hash index? Since order numbers are assigned

sequentially and thus correspond to the order date, sorting by ordernum effectively

sorts by order date as well Thus DBDudes decides to build a clustered B+ tree index

on ordernum Although the operational requirements that have been mentioned until

know favor neither a B+ tree nor a hash index, B&N will probably want to monitordaily activities, and the clustered B+ tree is a better choice for such range queries Ofcourse, this means that retrieving all orders for a given customer could be expensive

for customers with many orders, since clustering by ordernum precludes clustering by other attributes, such as cid.

The Orderlists relation mostly involves insertions, with an occasional update of ashipment date or a query to list all components of a given order If Orderlists is kept

sorted on ordernum, all insertions are appends at the end of the relation and thus very efficient A clustered B+ tree index on ordernum maintains this sort order and also

speeds up retrieval of all items for a given order To update a shipment date, we need

to search for a tuple by ordernum and isbn The index on ordernum helps here as well.

Although an index on hordernum, isbni would be better for this purpose, insertions

would not be as efficient as with an index on just ordernum; DBDudes therefore decides

to index Orderlists on just ordernum.

838 Appendix A

A.5.1 Tuning the Database

We digress from our discussion of the initial design to consider a problem that arisesseveral months after the launch of the B&N site DBDudes is called in and told thatcustomer enquiries about pending orders are being processed very slowly B&N hasbecome very successful, and the Orders and Orderlists tables have grown huge.Thinking further about the design, DBDudes realizes that there are two types of orders:

completed orders, for which all books have already shipped, and partially completed ders, for which some books are yet to be shipped Most customer requests to look up

or-an order involve partially completed orders, which are a small fraction of all orders.DBDudes therefore decides to horizontally partition both the Orders table and the Or-

derlists table by ordernum This results in four new relations: NewOrders, OldOrders,

NewOrderlists, and OldOrderlists

An order and its components are always in exactly one pair of relations—and we

can determine which pair, old or new, by a simple check on ordernum—and queries

involving that order can always be evaluated using only the relevant relations Somequeries are now slower, such as those asking for all of a customer’s orders, since theyrequire us to search two sets of relations However, these queries are infrequent andtheir performance is acceptable

Returning to our discussion of the initial design phase, recall that DBDudes completedphysical database design Next, they address security There are three groups of users:customers, employees, and the owner of the book shop (Of course, there is also thedatabase administrator who has universal access to all data and who is responsible forregular operation of the database system.)

The owner of the store has full privileges on all tables Customers can query the Bookstable and can place orders online, but they should not have access to other customers’records nor to other customers’ orders DBDudes restricts access in two ways First,they design a simple Web page with several forms similar to the page shown in Figure22.1 in Chapter 22 This allows customers to submit a small collection of valid requestswithout giving them the ability to directly access the underlying DBMS through anSQL interface Second, they use the security features of the DBMS to limit access tosensitive data

The Web page allows customers to query the Books relation by ISBN number, name ofthe author, and title of a book The Web page also has two buttons The first buttonretrieves a list of all of the customer’s orders that are not completely fulfilled yet Thesecond button will display a list of all completed orders for that customer Note that

Design Case Study: An Internet Shop 839

customers cannot specify actual SQL queries through the Web; they can only fill insome parameters in a form to instantiate an automatically generated SQL query Allqueries that are generated through form input have a WHERE clause that includes the

cid attribute value of the current customer, and evaluation of the queries generated

by the two buttons requires knowledge of the customer identification number Sinceall users have to log on to the Web site before browsing the catalog, the business logic(discussed in Section A.7) must maintain state information about a customer (i.e., thecustomer identification number) during the customer’s visit to the Web site

The second step is to configure the database to limit access according to each usergroup’s need to know DBDudes creates a special customer account that has thefollowing privileges:

SELECT ON Books, NewOrders, OldOrders, NewOrderlists, OldOrderlistsINSERT ON NewOrders, OldOrders, NewOrderlists, OldOrderlists

