2.1.1 Static Context

[Definition: The static context of an expression is the information that is available during static analysis of the expression, prior to its evaluation.] This information can be used to decide whether the expression contains a static error. If analysis of an expression relies on some component of the static context that has not been assigned a value, a static error is raised [err:XPST0001].

The individual components of the static context are summarized below. Rules governing the scope and initialization of these components can be found in C.1 Static Context Components.

• [Definition: XPath 1.0 compatibility mode. This component must be set by all host languages that include XPath 2.0 as a subset, indicating whether rules for compatibility with XPath 1.0 are in effect. XQuery sets the value of this component to false. ]

• [Definition: Statically known namespaces. This is a set of (prefix, URI) pairs that define all the namespaces that are known during static processing of a given expression.] The URI value is whitespace normalized according to the rules for the xs:anyURI type in [XML Schema]. Note the difference between in-scope namespaces, which is a dynamic property of an element node, and statically known namespaces, which is a static property of an expression.

  Some namespaces are predefined; additional namespaces can be added to the statically known namespaces by namespace declarations in a Prolog and by namespace declaration attributes in direct element constructors.

• [Definition: Default element/type namespace. This is a namespace URI or "none". The namespace URI, if present, is used for any unprefixed QName appearing in a position where an element or type name is expected.] The URI value is whitespace normalized according to the rules for the xs:anyURI type in [XML Schema].

• [Definition: Default function namespace. This is a namespace URI or "none". The namespace URI, if present, is used for any unprefixed QName appearing in a position where a function name is expected.] The URI value is whitespace normalized according to the rules for the xs:anyURI type in [XML Schema].

• [Definition: In-scope schema definitions. This is a generic term for all the element declarations, attribute declarations, and schema type definitions that are in scope during processing of an expression.] It includes the following three parts:

  • [Definition: In-scope schema types. Each schema type definition is identified either by an expanded QName (for a named type) or by an implementation-dependent type identifier (for an anonymous type). The in-scope schema types include the predefined schema types described in 2.5.1 Predefined Schema Types. If the Schema Import Feature is supported, in-scope schema types also include all type definitions found in imported schemas. ]

  • [Definition: In-scope element declarations. Each element declaration is identified either by an expanded QName (for a top-level element declaration) or by an implementation-dependent element identifier (for a local element declaration). If the Schema Import Feature is supported, in-scope element declarations include all element declarations found in imported schemas. ] An element declaration includes information about the element's substitution group affiliation.

    [Definition: Substitution groups are defined in [XML Schema] Part 1, Section 2.2.2.2. Informally, the substitution group headed by a given element (called the head element) consists of the set of elements that can be substituted for the head element without affecting the outcome of schema validation.]

  • [Definition: In-scope attribute declarations. Each attribute declaration is identified either by an expanded QName (for a top-level attribute declaration) or by an implementation-dependent attribute identifier (for a local attribute declaration). If the Schema Import Feature is supported, in-scope attribute declarations include all attribute declarations found in imported schemas.]

• [Definition: In-scope variables. This is a set of (expanded QName, type) pairs. It defines the set of variables that are available for reference within an expression. The expanded QName is the name of the variable, and the type is the static type of the variable.]

  Variable declarations in a Prolog are added to in-scope variables. An expression that binds a variable (such as a let, for, some, or every expression) extends the in-scope variables of its subexpressions with the new bound variable and its type. Within a function declaration, the in-scope variables are extended by the names and types of the function parameters.

  The static type of a variable may be either declared in a query or (if the Static Typing Feature is enabled) inferred by static type inference rules as described in [XQuery 1.0 and XPath 2.0 Formal Semantics].

• [Definition: Context item static type. This component defines the static type of the context item within the scope of a given expression.]

• [Definition: Function signatures. This component defines the set of functions that are available to be called from within an expression. Each function is uniquely identified by its expanded QName and its arity (number of parameters).] In addition to the name and arity, each function signature specifies the static types of the function parameters and result.

  The function signatures include the signatures of constructor functions, which are discussed in 3.12.5 Constructor Functions.

• [Definition: Statically known collations. This is an implementation-defined set of (URI, collation) pairs. It defines the names of the collations that are available for use in processing queries and expressions.] [Definition: A collation is a specification of the manner in which strings and URIs are compared and, by extension, ordered. For a more complete definition of collation, see [XQuery 1.0 and XPath 2.0 Functions and Operators].]

• [Definition: Default collation. This identifies one of the collations in statically known collations as the collation to be used by functions and operators for comparing and ordering values of type xs:string and xs:anyURI (and types derived from them) when no explicit collation is specified.]

