Home > Articles > Web Services > XML

This chapter is from the book

LOB or Compose?

The most basic dichotomy in mapping between XML and relational data is the choice between representing an XML document in a single column or in multiple columns: LOB representation or composed representation. Additionally, relational databases that have been extended to add XML functionality may implement a native XML type.

In an LOB representation, a column in a database table contains a literal XML string, usually in a CLOB (character large object) or similar datatype—hence the name. A single application might store multiple types of XML documents in different columns or even in the same column. Listing 6.1 shows an example of LOB XML. In this example, we have a database table that stores purchase orders as XML documents in one column and includes two other columns: id and receivedate, that are part of the management of the purchase order document. (Note: in this and all other examples of XML used in this chapter, whitespace has been added for clarity; it is not necessarily part of the actual XML data.)

Listing 6.1 Example of LOB-Encoded XML

id        receivedate     purchaseorder

4023      2001-12-01       <purchaseOrder 
                             <originator billId="0013579">
                                 Fred Allen
                                 <street>123 2nd Ave. NW</street>
                               <phone>(330) 555-1212</phone>
                                 AS 1132
                               <item code="34xdl 1278 12ct"
                               <item code="57xdl 7789"
                             <shipAddress sameAsContact="true"/>
                             <shipCode carrier="02"/> 
5327      2002-04-23       <purchaseOrder

The other major approach to mapping between XML and relational data employs a composed representation of XML data. In a composed representation, the XML document does not exist in its XML form in the relational database. Instead, an XML document is composed from individual data items that are stored in many columns, and often in multiple tables, in the relational database. Typically each XML element that has simple data corresponds to the value of some column in some row of the relational data.

For example, the same data that we saw in Listing 6.1 might look as shown in Listing 6.2 in a composed representation. A key observation for the composed representation is that the same relational data could be used to represent many different XML documents by choosing different columns, arranged in different ways, and different corresponding element names. There must be some explicit or implicit mechanism to determine what the XML for a set of relational tables is supposed to look like. We call such a mechanism a composition technique. A variety of composition techniques exist, and we explore a few of the variations in the next section.

Listing 6.2 Example of Composed XML

purchaseorder table
id        receivedate

4023      2001-12-01
5327      2002-04-23

originator table
poid      billId     contactName     street             ...

4023      0013579    Fred Allen      123 2nd Ave. NW

order table
poid      code             quant   color   size

4023      34xdl 1278 12ct      1    null   null
4023      57xdl 7789           1     012   null

ship table
poid      carrier    ... 

 4023      02    

An important aspect of the composed representation concerns the use case where the original data is in XML form. In this case, the XML document must be decomposed into values that can be placed into the relational tables—a process often called by the evocative name “shredding.” This shredded, or decomposed, representation is then recomposed on the fly to generate the final XML. The shredding, or decomposition, process can also be configurable; ideally it uses the same information that is used in the composition technique that will be used to recompose the data. Shredding creates a variety of additional issues and choices that we discuss in a separate section below.

LOB and composed approaches can coexist, of course: part of an XML document can be (de)composed, while certain subelements are maintained in LOB form. For example, a table that mixes composed and LOB data might look like the one shown in Listing 6.3. In this case, the document data is divided between decomposed, or shredded, elements and XML LOB contents. In other scenarios, the division might be redundant: Certain elements of the LOB information might be duplicated as individual database columns.

Listing 6.3 A Table with Both Composed and LOB Data

originator table
poid      billId     contactName     address

4023      0013579    Fred Allen      <contactAddress>
                                       <street>1234 2nd Ave. NW</street>

Finally, when the database itself is extended to include native XML functionality, then another representation option is a native XML type. That is, the SQL language is extended to allow “XML” to be a primitive datatype. If we look behind the implementation of such a type, we will generally find a dichotomy similar to the one we have outlined here: either an LOB data object or some structured representation is used. But of course the SQL database may also have additional options, especially with regard to details like indexing. Options for an XML native datatype are discussed in more detail in Chapter 7.

Different vendors use different approaches to support mapping between XML and relational data: Some use an LOB representation, some use a composed representation, and some allow either, or a mixture of both. (The same holds true when the database supports a native XML datatype: The database designer may have a choice of underlying representations.)

With respect to the kinds of applications supported by each representation, we can make the following observations:

  • Clearly if the source data is relational, a composed representation is used.

  • LOB representations are the most flexible with respect to wide varieties of XML inputs, including marked-up text and variable- or no-schema documents. Composed representations with shredding can manage structured or semi-structured XML documents with a given schema much more easily than they can the other types of XML sources.

  • LOB representations obviously make it easy to implement the “emit documents” application type. All the other application types, however, require some ability to look inside the document, which must be implemented as an additional layer of processing that cannot take advantage of the database's capabilities, unless implemented as part of a native implementation. This is a significant restriction. By comparison, the composed technique exposes the XML data in such a way that it is possible to use the SQL engine's own capabilities to implement important operations such as selection, at the cost of requiring additional complexity to generate XML output.

  • Hybrid and redundant representations can help obtain the benefits of both approaches by enabling indexing and easy access to some elements, while preserving complex XML representations. The cost is a more complex representation of the data, essentially requiring that any access or update methods be able to handle both cases. Redundant representations also add the overhead of keeping multiple representations of the same data in sync.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020