Databases and Information Systems XML storage in RDBMS and core XPath implementation. Prof. Dr. Stefan Böttcher

Transcription

1 Databases and Information Systems 1 8. XML storage in RDBMS and core XPath implementation Prof. Dr. Stefan Böttcher

2 XML storage and core XPath implementation 8.1. XML in commercial DBMS 8.2. Mapping XML to relational databases and Datalog how to store an XML document in a relational database? how to answer XPath queries on these XML documents? minimum information needed to store and to query XML documents? 10. A succint data format for XML implementing core XPath use this for XML compression 2/24

3 XML extensions of relational databases - data are already stored in relational database (e.g. Oracle) - extra data type generates XML: select <kunde name = Kunde.name> <auftrag> Kunde.auftrag </auftrag> <adresse> Kunde.adresse </adresse> </kunde> from Kunde where name = Meier kunde doc kunde auftrag adresse auftrag adresse name Kunde auftrag adresse 3/24

4 XML extensions of relational databases - load XML data into relational database requires mapping (defining how to store data in RDB) mapping uses annotations in DTD or XML Schema requires DTD or XML Schema only for strongly structured XML name = Meier kunde doc kunde auftrag adresse auftrag adresse Kunde name auftrag adresse 4/24

5 XML columns in relational databases extra columns of type XML-Document (e.g. Microsoft) + arbitrarily structured XML documente storable + search in imported XML documents possible + XPath queries internally mapped to ordinary queries date orderid XML document regarding the order doc name = Meier kunde kunde auftrag adresse auftrag adresse 5/24

6 XML - storage in databases hybrid XML-database system (e.g. DB2 Universal Server): DB2 client application SQL XQuery relational interface XML interface DB2 Query Engine DB2 Memory relational XML + relational data in relational database efficient query evaluation in relational part of database + XML data stored separately imported XML documents can be stored + both datenbase parts can be joined in queries efficient search in imported XML documents 6/24

7 Implementing XML by relational databases many different approaches one approach is based on label, first-child (fc) and next-sibling(ns) <Auftrag> <Kunde>Meier</Kunde> <PC>pc500</PC> </Auftrag> 1 2 root Auftrag 3 Kunde 5 PC 4 Meier 6 pc500 7/24

8 Implementing XML by relational databases many different approaches one based on label, first-child (fc) and next-sibling(ns) <Auftrag> <Kunde>Meier</Kunde> <PC>pc500</PC> </Auftrag> label( ID, Label) ID 1 root Label(ID) 2 Auftrag 3 Kunde 4 Meier 5 PC 6 pc500 fc(p,fc) ID fc(id) 5 6 ID ns(id) ns(id,ns) 1 2 Kunde Meier root Auftrag 5 6 PC pc500 8/24

9 Implementing XML axes by Datalog programs following-sibling(x,y) = ns + (X,Y) ; // implementation in datalog as usual following-sibling(x,y) :- ns( X, Y ). following-sibling(x,y) :- ns( X, NS ), following-sibling(ns,y). child(x,y) :- fc(x,y). child(x,y) :- fc(x,fc), following-sibling( FC, Y ). ID Label(ID) 1 Root 2 Auftrag 3 Kunde 4 Meier 5 PC 6 pc500 ID fc(id) ns(id,ns) ID ns(id) 1 2 Kunde Meier root Auftrag 5 6 PC pc500 9/24

10 Implementing XML axes by Datalog programs // following-sibling(x,y) = ns + (X,Y). // implementation in datalog as usual following-sibling(x,y) :- ns( X, Y ). following-sibling(x,y) :- ns( X, NS ), following-sibling(ns,y). child(x,y) :- fc(x,y). child(x,y) :- fc(x,fc), following-sibling( FC, Y ). 1 2 root Auftrag // descendant( X, Y ) = child + (X,Y). descendant(x,y) :- child( X, Y ). descendant(x,y) :- child( X, C ), descendant(c,y). 3 4 Kunde Meier 5 6 PC pc500 10/24

11 Implementing XML axes by Datalog programs reverse-axis(x,y) = reverse-axis -1 (Y,X) ancestor( D, A ) :- descendant( A, D ). parent( C, P ) :- child ( P, C ). parent( A, P ) :- attribute ( P, A ). parent( N, P ) :- namespace ( P, N ). Axis-or-self(X,Y) = Axis(X,Y) Axis-or-self(X,Y) = self(x,y) self( X, X ). descendant-or-self( A, D ) :- descendant( A, D ). descendant-or-self( A, D ) :- self( A, D ). ancestor-or-self( D, A ) :- ancestor( D, A ). ancestor-or-self( D, A ) :- self( D, A ) Kunde Meier root Auftrag 5 6 PC pc500 11/24

12 Implementing XML axes by Datalog programs following::* = ancestor-or-self::* / following-sibling::* / descendant-or-self::* following(x,y) :- ancestor-or-self(x,a), following-sibling(a,s), descendant-or-self(s,y). 1 root preceding(x,y) :- following(y,x). 2 Auftrag 3 Kunde 5 PC 4 Meier 6 pc500 12/24

