Optimizing XSLT based on input XML document structure description and translating XSLT into equivalent XQuery expressions

[0001] This application is related to and claims the benefit of priority to U.S. Provisional Patent Application No. 60/673,997 filed on Apr. 22, 2005, entitled “IDEA AND TECHNIQUE OF REWRITING XSLT INTO XQUERY”; the entire contents of which is incorporated by this reference for all purposes as if fully disclosed herein.

[0002] This application is related to U.S. patent application Ser. No. 10/428,878 filed on May 1, 2003, entitled “Techniques For Rewriting XML Queries Directed to Relational Database Constructs”; and is related to U.S. patent application Ser. No. 10/984,471 filed Nov. 8, 2004, entitled “Techniques For Partial Rewrite of XPATH Queries In A Relational Database”; and is related to U.S. patent application Ser. No. 10/948,523 filed Sep. 22, 2004, entitled “Efficient Evaluation of Queries Using Translation”; the entire contents of all of which are incorporated by this reference for all purposes as if fully disclosed herein. These three applications are referred to herein as the “Query Rewrite” references.

[0003] The present invention relates generally to data management and, more specifically, to optimizing and translating XSLT into equivalent XQuery expressions.

[0004] XML is a versatile markup language, capable of labeling the information content of diverse data sources including structured and semi-structured documents, relational databases, and object repositories. A query language that uses the structure of XML intelligently can express queries across all these kinds of data, whether physically stored in XML or viewed as XML via middleware. As increasing amounts of information are stored, exchanged, and presented using XML, the ability to intelligently query XML data sources becomes increasingly important. One of the great strengths of XML is its flexibility in representing many different kinds of information from diverse sources. To exploit this flexibility, an XML query language must provide features for retrieving and interpreting information from these diverse sources.

The XQuery Query Language

[0005] The XQuery language is described in “XQuery 1.0: An XML Query Language, W3C Working Draft 4 Apr. 2005” available from the W3C organization; the entire contents of which are incorporated by this reference for all purposes as if fully disclosed herein. XQuery is designed to be a language in which queries are concise and easily understood. XQuery is also flexible enough to query a broad spectrum of XML information sources, including both databases and documents. XQuery operates on the abstract, logical structure (i.e., the data model) of an XML document, rather than its surface syntax. Note that an XQuery expression comprises one or more expressions (i.e., sub-expressions).

The XSLT Transformation Language

[0006] The XSLT language is described in “XSL Transformations (XSLT) Version 1.0, W3C Recommendation 16 Nov. 1999” available from the W3C organization; the entire contents of which are incorporated by this reference for all purposes as if fully disclosed herein. XSLT is a language for transforming XML documents into other XML documents or other markup language documents (e.g., HTML). XSLT is designed for use as part of XSL, which is a stylesheet language for XML. In addition to XSLT, XSL includes an XML vocabulary for specifying formatting. XSL specifies the styling of an XML document by using XSLT to describe how the document is transformed into another document that uses the formatting vocabulary. XSLT is also designed for use independently of XSL.

[0007] A transformation in the XSLT language is expressed as a well-formed XML document. A transformation expressed in XSLT describes rules for transforming a source tree into a result tree. The transformation is achieved by associating patterns with templates. A pattern is matched against nodes in the source tree. A template is instantiated to create part of the result tree. The result tree is separate from the source tree and the structure of the result tree can be completely different from the structure of the source tree. In constructing the result tree, nodes from the source tree can be filtered and reordered, and arbitrary structure can be added.

[0008] A transformation expressed in XSLT is called a stylesheet because, in the case when XSLT is transforming into the XSL formatting vocabulary, the transformation functions as a stylesheet. A stylesheet contains a set of template rules. A template rule has two parts: (1) a pattern which is matched against nodes in the source tree, and (2) a template which can be instantiated to form part of the result tree. This allows a stylesheet to be applicable to a wide class of documents that have similar source tree structures.

[0009] A template is instantiated for a particular source node to create part of the result tree. A template can contain elements that specify literal result element structure. A template can also contain elements from the XSLT namespace that are instructions for creating result tree fragments. When a template is instantiated, each instruction is executed and replaced by the result tree fragment that it creates. Instructions can select and process descendant source nodes. Processing a descendant node creates a result tree fragment by finding the applicable template rule and instantiating its template. Note that nodes are only processed when they have been selected by the execution of an instruction. The result tree is constructed by finding the template rule for the root node and instantiating its template.