Employees should be able to add new books to the catalog, update the quantity of abook in stock, revise customer orders if necessary, and update all customer information

except the credit card information In fact, employees should not even be able to see a

customer’s credit card number Thus, DBDudes creates the following view:

CREATE VIEW CustomerInfo (cid,cname,address)

AS SELECT C.cid, C.cname, C.addressFROM Customers C

They give the employee account the following privileges:

SELECT ON CustomerInfo, Books,

NewOrders, OldOrders, NewOrderlists, OldOrderlistsINSERT ON CustomerInfo, Books,

NewOrders, OldOrders, NewOrderlists, OldOrderlistsUPDATE ON CustomerInfo, Books,

NewOrders, OldOrders, NewOrderlists, OldOrderlistsDELETE ON Books, NewOrders, OldOrders, NewOrderlists, OldOrderlists

In addition, there are security issues when the user first logs on to the Web site usingthe customer identification number Sending the number unencrypted over the Internet

is a security hazard, and a secure protocol such as the SSL should be used

There are companies such as CyberCash and DigiCash that offer electronic commercepayment solutions, even including ‘electronic’ cash Discussion of how to incorporatesuch techniques into the Website are outside the scope of this book

840 Appendix A

DBDudes now moves on to the implementation of the application layer and considersalternatives for connecting the DBMS to the World-Wide Web (see Chapter 22).DBDudes note the need for session management For example, users who log in tothe site, browse the catalog, and then select books to buy do not want to re-entertheir customer identification number Session management has to extend to the wholeprocess of selecting books, adding them to a shopping cart, possibly removing booksfrom the cart, and then checking out and paying for the books

DBDudes then considers whether Web pages for books should be static or dynamic

If there is a static Web page for each book, then we need an extra database field inthe Books relation that points to the location of the file Even though this enablesspecial page designs for different books, it is a very labor intensive solution DBDudesconvinces B&N to dynamically assemble the Web page for a book from a standardtemplate instantiated with information about the book in the Books relation

This leaves DBDudes with one final decision, namely how to connect applications tothe DBMS They consider the two main alternatives that we presented in Section 22.2:CGI scripts versus using an application server infrastructure If they use CGI scripts,they would have to encode session management logic—not an easy task If they use

an application server, they can make use of all the functionality that the applicationserver provides Thus, they recommend that B&N implement server-side processingusing an application server

B&N, however, refuses to pay for an application server and decides that for theirpurposes CGI scripts are fine DBDudes accepts B&N’s decision and proceeds to buildthe following pieces:

The top level HTML pages that allow users to navigate the site, and various formsthat allow users to search the catalog by ISBN, author name, or author title Anexample page containing a search form is shown in Figure 22.1 in Chapter 22 Inaddition to the input forms, DBDudes must develop appropriate presentations forthe results

The logic to track a customer session Relevant information must be stored either

in a server-side data structure or be cached in hte customer’s browser using a

mechanism like cookies Cookies are pieces of information that a Web server

can store in a user’s Web browser Whenever the user generates a request, thebrowser passes along the stored information, thereby enabling the Web server to

‘remember’ what the user did earlier

The scripts that process the user requests For example, a customer can use aform called ‘Search books by title’ to type in a title and search for books with that

Design Case Study: An Internet Shop 841

title The CGI interface communicates with a script that processes the request

An example of such a script written in Perl using DBI for data access is shown inFigure 22.4 in Chapter 22

For completeness, we remark that if B&N had agreed to use an application server,DBDudes would have had the following tasks:

As in the CGI-based architecture, they would have to design top level pages thatallow customers to navigate the Web site as well as various search forms and resultpresentations

Assuming that DBDudes select a Java-based application server, they have to writeJava Servlets to process form-generated requests Potentially, they could reuseexisting (possibly commercially available) JavaBeans They can use JDBC as adatabase interface; examples of JDBC code can be found in Section 5.10 Instead

of programming Servlets, they could resort to Java Server Pages and annotatepages with special JSP markup tags An example of a Web page that includesJSP commands is shown in Section 22.2.1

