XML-to-SQL Query Mapping in the Presence of Multi-valued Schema

Mappings and Recursive XML Schemas

Speaker: Artem Chebotko

Mustafa Atay, Artem Chebotko, Shiyong Lu, and Farshad Fotouhi

Department of Computer ScienceWayne State UniversityDetroit, MI 48202 USA

{matay, artem, shiyong, fotouhi}@wayne.edu

September 5, 2007 DEXA'07, Regensburg, Germany

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Outline of Talk Motivation

XML-to-Relational Mappings Recursion in Query Mapping

Problem Statement and Contributions Our Proposed Solution

Path-based XML-to-Relational Mapping Unfolded XML Schema Graph Proposed generic query mapping algorithm

Conclusions and Future Work

September 5, 2007 DEXA'07, Regensburg, Germany

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XML-to-Relational Mappings

Single-valued Each XML element type is mapped to exactly one

relation e.g., Shared, Shanmugasundaram et al., VLDB,

1999 e.g., ODTDMap, Atay et al., Information Systems,

2007 Multi-valued

Each XML element type can be mapped to multiple relations

e.g., Basic and Hybrid, Shanmugasundaram et al., VLDB, 1999

September 5, 2007 DEXA'07, Regensburg, Germany

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Single-valued vs. Multi-valued

September 5, 2007 DEXA'07, Regensburg, Germany

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Single-valued vs. Multi-valued Motivating Example XPath expression

/A/B1/C/D3 SQL query

Select T4.IDFrom (A) T1, (B1) T2, (C) T3, (D3) T4Where T1.ID=T2.parentID And T2.ID=T3.parentID And T3.ID=T4.parentID

September 5, 2007 DEXA'07, Regensburg, Germany

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Recursion in Query Mapping Challenge: When there is recursion

both in an XML query and in its underlying schema, there might be infinitely many matching paths

Solutions Krishnamurthy et al., ICDE, 2004

requires with construct of SQL’99 Fan et al., VLDB, 2005

requires LFP (least fixpoint operator)

September 5, 2007 DEXA'07, Regensburg, Germany

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Problem Statement

i. Existing query mapping algorithms only support single-valued mappings

A query mapping algorithm supporting multi-valued mappings is missing

ii. Existing query mapping algorithms need special operators to deal with recursion

There is a need for a query mapping algorithm which can be implemented for any RDBMS

September 5, 2007 DEXA'07, Regensburg, Germany

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Contributions We propose a generic query mapping

algorithm which supports both single-valued and multi-valued mappings

Our proposed algorithm only requires the traditional relational operators to handle the recursion It can be implemented in any RDBMS

September 5, 2007 DEXA'07, Regensburg, Germany

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Our Proposed Solution Path-based XML-to-Relational Mapping

p-Mapping supports multi-valued mappings (i)

Unfolded XML Schema Graph UXG helps identifying finite number of paths for a

given recursive query (ii) Proposed generic query mapping algorithm

ID-XMLtoSQL

September 5, 2007 DEXA'07, Regensburg, Germany

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p-Mapping

Provides solution to the problem of supporting multi-valued mappings

Combines the followings to find a mapping XML-to-Relational Mapping (-Mapping) Path structure of input XML query p=e1/e2/…/en

September 5, 2007 DEXA'07, Regensburg, Germany

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p-Mapping Example/A/B1/C/D3

SQL query Select T4.ID

From (A) T1, (B1) T2, (C) T3, (D3) T4Where T1.ID=T2.parentID And T2.ID=T3.parentID And T3.ID=T4.parentID

September 5, 2007 DEXA'07, Regensburg, Germany

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UXG (Unfolded XML Schema Graph)

UXG provides solution to the problem of finding a finite number of matching paths for a recursive XML query

We convert a cyclic XML schema graph to a directed acyclic graph by unfolding the cycles in the original graph (UXG)

Static and dynamic approaches UXG always guarantees a finite number

of matching paths for an arbitrary XML query

September 5, 2007 DEXA'07, Regensburg, Germany

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UXG Example

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Algorithm ID-XMLtoSQL Given a path expression P and UXG Gu

1. Extract all matching paths2. Identify the p-Mappings for each pi

3. Call SPathToSQL() to generate an SQL query for each pi

4. Get the union of output SQL queries

September 5, 2007 DEXA'07, Regensburg, Germany

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Clustering Cluster is the set of consecutive elements in

path expression which are mapped to the same relation

We use the notion of a cluster for optimizing the output SQL query in SPathToSQL()

e.g., /A/B1/C/D3/E

A B1 B1 B1 E c1 c2 c3

September 5, 2007 DEXA'07, Regensburg, Germany

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Algorithm SPathToSQL

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Example /A/D3//E is given

extracted paths /A/D3/E /A/D3/E/D1/E

identified p-Mappings

{(A,A), (D3,A), (E,E)}

{(A,A), (D3,A), (E,E), (D1,D1),(E,E)}

Output SQL query

Select E.IDFrom A, EWhere

A.D3.ID=E.parentIDUNION ALLSelect E.IDFrom A, E T1, D1, E T2Where

A.D3.ID=T1.parentID And T1.ID=D1.parentID And D1.ID=T2.parentID

September 5, 2007 DEXA'07, Regensburg, Germany

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Performance Study

We compared our ID-XMLtoSQL to SQLGen of Krishnamurthy et al., ICDE, 2004

We selected 9 queries from the XMark benchmark ID-XMLtoSQL outperformed SQLGen in all the test

queries

September 5, 2007 DEXA'07, Regensburg, Germany

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Conclusions and Future Work We proposed a generic query mapping

algorithm for a schema-based relational XML storage

We proposed an efficient way of handling recursion in query mapping which can be applicable to all RDBMSs

We consider augmenting our proposed ID-based algorithm with interval-based and path-based mapping schemes as a potential future work

Thank you!

Questions?