[0010] XSLT and XQuery share the same XPath as a common core. Additionally, both languages have constructs to construct new nodes in an XML result tree, provide constructs for iterations, conditional testing, allow read-only variables usage (functional language without assignment semantics), support user defined functions, etc. A significant difference between the two languages is that XSLT has the powerful template rule matching driven style of execution, which XQuery does not have. In this sense, XSLT is even more declarative than XQuery as it does not require programmers to state procedurally how each template is applied.

[0011] However, the declarative nature of XSLT stylesheets is at the expense of performing what is often a complex template matching routine. This runtime matching routine is required because of the lack of advance knowledge about the input content and, therefore, which particular templates are most appropriate and should be invoked to transform the input content. Furthermore, sequential languages such as XQuery are more computationally efficient than template-matching driven languages such as XSLT because a processor executing a sequence of instructions in the sequential language simply knows what to do, without having to perform rule-based matching to determine what to do.

[0012] One approach to transforming XML data based on an XSLT stylesheet, in the context of a relational database system, is referred to as functional evaluation. FIG. 1 is a block diagram that illustrates a functional evaluation process flow for an XSLT transformation request. Functional evaluation of an XSLT transformation request 102, in view of the XML input data and the particular XSLT stylesheet specified in the request 102 (e.g., an xmltransform( ) operator), generally involves the following: (a) constructing a structured XML document 106 from the XML data storage layer (e.g., if the XML data is shredded and stored in relational and/or object-relational tables of a relational database management system (RDBMS), then the actual construction of the XML document from relational or object-relational tables is required); (b) materializing the XML document into a DOM-like (document object model) structure, which is a tree-like structure used to represent the structure of the data; (c) calling into an XSLT engine 108 to apply the XSLT stylesheet 107 and its associated set of transformation templates to the input DOM, which includes performing the template matching routine; (d) generating a new DOM that represents the transformed XML data; and (e) re-mapping the transformed data into new layouts or table structures (i.e., table translation), via a DOM interface 109 and RDBMS engine 104. This conceptual functional evaluation of XMLTransform( ) with XSLT style sheet is expensive because XML data needs to be constructed from various pieces from different storage tables and then passed to a standalone XSLT engine. Based on the foregoing, there is a need for a better process for transforming, via XSLT stylesheets, XML data stored in a relational database system.

[0013] The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.

[0014] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

[0015] FIG. 1 is a block diagram that illustrates a functional evaluation process flow for an XSLT transformation request;

[0016] FIG. 2 is a block diagram that illustrates an XSLT transformation process flow, according to an embodiment of the invention;

[0017] FIG. 3 is a flow diagram that illustrates a method for managing XSLT transformation requests, according to an embodiment of the invention; and

[0018] FIG. 4 is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented.

[0019] In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

Functional Overview of Embodiments

[0020] According to one aspect of the invention, transformation of a set of XML documents via an XSLT stylesheet is made more efficient by processing the XSLT transformation request, at compile-time, into an optimized XSLT stylesheet using the structural descriptions of the set of XML documents. Structural descriptions of the set of XML documents can be, for example, an XML Schema or a DTD (Document Type Definition) to which this set of XML documents conforms. In the optimized XSLT stylesheet, the template instantiation is more deterministic than the original XSLT stylesheet. Thus, evaluation of the optimized XSLT stylesheet requires fewer, or no, dynamic XSLT template matching processes than with evaluation of the original XSLT stylesheet. For example, the ‘xsl:apply-templates’ instructions in the original XSLT stylesheet are replaced by ‘xsl:call-template’ instructions.

[0021] According to one aspect of the invention, transformation of one or more XML documents via an XSLT stylesheet is made more efficient by processing the XSLT transformation request, at compile-time, into corresponding one or more XQuery expressions. Consequently, further existing optimization techniques can be applied to the XQuery expression, such as rewriting the XQuery expression into a corresponding SQL or SQL/XML statement for efficient execution by a relational database query engine, against underlying relational and/or object-relational constructs. Furthermore, when stored XML data has an XPath index and/or a value index, then XQuery expressions can be executed more efficiently by leveraging these indexes. The consequence is that only the portions of the XML data that are needed for the XSLT result construction are extracted, and the extraction process itself can be done very quickly via various indexing mechanisms. This is significantly faster than formulating the entire XML document from various pieces and passing that into the XSLT engine. Thus, the XSLT-to-XQuery translation process leverages the vast XML processing capabilities of the relational database management system.