• [Definition: Construction mode. The construction mode governs the behavior of element and document node constructors. If construction mode is preserve, the type of a constructed element node is xs:anyType, and all attribute and element nodes copied during node construction retain their original types. If construction mode is strip, the type of a constructed element node is xs:untyped; all element nodes copied during node construction receive the type xs:untyped, and all attribute nodes copied during node construction receive the type xs:untypedAtomic.]

• [Definition: Ordering mode. Ordering mode, which has the value ordered or unordered, affects the ordering of the result sequence returned by certain path expressions, union, intersect, and except expressions, and FLWOR expressions that have no order by clause.] Details are provided in the descriptions of these expressions.

• [Definition: Default order for empty sequences. This component controls the processing of empty sequences and NaN values as ordering keys in an order by clause in a FLWOR expression, as described in 3.8.3 Order By and Return Clauses.] Its value may be greatest or least.

• [Definition: Boundary-space policy. This component controls the processing of boundary whitespace by direct element constructors, as described in 3.7.1.4 Boundary Whitespace.] Its value may be preserve or strip.

• [Definition: Copy-namespaces mode. This component controls the namespace bindings that are assigned when an existing element node is copied by an element constructor, as described in 3.7.1 Direct Element Constructors. Its value consists of two parts: preserve or no-preserve, and inherit or no-inherit.]

• [Definition: Base URI. This is an absolute URI, used when necessary in the resolution of relative URIs (for example, by the fn:resolve-uri function.)] The URI value is whitespace normalized according to the rules for the xs:anyURI type in [XML Schema].

• [Definition: Statically known documents. This is a mapping from strings onto types. The string represents the absolute URI of a resource that is potentially available using the fn:doc function. The type is the static type of a call to fn:doc with the given URI as its literal argument. ] If the argument to fn:doc is a string literal that is not present in statically known documents, then the static type of fn:doc is document-node()?.

  Note:

  The purpose of the statically known documents is to provide static type information, not to determine which documents are available. A URI need not be found in the statically known documents to be accessed using fn:doc.

• [Definition: Statically known collections. This is a mapping from strings onto types. The string represents the absolute URI of a resource that is potentially available using the fn:collection function. The type is the type of the sequence of nodes that would result from calling the fn:collection function with this URI as its argument.] If the argument to fn:collection is a string literal that is not present in statically known collections, then the static type of fn:collection is node()*.

  Note:

  The purpose of the statically known collections is to provide static type information, not to determine which collections are available. A URI need not be found in the statically known collections to be accessed using fn:collection.

• [Definition: Statically known default collection type. This is the type of the sequence of nodes that would result from calling the fn:collection function with no arguments.] Unless initialized to some other value by an implementation, the value of statically known default collection type is node()*.

2.1.2 Dynamic Context

[Definition: The dynamic context of an expression is defined as information that is available at the time the expression is evaluated.] If evaluation of an expression relies on some part of the dynamic context that has not been assigned a value, a dynamic error is raised [err:XPDY0002].

The individual components of the dynamic context are summarized below. Further rules governing the semantics of these components can be found in C.2 Dynamic Context Components.

The dynamic context consists of all the components of the static context, and the additional components listed below.

[Definition: The first three components of the dynamic context (context item, context position, and context size) are called the focus of the expression. ] The focus enables the processor to keep track of which items are being processed by the expression.

Certain language constructs, notably the path expression E1/E2 and the predicate E1[E2], create a new focus for the evaluation of a sub-expression. In these constructs, E2 is evaluated once for each item in the sequence that results from evaluating E1. Each time E2 is evaluated, it is evaluated with a different focus. The focus for evaluating E2 is referred to below as the inner focus, while the focus for evaluating E1 is referred to as the outer focus. The inner focus exists only while E2 is being evaluated. When this evaluation is complete, evaluation of the containing expression continues with its original focus unchanged.

• [Definition: The context item is the item currently being processed. An item is either an atomic value or a node.][Definition: When the context item is a node, it can also be referred to as the context node.] The context item is returned by an expression consisting of a single dot (.). When an expression E1/E2 or E1[E2] is evaluated, each item in the sequence obtained by evaluating E1 becomes the context item in the inner focus for an evaluation of E2.

• [Definition: The context position is the position of the context item within the sequence of items currently being processed.] It changes whenever the context item changes. When the focus is defined, the value of the context position is an integer greater than zero. The context position is returned by the expression fn:position(). When an expression E1/E2 or E1[E2] is evaluated, the context position in the inner focus for an evaluation of E2 is the position of the context item in the sequence obtained by evaluating E1. The position of the first item in a sequence is always 1 (one). The context position is always less than or equal to the context size.