If DBDudes select an application server that uses proprietary markup tags, theyhave to develop Web pages by using such tags An example using Cold Fusionmarkup tags can be found in Section 22.2.1

Our discussion thus far only covers the ‘client-interface’, the part of the Web site that

is exposed to B&N’s customers DBDudes also need to add applications that allowthe employees and the shop owner to query and access the database and to generatesummary reports of business activities

This completes our discussion of Barns and Nobble While this study only describes

a small part of a real problem, we saw that a design even at this scale involved trivial tradeoffs We would like to emphasize again that database design is an iterativeprocess and that therefore it is very important not to lock oneself down early on in afixed model that is too inflexible to accomodate a changing environment Welcome tothe exciting world of database management!

non-B THE MINIBASE SOFTWARE

Practice is the best of all instructors

—Publius Syrus, 42 B.C

Minibase is a small relational DBMS, together with a suite of visualization tools, that

has been developed for use with this book While the book makes no direct reference tothe software and can be used independently, Minibase offers instructors an opportunity

to design a variety of hands-on assignments, with or without programming To see anonline description of the software, visit this URL:

http://www.cs.wisc.edu/˜ dbbook/minibase.html

The software is available freely through ftp By registering themselves as users atthe URL for the book, instructors can receive prompt notification of any major bugreports and fixes Sample project assignments, which elaborate upon some of the

briefly sketched ideas in the project-based exercises at the end of chapters, can be seen

at

http://www.cs.wisc.edu/˜ dbbook/minihwk.html

Instructors should consider making small modifications to each assignment to age undesirable ‘code reuse’ by students; assignment handouts formatted using Latexare available by ftp Instructors can also obtain solutions to these assignments bycontacting the authors (raghu@cs.wisc.edu, johannes@cs.cornell.edu)

Minibase is intended to supplement the use of a commercial DBMS such as Oracle orSybase in course projects, not to replace them While a commercial DBMS is idealfor SQL assignments, it does not allow students to understand how the DBMS works.Minibase is intended to address the latter issue; the subset of SQL that it supports isintentionally kept small, and students should also be asked to use a commercial DBMSfor writing SQL queries and programs Minibase is provided on an as-is basis with nowarranties or restrictions for educational or personal use It includes the following:

842

The Minibase Software 843

Code for a small single-user relational DBMS, including a parser and query mizer for a subset of SQL, and components designed to be (re)written by students

opti-as project opti-assignments: heap files, buffer manager, B+ trees, sorting, and joins.

Graphical visualization tools to aid in students’ exploration and understanding of

the behavior of the buffer management, B+ tree, and query optimization

compo-nents of the system There is also a graphical tool to refine a relational database

design using normalization.

Several assignments involving the use of Minibase are described below Each of thesehas been tested in a course already, but the details of how Minibase is set up might vary

at your school, so you may have to modify the assignments accordingly If you plan touse these assignments, you are advised to download and try them at your site well inadvance of handing them to students We have done our best to test and documentthese assignments, and the Minibase software, but bugs undoubtedly persist Pleasereport bugs at this URL:

http://www.cs.wisc.edu/˜ dbbook/minibase.comments.html

I hope that users will contribute bug fixes, additional project assignments, and sions to Minibase These will be made publicly available through the Minibase site,together with pointers to the authors

exten-B.2.1 Overview of Programming Projects

In several assignments, students are asked to rewrite a component of Minibase Thebook provides the necessary background for all of these assignments, and the assign-ment handout provides additional system-level details The online HTML documen-tation provides an overview of the software, in particular the component interfaces,and can be downloaded and installed at each school that uses Minibase The projectslisted below should be assigned after covering the relevant material from the indicatedchapter

Buffer manager (Chapter 7): Students are given code for the layer that

man-ages space on disk and supports the concept of pman-ages with page ids They areasked to implement a buffer manager that brings requested pages into memory ifthey are not already there One variation of this assignment could use differentreplacement policies Students are asked to assume a single-user environment,with no concurrency control or recovery management