[0022] For example, when a set of XQuery expressions is processed by a database server, depending on the nature of the input XML (e.g., if the input XML is from schema based XMLType table/view, SQL/XML view over relational tables/views, XMLIndex indexed XMLType value), the query performance can be orders of magnitude better than processing the corresponding XSLT statement via an XSLT engine. Therefore, applying the described techniques to an XMLTransform( ) function, for example, the performance of the XMLTransform( ) function can be orders of magnitude better.

[0023] One key to translating from XSLT to XQuery is how to effectively and efficiently translate XSLT template processing into XQuery constructs. According to one embodiment, at XSLT statement compile-time, the particular transformation templates that are appropriate for transforming the particular elements contained in the XML document (also referred to as source tree nodes or, simply, nodes) are determined, and XSLT calls to the templates are converted to calls to corresponding XQuery language functions. Furthermore, data from the template is translated into corresponding XQuery constructors, and instructions from the template are translated into XQuery expressions.

[0024] According to one embodiment, the appropriate templates are determined at compile time based on the structure of a set of XML documents, by generating a corresponding sample document and submitting the sample document to an XSLT engine for tracing execution paths based on the particular XSLT stylesheet and the sample document.

Operating Environment

[0025] The techniques described herein are described in the general context of a database system. A database system typically comprises one or more clients that are communicatively coupled to a server that is connected to a shared database. “Server” may refer collectively to a cluster of server instances and machines on which the instances execute. Generally, a server is combination of integrated software components and an allocation of computational resources, such as memory, a node, and processes on the node for executing the integrated software components on a processor, where the combination of the software and computational resources are dedicated to providing a particular type of function on behalf of clients of the server. Among other functions of database management, a database server governs and facilitates access to a particular database, processing requests by clients to access the database.

[0026] A database comprises data and metadata that is stored on a persistent memory mechanism, such as a set of hard disks. Such data and metadata may be stored in a database logically, for example, according to relational and/or object-relational database constructs. Database applications interact with a database server by submitting to the database server commands that cause the database server to perform operations on data stored in a database. A database command that is sent from a database application to a database server contains an original statement of the database command. For the database server to process the commands, the commands must conform to a database language supported by the database server. One database language supported by many database servers is known as the Structured Query Language (SQL).

[0027] A relational database system may be extended to provide native support for storage, management, and query of particular types of data. For example, a traditional relational database system may be augmented with features and technologies to enable storage of XML documents directly in the database, and access to such XML data in either an XML-centric (e.g., using the XQuery query language) or a relational-centric (e.g., using the SQL query language) manner.

XSLT to XQuery Translation Process Flow

[0028] FIG. 2 is a block diagram that illustrates an XSLT transformation process flow, according to an embodiment of the invention. One notable difference between the process flow of FIG. 1 and the process flow of FIG. 2 is that, rather than performing the entire transformation at transformation statement runtime (FIG. 1), most of the process described herein is performed at transformation statement compile time.

[0029] An XSLT transformation request 102 is received at a database server relational engine 104. XSLT transformation requests include references to a set of XML document instances (which are typically stored in XMLType tables or XMLType table columns, or SQL/XML view columns generated from relational or object relational data) and an XSLT stylesheet 107. According to one embodiment, based on the input XMLType metadata, a corresponding sample document 206 is constructed or accessed if already constructed. The sample document 206 represents all possible data structures with which the corresponding set of XML document instances may be associated, and is based on the structure of a class of documents of which the XML document is part. For non-limiting examples, the sample document 206 may be constructed based on (a) an XML schema or DTD information to which the corresponding set of XML document instances conforms, (b) a relational or object relational schema from which the set of XML document instances is constructed, and (c) the data guide for the set of XML document instances, which is based on analysis of the set of XML document instances and represents all possible data structures with which any corresponding XML document instance may be associated.

[0030] One approach to representing a class of XML documents is described in U.S. patent application Ser. No. 11/044,472 entitled “Technique of Using XMLType Tree as the Type Infrastructure for XML”, the content of which is incorporated by this reference in its entirety for all purposes as if fully disclosed herein. This approach utilizes a universal format for a type representation of type structure of XMLType instances that are generated in various ways from various data sources, SQL expressions and queries, and storage mechanisms. This format or type representation is referred to herein as an XMLType type tree. Therefore, sample document 206 may be constructed based on an XMLType type tree associated with a corresponding input XML document.

[0031] An XSLT engine 108 uses the sample document 206 and XSLT stylesheet 107 to trace possible execution paths of the XSLT transformation for documents of the class corresponding to the sample document 206. Note that the sample document 206 that corresponds to a given input set of XML document instances that is the subject of the XSLT transformation request 102 (i.e., the document being transformed) may have already been constructed, at compile time, in response to a prior XSLT transformation request that included the same or a different input XML document from the same class of documents. Similarly, the execution path trace process performed by XSLT engine 108 may also have been previously performed at compile time in response to a prior XSLT transformation request that included the same or a different input XML document from the same class of documents.