• [Definition: The context size is the number of items in the sequence of items currently being processed.] Its value is always an integer greater than zero. The context size is returned by the expression fn:last(). When an expression E1/E2 or E1[E2] is evaluated, the context size in the inner focus for an evaluation of E2 is the number of items in the sequence obtained by evaluating E1.

• [Definition: Variable values. This is a set of (expanded QName, value) pairs. It contains the same expanded QNames as the in-scope variables in the static context for the expression. The expanded QName is the name of the variable and the value is the dynamic value of the variable, which includes its dynamic type.]

• [Definition: Function implementations. Each function in function signatures has a function implementation that enables the function to map instances of its parameter types into an instance of its result type. For a user-defined function, the function implementation is an XQuery expression. For a built-in function or external function, the function implementation is implementation-dependent.]

• [Definition: Current dateTime. This information represents an implementation-dependent point in time during the processing of a query, and includes an explicit timezone. It can be retrieved by the fn:current-dateTime function. If invoked multiple times during the execution of a query, this function always returns the same result.]

• [Definition: Implicit timezone. This is the timezone to be used when a date, time, or dateTime value that does not have a timezone is used in a comparison or arithmetic operation. The implicit timezone is an implementation-defined value of type xs:dayTimeDuration. See [XML Schema] for the range of legal values of a timezone.]

• [Definition: Available documents. This is a mapping of strings onto document nodes. The string represents the absolute URI of a resource. The document node is the root of a tree that represents that resource using the data model. The document node is returned by the fn:doc function when applied to that URI.] The set of available documents is not limited to the set of statically known documents, and it may be empty.

  If there are one or more URIs in available documents that map to a document node D, then the document-uri property of D must either be absent, or must be one of these URIs.

  Note:

  This means that given a document node $N, the result of fn:doc(fn:document-uri($N)) is $N will always be True, unless fn:document-uri($N) is an empty sequence.

• [Definition: Available collections. This is a mapping of strings onto sequences of nodes. The string represents the absolute URI of a resource. The sequence of nodes represents the result of the fn:collection function when that URI is supplied as the argument. ] The set of available collections is not limited to the set of statically known collections, and it may be empty.

  For every document node D that is in the target of a mapping in available collections, or that is the root of a tree containing such a node, the document-uri property of D must either be absent, or must be a URI U such that available documents contains a mapping from U to D."

  Note:

  This means that for any document node $N retrieved using the fn:collection function, either directly or by navigating to the root of a node that was returned, the result of fn:doc(fn:document-uri($N)) is $N will always be True, unless fn:document-uri($N) is an empty sequence. This implies a requirement for the fn:doc and fn:collection functions to be consistent in their effect. If the implementation uses catalogs or user-supplied URI resolvers to dereference URIs supplied to the fn:doc function, the implementation of the fn:collection function must take these mechanisms into account. For example, an implementation might achieve this by mapping the collection URI to a set of document URIs, which are then resolved using the same catalog or URI resolver that is used by the fn:doc function.

• [Definition: Default collection. This is the sequence of nodes that would result from calling the fn:collection function with no arguments.] The value of default collection may be initialized by the implementation.

2.2.1 Data Model Generation

Before a query can be processed, its input data must be represented as an XDM instance. This process occurs outside the domain of XQuery, which is why Figure 1 represents it in the external processing domain. Here are some steps by which an XML document might be converted to an XDM instance:

1. A document may be parsed using an XML parser that generates an XML Information Set (see [XML Infoset]). The parsed document may then be validated against one or more schemas. This process, which is described in [XML Schema], results in an abstract information structure called the Post-Schema Validation Infoset (PSVI). If a document has no associated schema, its Information Set is preserved. (See DM1 in Fig. 1.)

2. The Information Set or PSVI may be transformed into an XDM instance by a process described in [XQuery/XPath Data Model (XDM)]. (See DM2 in Fig. 1.)

The above steps provide an example of how an XDM instance might be constructed. An XDM instance might also be synthesized directly from a relational database, or constructed in some other way (see DM3 in Fig. 1.) XQuery is defined in terms of the data model, but it does not place any constraints on how XDM instances are constructed.

[Definition: Each element node and attribute node in an XDM instance has a type annotation (referred to in [XQuery/XPath Data Model (XDM)] as its type-name property.) The type annotation of a node is a schema type that describes the relationship between the string value of the node and its typed value.] If the XDM instance was derived from a validated XML document as described in Section 3.3 Construction from a PSVI^DM, the type annotations of the element and attribute nodes are derived from schema validation. XQuery does not provide a way to directly access the type annotation of an element or attribute node.

