This document discusses efficiently publishing relational data stored in databases as XML documents. It addresses the primary issues in specifying the structure and tags for representing relational data hierarchically in XML. It also evaluates different implementation techniques for constructing XML documents, including when to add tags and structure as well as how much processing can be done within the relational database engine.

1. Efficiently Publishing Relational Data as XML Documents Authors:- J Shanmugasundaram, Michael Carey etc (IBM Almaden Research Center) Presented By Harshavardhan Achrekar (University of Massachusetts-Lowell)

2. What drove them? XML emerging as standard for business data exchange on World Wide Web. Need a mechanism to publish currently stored relational data as XML Documents.

3. Primary Issues Language Specifications - structure and tag data from tables as hierarchical XML Documents. Best Implementation Technique – study characteristics and performances of various alternatives for constructing XML documents. When to add tags & structure How much of processing is done within relational engine?

4. RoadMap Language specification based on SQL Implementation Early tagging, structuring Late tagging, structuring Early structure, late tagging Performance Evaluation

5. Sample XML Document for Customer <customer id=”C1”> <name> John Doe </name> <accounts> <account id=”A1”> 1894654 </account> <account id=”A2”> 3849342 </account> </accounts> <porders> <porder id=”PO1” acct=”A1”> // first purchase order <date>1 Jan 2000</date> <items> <item id=”I1”> Shoes </item> <item id=”I2”> Bungee Ropes </item> </items> <payments> <payment id=”P1”> due Jan 15 </payment> <payment id=”P2”> due Jan 20 </payment> <payment id=”P3”> due feb 15 </payment> </payments> </porder> <porder id=”PO2” acct=”A2”> // second purchase order … </porder> </porders> </customer> Note the Elements Names/Tags ID Refs Attribute Nested sub-element

6. Underlying tables Customer ( id int, name varchar) Account ( id varchar, custID int, acctnum int) Item ( id int, poID int, desc varchar) PurchOrder ( id int, custID int, acctID varchar, date varchar) Payment ( id int, poID int, desc varchar)

7. SQL-based language specifications Sqlfunctions: Define XMLConstruct CUST (Custid: integer, CustName: varchar) AS { <Customer id=$Custid>$CustName </Customer>} Sqlaggregates: Select XMLAGG ( ITEM (item.id, item.desc) ) From Item item // returns an XML aggregation of items

8. Customer Definition of XML Constructor Define XML Constructor CUST (custId: integer, custName: varchar(20), acctList: xml, porderList: xml) AS { < customer id=$custId> < name > $custName </ name > < accounts > $acctList </ accounts > < porders > $porderList </ porders > </ customer > } Input Output Output - A Customer XML Element Aggregate function XMLAGG – Concatenates XML Fragments produced by XML Constructor

9. Sample SQL query constructs XML from relational tables Select cust.name, CUST(cust.id, cust.name, (Select XMLAGG(ACCT(acct.id, acct.acctnum)) From Account acct Where acct.custId=cust.id), (Select XMLAGG(PORDER(porder.id, porder.acct, porder.date, (Select XMLAGG(ITEM(item.id, item.desc)) From Item item Where item.poid=porder.id) (Select XMLAGG(PAYMENT(pay.id,pay.desc)) From Payment pay, Where pay.poid=porder.id))) From PurchOrder porder Where porder.custID=cust.id)) From Customer cust Correlated sub-query for customer’s Accounts Correlated sub-query For purchase orders Top Level query returns each customer from customer table Correlated sub-query returns XML fragment LINES 1-14 produces Scalar function returning Customer XML

10. Implementation Alternatives Two main differences: Nesting (structuring) Tagging Space of alternatives: Late Tagging Early Tagging Late Structuring Early Structuring Inside Engine Inside Engine Inside Engine Outside Engine Outside Engine Outside Engine Stored Procedures CLOB

11. Early tagging and structuring Stored Procedure - Outside the engine Approach Explicitly issue nested queries Algorithm:- First query & retrieve root elements (customers id, name) Using Customer id ,issue a query to retrieve account info. Next, for same customer id, issue a query to retrieve customers purchase order For each purchase order retrieved, query to get item and payment info. Once done Processing of one customer is over. Repeat same for next customer till entire XML Document is ready. Fixed order Nested Loop Join outside the ENGINE Tag/Structure as soon as structure is ready Many SQL queries issued/tuple for tables with nested structure.

12. Early tagging and structuring Correlated CLOB - Inside the engine Approach Push queries into the engine Plug in XMLAGG, XMLCONSTRUCT support into engine Character Large Objects- CLOBS XML Fragments Performance Issues -handle huge CLOBS in engine Fixed join order – implies nested loop join strategy

13. Efficiently Publishing Relational Data as XML Documents

14. Early tagging and structuring De-Correlated CLOB - Inside the engine Approach\ Decorrelate and use Outer Joins – no longer fixed order Compute Account lists associated with all customers Compute Purchase order lists associated with all customers Join results above on customer id. Still carry around CLOBs (due to early tagging!)

15. Efficiently Publishing Relational Data as XML Documents