[0032] The execution path trace process, for a set of documents, results in a mapping 212 of (a) XML nodes that may be contained in an XML document of the set, to (b) respective corresponding transformation templates, which are referenced in the input XSLT stylesheet 107. The execution path trace process effectively performs, at XSLT request compile time, the rule-based pattern matching and template selection process that is required by an XSLT transformation, i.e., for transforming a source tree into a result tree. Therefore, when a corresponding XML document needs to be transformed via the XSLT stylesheet, i.e., at XSLT transformation runtime, the transformation templates and the order of instantiation of these templates, for that document have already been identified via the trace process.

[0033] Relational engine 104 takes the set of input XML document instances stored in the database and uses the corresponding mapping 212 to translate the original XSLT stylesheet 107 of the transformation request 102 into a corresponding XQuery expression 220, where the mapping is used to translate template calls to XQuery user-defined function calls. Translation of the XSLT transformation request 102 into a corresponding XQuery expression 220 is described in greater detail herein. Using other independent techniques beyond the scope of this description, XQuery expression 220 can then be further translated into an SQL or SQL/XML statement 221, for efficient execution by relational engine 104 against the underlying relational or object relational tables used for the storage of the actual data or index on the input XML document instances in the database. This is because XML data or index stored in relational and object-relational constructs provides an efficient extraction of the individual pieces of XML data than constructing XML data in DOM form. In summary, the runtime functional evaluation flow depicted in FIG. 1, with which an XML document is formulated, a corresponding DOM constructed, etc., is replaced with runtime evaluation of a SQL or SQL/XML statement that simply accesses the XML data directly from the underlying relational/object-relational storage table(s).

XSLT To XQuery Translation

[0034] FIG. 3 is a flow diagram that illustrates a method for managing XSLT transformation requests, according to an embodiment of the invention. More specifically, the technique illustrated in FIG. 3 can be used to optimize XSLT transformation queries on XML data stored in a relational database. The process illustrated in FIG. 3 is implemented for automated performance by a conventional computing system, such as computer system 400 of FIG. 4.

[0035] At block 302, a request to transform a set of XML document instances stored in the database, based on an XSLT stylesheet, is received. For example, a database statement that contains an XMLTransform( ) operator is received at a database server, where the operator includes a reference to an XMLType column (e.g., XMLType data stored in a relational database) and an XSLT stylesheet as input to the operator.

[0036] The following is an example of a SQL statement that calls an XMLTransform( ) operator to transform each XML document instance stored in the table ‘poem.doc’ column into an HTML document. In this example, ‘poem’ is a table with an XMLType column named ‘doc’. Each row of ‘poem’ table stores an instance of the ‘poem’ XML document.

SELECT XMLTransform(poem.doc,
‘<?xml version=“1.0”?>
<xsl:stylesheet version = “1.0”
xmlns:xsl=”http://www.w3.org/1999/XSL/Transform”
xmlns:oracle=”http://www.oracle.com/XSL/Transform/java”>
<xsl:template match=“poem”>
<html>
<head>
<title><xsl:value-of select=“title”/></title>
</head>
<body>
<xsl:apply-templates select=“title”/>
<xsl:apply-templates select=“author”/>
</body>
</html>
</xsl:template>
<xsl:template match=“title”>
<div align=“center”><h1><xsl:value-of select=“.”/></h1></div>
</xsl:template>
<xsl:template match=“author”>
<div align=“center”><h1><xsl:value-of select=“.”/></h1></div>
</xsl:template>
</xsl:stylesheet>’
FROM poem_table.

[0037] Conventionally, the XSLT stylesheet includes calls to one or more transformation templates, where each template specifies how to transform data associated with a corresponding XML element (i.e., node), and rule-based pattern matching criteria to match particular XML elements with particular corresponding transformation templates.