The value of an attribute is represented directly within the attribute node. An attribute node whose type is unknown (such as might occur in a schemaless document) is given the type annotation xs:untypedAtomic.

The value of an element is represented by the children of the element node, which may include text nodes and other element nodes. The type annotation of an element node indicates how the values in its child text nodes are to be interpreted. An element that has not been validated (such as might occur in a schemaless document) is annotated with the schema type xs:untyped. An element that has been validated and found to be partially valid is annotated with the schema type xs:anyType. If an element node is annotated as xs:untyped, all its descendant element nodes are also annotated as xs:untyped. However, if an element node is annotated as xs:anyType, some of its descendant element nodes may have a more specific type annotation.

2.2.2 Schema Import Processing

2.2.3 Expression Processing

XQuery defines two phases of processing called the static analysis phase and the dynamic evaluation phase (see Fig. 1). During the static analysis phase, static errors, dynamic errors, or type errors may be raised. During the dynamic evaluation phase, only dynamic errors or type errors may be raised. These kinds of errors are defined in 2.3.1 Kinds of Errors.

Within each phase, an implementation is free to use any strategy or algorithm whose result conforms to the specifications in this document.

2.2.4 Serialization

[Definition: Serialization is the process of converting an XDM instance into a sequence of octets (step DM4 in Figure 1.) ] The general framework for serialization is described in [XSLT 2.0 and XQuery 1.0 Serialization].

2.2.5 Consistency Constraints

In order for XQuery to be well defined, the input XDM instance, the static context, and the dynamic context must be mutually consistent. The consistency constraints listed below are prerequisites for correct functioning of an XQuery implementation. Enforcement of these consistency constraints is beyond the scope of this specification. This specification does not define the result of a query under any condition in which one or more of these constraints is not satisfied.

Some of the consistency constraints use the term data model schema. [Definition: For a given node in an XDM instance, the data model schema is defined as the schema from which the type annotation of that node was derived.] For a node that was constructed by some process other than schema validation, the data model schema consists simply of the schema type definition that is represented by the type annotation of the node.

• For every node that has a type annotation, if that type annotation is found in the in-scope schema definitions (ISSD), then its definition in the ISSD must be equivalent to its definition in the data model schema. Furthermore, all types that are derived by extension from the given type in the data model schema must also be known by equivalent definitions in the ISSD.

• For every element name EN that is found both in an XDM instance and in the in-scope schema definitions (ISSD), all elements that are known in the data model schema to be in the substitution group headed by EN must also be known in the ISSD to be in the substitution group headed by EN.

• Every element name, attribute name, or schema type name referenced in in-scope variables or function signatures must be in the in-scope schema definitions, unless it is an element name referenced as part of an ElementTest or an attribute name referenced as part of an AttributeTest.

• Any reference to a global element, attribute, or type name in the in-scope schema definitions must have a corresponding element, attribute or type definition in the in-scope schema definitions.

• For each mapping of a string to a document node in available documents, if there exists a mapping of the same string to a document type in statically known documents, the document node must match the document type, using the matching rules in 2.5.4 SequenceType Matching.

• For each mapping of a string to a sequence of nodes in available collections, if there exists a mapping of the same string to a type in statically known collections, the sequence of nodes must match the type, using the matching rules in 2.5.4 SequenceType Matching.

• The sequence of nodes in the default collection must match the statically known default collection type, using the matching rules in 2.5.4 SequenceType Matching.

• The value of the context item must match the context item static type, using the matching rules in 2.5.4 SequenceType Matching.

• For each (variable, type) pair in in-scope variables and the corresponding (variable, value) pair in variable values such that the variable names are equal, the value must match the type, using the matching rules in 2.5.4 SequenceType Matching.

• For each variable declared as external: If the variable declaration includes a declared type, the external environment must provide a value for the variable that matches the declared type, using the matching rules in 2.5.4 SequenceType Matching. If the variable declaration does not include a declared type, the external environment must provide a type and a matching value, using the same matching rules.

• For each function declared as external: the function implementation must either return a value that matches the declared result type, using the matching rules in 2.5.4 SequenceType Matching, or raise an implementation-defined error.

• For a given query, define a participating ISSD as the in-scope schema definitions of a module that is used in evaluating the query. If two participating ISSDs contain a definition for the same schema type, element name, or attribute name, the definitions must be equivalent in both ISSDs. Furthermore, if two participating ISSDs each contain a definition of a schema type T, the set of types derived by extension from T must be equivalent in both ISSDs. Also, if two participating ISSDs each contain a definition of an element name E, the substitution group headed by E must be equivalent in both ISSDs.