HF page (Chapter 7): Students must write code that manages records on a

page using a slot-directory page format to keep track of records on a page Possible

844 Appendix B

variants include fixed-length versus variable-length records and other ways to keeptrack of records on a page

Heap files (Chapter 7): Using the HF page and buffer manager code, students

are asked to implement a layer that supports the abstraction of files of unorderedpages, that is, heap files

B+ trees (Chapter 9): This is one of the more complex assignments Students

have to implement a page class that maintains records in sorted order within apage and implement the B+ tree index structure to impose a sort order acrossseveral leaf-level pages Indexes storehkey, record-pointeri pairs in leaf pages, and

data records are stored separately (in heap files) Similar assignments can easily

be created for Linear Hashing or Extendible Hashing index structures

External sorting (Chapter 11): Building upon the buffer manager and heap

file layers, students are asked to implement external merge-sort The emphasis is

on minimizing I/O, rather than on the in-memory sort used to create sorted runs

Sort-merge join (Chapter 12): Building upon the code for external sorting,

students are asked to implement the sort-merge join algorithm This assignmentcan be easily modified to create assignments that involve other join algorithms

Index nested-loop join (Chapter 12): This assignment is similar to the

sort-merge join assignment, but relies on B+ tree (or other indexing) code, instead ofsorting code

B.2.2 Overview of Nonprogramming Assignments

Four assignments that do not require students to write any code (other than SQL, inone assignment) are also available

Optimizer exercises (Chapter 13): The Minibase optimizer visualizer offers

a flexible tool to explore how a typical relational query optimizer works It cepts single-block SQL queries (including some queries that cannot be executed

ac-in Mac-inibase, such as queries ac-involvac-ing groupac-ing and aggregate operators) dents can inspect and modify synthetic catalogs, add and drop indexes, enable ordisable different join algorithms, enable or disable index-only evaluation strate-gies, and see the effect of such changes on the plan produced for a given query.All (sub)plans generated by an iterative System R style optimizer can be viewed,ordered by the iteration in which they are generated, and details on a given plancan be obtained readily All interaction with the optimizer visualizer is through aGUI and requires no programming

Stu-The assignment introduces students to this tool and then requires them to answerquestions involving specific catalogs, queries, and plans generated by controllingvarious parameters

The Minibase Software 845

Buffer manager viewer (Chapter 12): This viewer lets students visualize

how pages are moved in and out of the buffer pool, their status (e.g., dirty bit,pin count) while in the pool, and some statistics (e.g., number of hits) The as-signment requires students to generate traces by modifying some trace-generationcode (provided) and to answer questions about these traces by using the visual-izer to look at them While this assignment can be used after covering Chapter

7, deferring it until after Chapter 12 enables students to examine traces that arerepresentative of different relational operations

B+ tree viewer (Chapter 9): This viewer lets students see a B+ tree as it is

modified through insert and delete statements The assignment requires students

to work with trace files, answer questions about them, and generate operationtraces (i.e., a sequence of inserts and deletes) that create specified kinds of trees

Normalization tool (Chapter 15): The normalization viewer is a tool for

nor-malizing relational tables It supports the concept of a refinement session, in

which a schema is decomposed repeatedly and the resulting decomposition tree isthen saved For a given schema, a user might consider several alternative decom-positions (more precisely, decomposition trees), and each of these can be saved

as a refinement session Refinement sessions are a very flexible and convenientmechanism for trying out several alternative decomposition strategies The nor-malization assignment introduces students to this tool and asks design-orientedquestions involving the use of the tool

Assignments that require students to evaluate various components can also be oped For example, students can be asked to compare different join methods, differentindex methods, and different buffer management policies

The Minibase software was inpired by Minirel, a small relational DBMS developed byDavid DeWitt for instructional use Minibase was developed by a large number ofdedicated students over a long time, and the design was guided by Mike Carey and R.Ramakrishnan See the online documentation for more on Minibase’s history

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