[0038] The following XSLT stylesheet is used as the parameter to the XMLTransform( ) operator in the SQL SELECT statement of the foregoing example:

<?xml version=“1.0”?>
<xsl:stylesheet version = ‘1.1’
xmlns:xsl=‘http://www.w3.org/1999/XSL/Transform’
xmlns:oracle=‘http://www.oracle.com/XSL/Transform/java’>
<xsl:template match=“poem”>
<html>
<head>
<title><xsl:value-of select=“title”/></title>
</head>
<body>
<xsl:apply-templates select=“title”/>
<xsl:apply-templates select=“author”/>
</body>
</html>
</xsl:template>
<xsl:template match=“title”>
<div align=“center”><h1><xsl:value-of select=“.”/></h1></div>
</xsl:template>
<xsl:template match=“author”>
<div align=“center”><h1><xsl:value-of select=“.”/></h1></div>
</xsl:template>
</xsl:stylesheet>

[0039] which, given an input row of ‘poem’ table containing the XML document ‘poem.xml’ with the following content:

	<poem>
	<author>Robert</author>
	<date>1912</date>
	<title>Song</title>
	</poem>

[0040] generates the following output XML (HTML) for that input row of the ‘poem’ table.

	<html>
	<head>
	<title>Song</title>
	</head>
	<body>
	<div align=“center”><h1>Song</h1></div>
	</div><div align=“center”><h1>Robert</h1></div>
	</body>
	</html>.

[0041] At block 304, a sample XML document, which captures all the possible structures of the corresponding set of XML document instances (e.g., stored in the ‘poem.doc’ column) is constructed and passed into the XSLT engine 108 (FIG. 2), together with the XSLT stylesheet. At block 304, the XSLT engine 108 determines the set of transformation templates that are used to transform the sample XML document, and the execution orders of these templates are identified, in a procedural manner. According to one embodiment, the mapping 212 (FIG. 2) of (a) XML nodes that may be contained in an XML document of the set, to (b) respective corresponding transformation templates, which are referenced in the input XSLT stylesheet 107 (FIG. 2), is used to select the appropriate template(s) for use in transforming the input set of XML document instances stored in the table.

[0042] At block 306, the XSLT stylesheet is translated into an XQuery language expression. According to one embodiment, rewriting the XSLT stylesheet into an XQuery expression includes converting each template into an XQuery user-defined function. According to one embodiment, for each template selected for use in transforming the XML document instances, the XSLT instructions in the template call are translated into corresponding XQuery expressions, and the output XML data in the template is translated into a corresponding XQuery direct constructor, as described in greater detail hereafter. The order of a series of XSLT template instantiations is converted into a sequence of XQuery user defined function calls.

[0043] According to one embodiment, at optional block 308, the non-recursive XQuery user-defined functions are “in-lined” so that the resultant XQuery expression is simplified, that is, the expression actually contains the body of the function rather than the call of the function. An XQuery compiler can perform this in-lining process. Alternatively, the XSLT to XQuery translation process can in-line non-recursive XSLT template calls so that all non-recursive XQuery user-defined function calls are not generated from XSLT to XQuery translation.

[0044] The following is the rewritten SQL/XML statement generated by blocks 306 and 308, where the XMLTransform( ) operator with the XSLT stylesheet is replaced by an XMLQuery( ) operator with an XQuery expression, from the above example:

	SELECT XMLQuery(
	‘declare namespace oracle =
	“http://www.oracle.com/XSL/Transform/java”;
	declare variable $var000 := .;
	(: builtin template :)
	(
	let $var002 := $var000/poem
	return
	(: <xsl:template match=“poem”> :)
	<html>
	<head>
	<title>{fn:string($var002/title)}</title>
	</head>
	<body>
	{
	for $var005 in $var002/title
	return
	(: <xsl:template match=“title”> :)
	<div align=“center”>
	<h1>{fn:string($var005)}</h1>
	</div>,
	for $var005 in $var002/author
	return
	(: <xsl:template match=“author”> :)
	<div align=“center”>
	<h1>{fn:string($var005)}</h1>
	</div>
	}
	</body>
	</html>
	) ‘
	FROM poem.

Translating XSLT Template Calls to XQuery Function Calls Explicit Language Translation

[0045] Translating XSLT transformation templates that are to be applied to the XML document resolves into selection of an XQuery user defined function to invoke. According to one embodiment, referred to herein as “explicit language translation,” each XSLT “xslt:apply-templates” instruction is translated into multiple XQuery conditional expressions. Each condition in the XQuery conditional expression is translated from the template matching pattern, with consideration of the template mode, priority, etc. Since each template rule matching expression is essentially an XPath expression, the translation of template matching patterns into respective conditional expressions is straightforward.

[0046] Each action in the XQuery conditional expression is the invocation of a specific user defined function corresponding to the specific XSLT template. This approach can be quite efficient if the XSLT compiler/optimizer can translate many “xslt:apply-templates” instruction into “xslt:call-templates” instruction by utilizing knowledge of the structure of the input XML documents (such as XML Schema information) during compile time. That is, it is desirable to translate template matching into direct template invocation as much as possible during XSLT compile time by taking advantage of knowing the structure or schema information of the input XML documents.