13 Implementing XPath queries by Datalog queries // Kunde?- goal(k). goal(k) :- label( R, root ), descendant( R, K ), label( K, Kunde ). Answer: K = 3. ( R = 1 ) 1 2 root Auftrag 3 Kunde 5 PC 4 Meier 6 pc500 13/24

14 Implementing XPath queries by Datalog queries // Auftrag [ PC ] / Kunde?- goal2( K ). goal2(k) :- label( R, root ), ( R = 1 ) descendant( R, A ), label( A, auftrag ), ( A = 2 ) child( A, P ), label( P, "PC" ), ( P = 5 ) child( A, K ), label( K, "kunde" ). ( K = 3 ) Kunde Meier root Auftrag 5 6 PC pc500 14/24

15 Evaluation of XPath implemented by Datalog 1. translate core XPath expression into a datalog program: using all the core XPath axes 2. datalog program contains tables fc(first-child), ns(next-sibling), label plus datalog rules for reducing other axes to these eleemntary axes, e.g. following-sibling( X,Y ) :- ns( X, Y ). following-sibling( X,Y ) :- ns( X, NS ), following-sibling( NS, Y ). 3. evaluate Datalog program using fast algorithm for Datalog evaluation, that combines Top-Down selection-first evaluation with set-oriented bottom-up computation, e.g. Process Model only unary and binary base relations subclass of Datalog programs that can be evaluated efficiently 15/24

16 Summary: XML/XPath implemented by Datalog base relations for binary XML tree: Label( ID, Label ), fc( Parent, FirstChild ), ns( Node, NextSibling ) recursive deductive database rule systems for following-sibling( Node, FS ) child( Parent, Child ), descendant( Node, Descendant ) combination rules for following( Node, Following ) descendant-or-self( Node, DOS ) ancestor-or-self( Node, AOS ) reverse-axis Datalog rules for ancestor( Node, Ancestor ) parent( Node, Parent ) preceding-sibling( Node, PrecedingSibling ) preceding( Node, Preceding ) 16/24

17 Generating and storing a binary XML tree How to transfrom an XML document such that we can answer core XPath queries with only fc, ns and label? transform XML document into Binary Simple SAX Events: XML file SAX Events Simple SAX Events (convert to elements only) Binary Simple SAX Events (Details given on the following slides) 17/24

18 How to treat different node types? escape text nodes: <E>"text"</E> <E> escape attribute nodes: <=text> </=text1> </E> <E a="value"></e> <E> <=value> </=value> </E> + escape root node, comments, PIs only elements remain 18/24

19 From SAX to a Simple SAX-Parser-API (1) public void startdocument() throws SAXException { } // SAX parser calls this once for each start tag of an element public void startelement( String namespaceuri, String localname, String qname, Attributes atts) throws SAXException { // code example: for(int i=0; i<atts.getlength(); i++) { // for each attribute // generate Simple SAX events, i.e. elements, for attribute name, // i.e. atts.getqname(i), and attribute value, i.e. atts.getvalue(i) } } // SAX parser calls this once for each end tag of an element public void endelement( String namespaceuri, String localname, String qname) throws SAXException { } // SAX parser calls this once when end of document is reached public void enddocument() throws SAXException { } 19/24

20 From SAX to a Simple SAX-Parser-API (2) // SAX parser calls this once // for each text constant found in the XML document public void characters(char[ ] ch, int start, int length) throws SAXException { String text = new String (ch, start, length); text = text.trim(); // generate Simple SAX event, i.e. element, for the text constant } 20/24

21 How to transform XML into a binary XML tree 1. Simplify single node type (element nodes) only 2. generate binary tree and store binary tree fc E1 fc E2 E3 E4 ns ns fc E5 ns E6 E7 ns E8 OR: generate and process binary SAX events ( next slide) 21/24

22 From SAX events to binary SAX events Pairs of SAX-Events Location Step Generate event end-element( _ ) start-element( a ) start-element( _ ) start-element( a ) end-element( _ ) end-element( _ ) start-element( _ ) end-element( _ ) next-sibling :: a first-child :: a parent :: * no location step next-sibling :: a first-child :: a (nothing) go back to parent (nothing) 22/24

23 Conclusions: XML/XPath implemented by Datalog only the following base relations or Simple SAX Events are needed: Label( ID, Label ), fc( Parent, FirstChild ), ns( Node, NextSibling ) remaining XPath evaluation can be done by evaluation of datalog rules when storage format is changed, only implement Label, fc, ns If compressed XML supports implementation of Label, fc, ns, then core XPath can be evaluated ( topic of Chapter 10 ) 23/24

24 Summary and Conclusions XPath evaluation can be implemented in Datalog, if XML storage format supports fc, ns, label XML storage in a relational database: store relationships fc, ns, label in table(s) Pre-processing of the XML input: XML document SAX-Events Binary Simple SAX Events 24/24