Efficiently Publishing Relational Data as XML Documents IBM Almaden Research Center Eugene Shekita Rimon Barr Michael Carey Bruce Lindsay Hamid Pirahesh Berthold Reinwald Univ. Wisconsin/IBM Almaden Jayavel Shanmugasundaram Univ. Wisconsin/IBM Almaden Joint work with:

Outline Why? How? Which? Hence

XML Example John Mary Internet Recycling

What is the big deal about XML? Elegantly models complex, hierarchical/ graph-structured data Domain-specific tags (unlike HTML) Standardized!  Fast emerging as dominant standard for data exchange on the WWW

Why Relational Data? Most business data stored in relational databases Unlikely to change in the near future –Scalability, Reliability, Performance, Tools  Need efficient means to publish relational data as XML documents

Usage Scenario Existing Database System (RDBMS) Application/User Query to produce XML Documents XML Result (processed or displayed in browser) The Internet

Outline Why? How? Which? Hence

Example Relational Schema Department DeptIdDeptName 10 Purchasing Project ProjId DeptIdProjName Internet 79510Recycling Employee EmpId DeptIdEmpName John 9110Mary Salary 50K 70K

XML Representation John Mary Internet Recycling

Main Issues Relational data is flat, XML is a tagged graph How do we specify translation from flat model to a graph model? –A query language to map from relations to XML How do we transform flat representations to tagged nested representations? –Efficient implementation strategies

Outline Why? How? –Language? –Mechanism? Which? Hence

SQL: Key Ideas Sub-queries to specify nesting Scalar functions to specify tags/attributes –XML Constructors Aggregate functions to group child elements

Example Relational Schema Department DeptIdDeptName 10 Purchasing Project ProjId DeptIdProjName Internet 79510Recycling Employee EmpId DeptIdEmpName John 9110Mary Salary 50K 70K

SQL: Query to publish XML Select DEPT(d.name,, ) From Department d

SQL: XML Constructor Define XML Constructor DEPT(dname: varchar(20), emplist: xml, projlist: xml) As ( {emplist} {projlist} )

SQL: Query to publish XML Select DEPT(d.name,, ) From Department d

SQL: Query to publish XML Select DEPT(d.name, (Select XMLAGG(EMP(e.name)) From Employee e Where e.deptno = d.deptno), (Select XMLAGG(PROJ(p.name)) From Project p Where p.deptno = d.deptno) ) From Department d

Query Result John Mary Internet Recycling ( )

Outline Why? How? –Language? –Mechanism? Which? Hence

Relations to XML: Issues Two main differences: –Nesting (structuring) –Tagging Space of alternatives: Late TaggingEarly Tagging Late Structuring Early Structuring Inside Engine Outside Engine

Stored Procedure Approach Issue queries for sub-structures and tag them Could be a Stored Procedure DBMS Engine Department Employee Project Problem: Too many SQL queries! (10, Purchasing) (John) (Mary) (Internet) (Recycling) Early Tagging, Early Structuring, Outside Engine

Correlated CLOB Approach Problem: Correlated execution of sub-queries Select DEPT(d.name, (Select XMLAGG(EMP(e.name)) From Employee e Where e.deptno = d.deptno), (Select XMLAGG(PROJ(p.name)) From Project p Where p.deptno = d.deptno) ) From Department d Early Tagging, Early Structuring, Inside Engine

De-Correlated CLOB Approach Compute employee lists associated with all departments Compute project lists associated with all departments Join results above on department id Early Tagging, Early Structuring, Inside Engine Problem: CLOBs during query processing

Late Tagging, Late Structuring XML document content produced without structure (in arbitrary order) Tagger enforces order as final step Relational Query Processing Unstructured content Tagging Result XML Document

Redundant Relation Approach How do we represent nested content as relations? (10, Purchasing) (10, Internet) (10, Recycling) (10, John) (10, Mary) (Purchasing, John, Internet) (Purchasing, John, Recycling) (Purchasing, Mary, Internet) (Purchasing, Mary, Recycling) Problem: Large relation due to data redundancy! Late Tagging, Late Structuring

Outer Union Approach How do we represent nested content as relations? Problem: Wide tuples (having many columns) Department EmployeeProject Department EmployeeProject Union (Purchasing, Internet) (Purchasing, Recycling) (Purchasing, John) (Purchasing, Mary) (10, Purchasing) (Purchasing, null, Internet, 0) (Purchasing, null, Recycling, 0) (Purchasing, John, null, 1) (Purchasing, Mary, null, 1) Late Tagging, Late Structuring

Hash-based Tagger Results not structured early –In arbitrary order Tagger has to enforce order during tagging –Hash-based approach Inside/Outside engine tagger Late Tagging, Late Structuring Problem: Requires memory for entire document

Late Tagging, Early Structuring Structured XML document content produced Tagger just adds tags (constant space) Relational Query Processing Structured content Tagging Result XML Document

Sorted Outer Union Approach A B C DEFG A B n n E n n A n C n n F n A n C n n n G Late Tagging, Early Structuring A B n D n n n Sort By: Aid, Bid, Cid Problem: Only partial ordering required

Constant Space Tagger Detects changes in XML document hierarchy Adds appropriate opening/closing tags Inside/outside engine Late Tagging, Late Structuring

Classification of Alternatives Late TaggingEarly Tagging Late Structuring Early Structuring Inside Engine De-Correlated CLOB Outside Engine Stored Procedure Inside Engine Outside Engine Sorted Outer Union (Tagging inside) Sorted Outer Union (Tagging outside) Unsorted Outer Union (Tagging inside) Unsorted Outer Union (Tagging outside) Outside Engine Correlated CLOB

Outline Why? How? –Language? –Mechanism? Which? Hence

Where Does Time Go?

Performance Evaluation Summary Late TaggingEarly Tagging Late Structuring Early Structuring Inside Engine De-Correlated CLOB Outside Engine Stored Procedure Inside Engine Outside Engine Sorted Outer Union (Tagging inside) Sorted Outer Union (Tagging outside) Unsorted Outer Union (Tagging inside) Unsorted Outer Union (Tagging outside) Outside Engine Correlated CLOB

Outline Why? How? –Language? –Mechanism? Which? Hence

Conclusion Publishing XML from relational sources important in Internet SQL-based language specification Implementation Alternatives –Inside engine >> Outside engine –Unsorted Outer Union : sufficient main memory –Sorted Outer Union : otherwise (most stable)

Related Work SilkRoute (WWW 2000) Oracle’s XML extensions (ICDE 2000) Microsoft’s XDR XPERANTO (VLDB demo tomorrow)

Performance Evaluation Query Depth Query Fan Out Database Size

Effect of Query Depth

De-Correlated CLOB Approach Problem: CLOBs during processing With EmpStruct (deptname, empinfo) AS ( Select d.deptname, XMLAGG(EMP(employee, e.empname)) From department d left join employee e on d.deptid = e.deptid Group By d.deptname) With ProjStruct (deptname, projinfo) AS ( Select d.deptname, XMLAGG(PROJ(employee, p.projname)) From department d left join project p on d.deptid = e.deptid Group By d.deptname) Select DEPT(name, d1.empinfo, d2.projinfo)) From EmpStruct d1 full join ProjStruct d2 on d1.deptname = d2.deptname Early Tagging, Early Structuring, Inside Engine