[0047] Implicit Language Translation

[0048] According to one embodiment, referred to herein as “implicit language translation,” an XQuery engine of the database server selects the template function to invoke. That is, the XQuery engine is enhanced with an XSLT style function invocation mode, which selects the function to invoke based on the set of criteria defined by the XSLT stylesheet. This can be quite efficient provided that the underlying XQuery engine can be enhanced with an efficient and scalable template selection algorithm. If the XQuery engine is enhanced to allow a user defined function to return a function reference, (e.g., in functional programming language, that means returning a lambda function; in conventional C/C++ programming language, that means returning a functional pointer), then even the template selection process can be translated as a user defined external function, which calls into the XQuery engine and returns a reference to the user defined function (for the corresponding template) to be invoked.

[0049] Hybrid Translation

[0050] According to one embodiment, referred to herein as “hybrid translation,” explicit language translation is performed for cases where the exact template is known (or a small number of templates) based on the input XML structural information (such as XML Schema) and the matching pattern itself. Otherwise, implicit language translation is performed for arbitrary template pattern matching cases.

[0051] Regardless of which of the foregoing template selection processes is used, template rule matching in XSLT is conceptually equivalent to the following XQuery expression with “xslt_xquery_eng:find_template( )” and “xslt_xquery_eng:invoke template( )” as the underlying XQuery engine implemented system function.

declare function local: template_for_nodes($x as node( )*) as node( )*
{
if (xslt_xquery_eng:find_template($x))
xslt_xquery_eng:invoke_template($x)
else
typeswitch ($x)
case $x as element( )
return for $d in $x/*
return local: template_for_nodes($d)
case $x as document-node( )
return for $d in $x/*
return local: template_for_nodes($d)
case $x as attribute( )
return fn:string($x)
case $x as text( )
return fn:string($x)
case $x as processing-instruction( )
return ( )
case $x as comment( )
return ( )
};
local: template_for_nodes(.)

Translating XSLT Template Instructions to XQuery Expressions

[0052] In the following description, XSLT instructions are grouped and for each XSLT instruction in the group, the corresponding XQuery expression is described.

[0053] (I) XSLT template Definition/Invocation Instruction to XQuery User Defined Function/Invocation Expression

[0054] (A) xsl:template

This is translated into an XQuery function. The text in the template body is translated into an XQuery constructor. The instruction in the template body is translated into an XQuery expression.

[0055] For example, the following XSLT template instructions,

	<xsl:template match=“title”>
	<div align=“center”><h1><xsl:value-of select=“.”/></h1></div>
	</xsl:template>

[0056] can be translated into the following XQuery function:

	declare function local:title_template($x as node()*) as node()*
	{
	<div align=“center”><h1>{fn:data($x/.)}</h1></div>
	}

[0057] The match pattern, name, priority, and mode can be used to convert “xslt:apply-templates” into “xslt:call-template” during translation-time, based on information about the input XML. Otherwise, the XQuery engine can determine which template to invoke, as described above.

[0058] (B) xsl:param (local variable to a template)

This is translated from the XSLT template into local variables in the XQuery function.

[0059] (C) xsl:call-template

This is translated into the direct invocation of the XQuery function corresponding to the named template specified by the “xsl:call-template” command.

[0060] (D) xsl:apply-templates

This translation is discussed above in reference to translating XSLT template calls to XQuery function calls.

[0061] (E) xsl:with-param

This is translated as parameter passing for the call of the XQuery user defined function.

[0062] (II) XSLT XML Result Constructor Instruction Translation to XQuery Constructor Expression

[0063] All plain text in XSLT is translated into an XQuery constructor.

[0064] (A) xsl:element

This is translated into an XQuery computed element constructor.

[0065] (B) xsl:attribute and xsl:attribute-set

This is translated into an XQuery attribute constructor.

[0066] (C) xsl:processing-instruction

This is translated into an XQuery processing instruction constructor.

[0067] (D) xsl:comment

This is translated into an XQuery comment constructor.

[0068] (E) xsl:text

This is translated into an Xquery text constructor.

[0069] (F) xsl:value-of

This is translated into an Xquery fn:string( ) function.

[0070] (III) XSLT Conditional Instruction Translation to XQuery Conditional Expression

[0071] (A) xsl:choose, xsl:otherwise, xsl:if, xsl:when

This is translated into an Xquery ‘if’ conditional expression.

