X2S XPath query processing for relational databases Diplomarbeit Am Institut für Programmstrukturen und Datenorganisation (IPD) Fakultät für Informatik der Universität Karlsruhe und FZI Forschungszentrum Informatik Tom Gelhausen Juli 2002 Verantwortlich: Prof. Dr. Peter C. Lockemann Betreuer: Aleksei Valikov Wassilios Kazakos Abstract This thesis shows a way to represent relational database content as a virtual XML document and how to process queries in natural XML access language (XPath) against this data. Design principals were both, efficiency and minimal sys- tem requirements to support also relational database systems without any type of build-in XML support. This is a read-only approach so far. The supported XPath features are • NodeTest: NameTest and NodeType • 8 axes (ancestor, ancestor-or-self, attribute, child, descendant, descendant-or-self, parent, self, unrolling is limited to mapping defini- tions depth) • relative navigation (starting from context node) • Constants: Numbers and Literals (Strings) • Operations: in principle all standard XPath 1.0 operations (additive (+, -), multiplicative (*, div, mod), and, or, equality (=, ≠), relational (>, ≥, <, ≤), union, sign) - 2 - Table of Contents ABSTRACT....................................................................................................................2 TABLE OF CONTENTS................................................................................................3 TABLE OF FIGURES ...................................................................................................5 TABLE OF ABBREVIATIONS .....................................................................................6 PREFACE ......................................................................................................................7 1 INTRODUCTION ............................................................................................9 1.1 MOTIVATION......................................................................................................................9 1.2 AN EXAMPLE ...................................................................................................................10 1.3 SCOPES ............................................................................................................................11 1.4 OVERVIEW .......................................................................................................................12 2 BASICS ..........................................................................................................13 2.1 RELATIONAL DATABASES ................................................................................................13 2.2 XML ................................................................................................................................14 2.3 XPATH .............................................................................................................................15 3 CONCEPTUAL CONSIDERATIONS ...........................................................17 3.1 PARADIGM GAPS ..............................................................................................................17 3.1.1 RECURSIVE STRUCTURE OF XML DOCUMENTS................................................................................................ 17 3.1.2 ORDER OF ELEMENTS IN AN XML DOCUMENT ................................................................................................. 18 3.1.3 NAVIGATION AND CONTEXT NODES ................................................................................................................. 19 3.2 CHOOSING THE MAPPING ................................................................................................19 3.3 RETURNED RESULTS .......................................................................................................21 3.4 CONSISTENCY..................................................................................................................22 4 STATE OF THE ART.....................................................................................25 4.1 MICROSOFT SQL SERVER 2000 XPATH QUERYING ........................................................25 4.1.1 RETRIEVING XML DATA VIA TRANSACT-SQL .................................................................................................. 25 4.1.2 RETRIEVING XML DATA VIA TEMPLATES......................................................................................................... 25 4.1.3 RETRIEVING XML DATA VIA XPATH................................................................................................................. 26 4.1.4 METHOD: THE EXPLICIT MODE..................................................................................................................... 27 4.1.5 DISTINCTIONS OF APPROACHES ....................................................................................................................... 29 4.2 IBM (XPERANTO/XTABLES) ......................................................................................30 4.3 JAIN/MAHAJAN/SUCIU (UNIVERSITY OF WASHINGTON) .................................................32 4.4 OTHER APPROACHES .......................................................................................................33 4.4.1 XREL................................................................................................................................................................. 33 4.4.2 XALAN (APACHE) .............................................................................................................................................. 34 4.4.3 ORACLE............................................................................................................................................................. 34 4.4.4 ROLEX ............................................................................................................................................................. 35 4.5 SUMMARY ........................................................................................................................35 5 DESIGN & REALIZATION ...........................................................................37 5.1 VIRTUAL DOM ................................................................................................................37 - 3 - 5.2 EXTENDED EXAMPLE.......................................................................................................40 5.3 THE BASIC OBJECT MODEL.............................................................................................42 5.4 DEMANDED TRANSLATION ..............................................................................................46 5.4.1 TRANSLATING ELEMENTS AND COLUMNS ........................................................................................................ 46 5.4.2 TRANSLATING CONDITIONS .............................................................................................................................. 46 5.4.3 TRANSLATING NESTED ELEMENTS................................................................................................................... 46 5.4.3.1 FIRST APPROACH .............................................................................................................................................. 47 5.4.3.2 CORRECTLY TRANSLATING NESTED ELEMENTS............................................................................................... 47 5.4.4 TRANSLATING EXISTENCE ................................................................................................................................48 5.4.5 TRANSLATING CROSS PRODUCTS...................................................................................................................... 49 5.5 THE SETUP ......................................................................................................................50 5.5.1 BASIC CONSIDERATIONS................................................................................................................................... 51 5.5.2 REQUIREMENTS ................................................................................................................................................ 51 5.5.3 ASYMMETRICAL (N:1) RELATIONS .................................................................................................................... 52 5.5.4 EXAMPLE .......................................................................................................................................................... 53 5.6 EXPRESSIONS ..................................................................................................................54 5.6.1 MATRIX OPERANDS........................................................................................................................................... 55 5.6.2 UNNESTING MATRIX OPERANDS ...................................................................................................................... 56 5.6.3 THE EXPRESSION OBJECT MODEL ................................................................................................................... 58 5.6.4 DOUBLE TREE ILLUSTRATION .......................................................................................................................... 60 5.7 THE PATH TRACING GRAPH (PTG)..................................................................................61 5.7.1 CLEAR POSITION WITHIN THE TEMPLATE DOCUMENT....................................................................................