[0072] (IV) XSLT Loop Iteration Instruction Translation into XQuery Iteration Expression Expression

[0073] (A) xsl:for-each, with optional xsl:sort

[0074] This is translated into an Xquery FOWR expression (where the general structure of an XQuery is described by the acronym FLOWR, which stands for FOR, LET, ORDER BY, WHERE and RETURN. The xsl:sort( ) is translated into XQuery ‘order by’ clause. The ‘number’ and ‘text’ datatype for xsl:sort( ) can be translated into XQuery order by using xs:double( ) and xs:string( ) functions, respectively.

[0075] (V) XSLT Variable Instruction Translation into XQuery Let Expression Expression

[0076] (A) xsl:variable

This is translated into an Xquery Let expression, where an XSLT variable reference is translated into its corresponding XQuery variable reference.

[0077] (VI) XSLT Global Variable Instruction Translation into XQuery Global Variable Expression

[0078] (A) xsl:param (top level)

This is translated into an XQuery global variable declaration.

[0079] (VII) XSLT Error Reporting Instruction Translation into XQuery fn:error( ) Expression

[0080] (A) xsl:message

This is translated into an Xquery fn:error( ).

[0081] For certain XSLT instructions that do not have a corresponding direct XQuery expression construct, such instructions are translated into XQuery functions that are implemented natively by the underlying XQuery processors.

[0082] As an example, an XSLT transformation request based on the foregoing XSLT stylesheet, and according to the techniques described herein, may be translated into the following XQuery Expression:

declare function local:title_template($x as node( )*) as item( )*
{
<div align=“center”><h1>{fn:string($x/.)}</h1></div>
};
declare function local:author_template($x as node( )*) as item( )*
{
<div align=“center”><h1>{fn:string($x/.)}</h1></div>
};
declare function local:poem_template($x as node( )*) as item( )*
{
<html><head><title>{fn:string($x/title)}</title></head><body>
{local:title_template($x/title)} {local:author_template($x/author)}
</body></html>
};
local:poem_template(fn:doc(‘poem.xml’))

[0083] After applying either XSLT template in-lining or XQuery function in-lining, the resulting XQuery expression can be as simple as the following. Note there are no user-defined XQuery function calls corresponding to the original XSLT template instantiation.

	Let $doc = fn:doc(‘poem.xml’)
	return
	<html>
	<head><title>fn:string($doc/title)</title></head>
	<body>
	<div algin=”center”><h1>fn:string($doc/title)</h1></div>
	<div algin=”center”><h1>fn:string($doc/author)</h1></div>
	</body>
	</html>.

Transforming an XML Document with Embedded XSLT

[0084] The techniques described herein can also effectively translate an XML document embedded with an XSLT stylesheet into XQuery expressions. For example, consider the following XML document with XSLT stylesheet embedded:

	<chapter xsl:version=“2.0”
	xmlns:xsl=“http://www.w3.org/1999/XSL/Transform”>
	<title>doc1.xml</title>
	<p>First paragraph</p>
	<xsl:copy-of select=“document(‘doc2.xml’)/doc/p[@id=‘p2’]”/>
	<p>Last paragraph</p>
	</chapter>

[0085] The foregoing document can be translated into the following equivalent XQuery expression:

	<chapter>
	<title>doc1.xml</title>
	<p>First paragraph</p>
	{document(‘doc2.xml’)/doc/p[@id=‘p2’]”}
	<p>Last paragraph</p>
	</chapter>

The forgoing XQuery can be executed efficiently by the database system that executes XQuery natively.
Rewriting XQuery Expressions for Accessing Relational Database Constructs

[0086] The techniques described herein effectively translate a dynamic XSLT transformation process (i.e., dynamic pattern matching template selection) into a static process by mapping XSLT templates to corresponding user defined functions, mapping XSLT template calls to function calls, and mapping XSLT transformation instructions to corresponding XQuery constructors. In addition to translating an XSLT transformation request into a corresponding XQuery expression, according to one embodiment, the XQuery expression is further rewritten to a form that is directed to the underlying relational database construct (e.g., directed to the storage table column or the pieces of the XML data). Execution of such a query may utilize efficient access paths for evaluation of the underlying tables, which is performed in accordance with regular query optimization (e.g., by a traditional database query optimizer). Some techniques for rewriting XML queries directed to relational database constructs are described in the Query Rewrite references. The techniques disclosed in the Query Rewrite references may be used in conjunction with the embodiments described herein, but use of the Query Rewrite techniques with the embodiments described herein is not required.

[0087] The following database statement is an example of applying the Query Rewrite techniques to the foregoing XQuery SQL statement, which results in the following simplified SQL/XML query for runtime execution. Note that the final SQL query has no XSLT or XQuery invocation. Rather, the query simply accesses the underlying storage column for the ‘title’ and ‘author’ of the XML data and generates the HTML tag using XMLElement( ) element constructor.

	SELECT xmlelement(“html”,
	xmelelment(“head”,
	xmlelement(“title”, poem.doc.title)),
	xmelelment(“body”,
	xmlelement(“div”, xmlattribute(‘center’ as “align”),
	xmlelement(“h1”, poem.doc.title)),
	xmlelement(“div”, xmlattribute(‘center’ as “align”),
	xmlelement(“h1”, poem.doc.author))))
	FROM poem;

[0088] On the other hand, if the ‘poem.doc’ is stored with a path index to access title and author data, then the above SQL statement is effectively executed with path index lookup, as follows:

SELECT xmlelement(“html”,
xmelelment(“head”,
xmlelement(“title”, extractValue(poem.doc.’/poem/title’)),
xmelelment(“body”,
xmlelement(“div”, xmlattribute(‘center’ as “align”),
xmlelement(“h1”, extractValue(poem.doc, ‘/poem/title’))),
xmlelement(“div”, xmlattribute(‘center’ as “align”),
xmlelement(“h1”, extractValue(poem.doc.’/poem/author’)))))
FROM poem.

Hardware Overview

[0089] FIG. 4 is a block diagram that illustrates a computer system 400 upon which an embodiment of the invention may be implemented. Computer system 400 includes a bus 402 or other communication mechanism for communicating information, and a processor 404 coupled-with bus 402 for processing information. Computer system 400 also includes a main memory 406, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 402 for storing information and instructions to be executed by processor 404. Main memory 406 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 404. Computer system 400 further includes a read only memory (ROM) 408 or other static storage device coupled to bus 402 for storing static information and instructions for processor 404. A storage device 410, such as a magnetic disk or optical disk, is provided and coupled to bus 402 for storing information and instructions.

[0090] Computer system 400 may be coupled via bus 402 to a display 412, such as a cathode ray tube (CRT), for displaying information to a computer user. An input device 414, including alphanumeric and other keys, is coupled to bus 402 for communicating information and command selections to processor 404. Another type of user input device is cursor control 416, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 404 and for controlling cursor movement on display 412. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.

[0091] The invention is related to the use of computer system 400 for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 400 in response to processor 404 executing one or more sequences of one or more instructions contained in main memory 406. Such instructions may be read into main memory 406 from another machine-readable medium, such as storage device 410. Execution of the sequences of instructions contained in main memory 406 causes processor 404 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software.

[0092] The term “machine-readable medium” as used herein refers to any medium that participates in providing data that causes a machine to operation in a specific fashion. In an embodiment implemented using computer system 400, various machine-readable media are involved, for example, in providing instructions to processor 404 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 410. Volatile media includes dynamic memory, such as main memory 406. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 402. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications.

[0093] Common forms of machine-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

[0094] Various forms of machine-readable media may be involved in carrying one or more sequences of one or more instructions to processor 404 for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 400 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 402. Bus 402 carries the data to main memory 406, from which processor 404 retrieves and executes the instructions. The instructions received by main memory 406 may optionally be stored on storage device 410 either before or after execution by processor 404.

[0095] Computer system 400 also includes a communication interface 418 coupled to bus 402. Communication interface 418 provides a two-way data communication coupling to a network link 420 that is connected to a local network 422. For example, communication interface 418 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 418 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 418 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0096] Network link 420 typically provides data communication through one or more networks to other data devices. For example, network link 420 may provide a connection through local network 422 to a host computer 424 or to data equipment operated by an Internet Service Provider (ISP) 426. ISP 426 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet” 428. Local network 422 and Internet 428 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 420 and through communication interface 418, which carry the digital data to and from computer system 400, are exemplary forms of carrier waves transporting the information.

[0097] Computer system 400 can send messages and receive data, including program code, through the network(s), network link 420 and communication interface 418. In the Internet example, a server 430 might transmit a requested code for an application program through Internet 428, ISP 426, local network 422 and communication interface 418.

[0098] The received code may be executed by processor 404 as it is received, and/or stored in storage device 410, or other non-volatile storage for later execution. In this manner, computer system 400 may obtain application code in the form of a carrier wave.

[0099] In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.