• In the statically known namespaces, the prefix xml must not be bound to any namespace URI other than http://www.w3.org/XML/1998/namespace, and no prefix other than xml may be bound to this namespace URI.

2.3.1 Kinds of Errors

As described in 2.2.3 Expression Processing, XQuery defines a static analysis phase, which does not depend on input data, and a dynamic evaluation phase, which does depend on input data. Errors may be raised during each phase.

[Definition: A static error is an error that must be detected during the static analysis phase. A syntax error is an example of a static error.]

[Definition: A dynamic error is an error that must be detected during the dynamic evaluation phase and may be detected during the static analysis phase. Numeric overflow is an example of a dynamic error. ]

[Definition: A type error may be raised during the static analysis phase or the dynamic evaluation phase. During the static analysis phase, a type error occurs when the static type of an expression does not match the expected type of the context in which the expression occurs. During the dynamic evaluation phase, a type error occurs when the dynamic type of a value does not match the expected type of the context in which the value occurs.]

The outcome of the static analysis phase is either success or one or more type errors, static errors, or statically-detected dynamic errors. The result of the dynamic evaluation phase is either a result value, a type error, or a dynamic error.

If more than one error is present, or if an error condition comes within the scope of more than one error defined in this specification, then any non-empty subset of these errors may be reported.

During the static analysis phase, if the Static Typing Feature is in effect and the static type assigned to an expression other than () or data(()) is empty-sequence(), a static error is raised [err:XPST0005]. This catches cases in which a query refers to an element or attribute that is not present in the in-scope schema definitions, possibly because of a spelling error.

Independently of whether the Static Typing Feature is in effect, if an implementation can determine during the static analysis phase that an expression, if evaluated, would necessarily raise a type error or a dynamic error, the implementation may (but is not required to) report that error during the static analysis phase. However, the fn:error() function must not be evaluated during the static analysis phase.

[Definition: In addition to static errors, dynamic errors, and type errors, an XQuery implementation may raise warnings, either during the static analysis phase or the dynamic evaluation phase. The circumstances in which warnings are raised, and the ways in which warnings are handled, are implementation-defined.]

In addition to the errors defined in this specification, an implementation may raise a dynamic error for a reason beyond the scope of this specification. For example, limitations may exist on the maximum numbers or sizes of various objects. Any such limitations, and the consequences of exceeding them, are implementation-dependent.

2.3.2 Identifying and Reporting Errors

The errors defined in this specification are identified by QNames that have the form err:XXYYnnnn, where:

• err denotes the namespace for XPath and XQuery errors, http://www.w3.org/2005/xqt-errors. This binding of the namespace prefix err is used for convenience in this document, and is not normative.

• XX denotes the language in which the error is defined, using the following encoding:

  • XP denotes an error defined by XPath. Such an error may also occur XQuery since XQuery includes XPath as a subset.

  • XQ denotes an error defined by XQuery.

• YY denotes the error category, using the following encoding:

  • ST denotes a static error.

  • DY denotes a dynamic error.

  • TY denotes a type error.

• nnnn is a unique numeric code.

The method by which an XQuery processor reports error information to the external environment is implementation-defined.

An error can be represented by a URI reference that is derived from the error QName as follows: an error with namespace URI NS and local part LP can be represented as the URI reference NS#LP. For example, an error whose QName is err:XPST0017 could be represented as http://www.w3.org/2005/xqt-errors#XPST0017.

2.3.3 Handling Dynamic Errors

Except as noted in this document, if any operand of an expression raises a dynamic error, the expression also raises a dynamic error. If an expression can validly return a value or raise a dynamic error, the implementation may choose to return the value or raise the dynamic error. For example, the logical expression expr1 and expr2 may return the value false if either operand returns false, or may raise a dynamic error if either operand raises a dynamic error.

If more than one operand of an expression raises an error, the implementation may choose which error is raised by the expression. For example, in this expression:

($x div $y) + xs:decimal($z)

both the sub-expressions ($x div $y) and xs:decimal($z) may raise an error. The implementation may choose which error is raised by the "+" expression. Once one operand raises an error, the implementation is not required, but is permitted, to evaluate any other operands.

[Definition: In addition to its identifying QName, a dynamic error may also carry a descriptive string and one or more additional values called error values.] An implementation may provide a mechanism whereby an application-defined error handler can process error values and produce diagnostic messages.

A dynamic error may be raised by a built-in function or operator. For example, the div operator raises an error if its operands are xs:decimal values and its second operand is equal to zero. Errors raised by built-in functions and operators are defined in [XQuery 1.0 and XPath 2.0 Functions and Operators].

A dynamic error can also be raised explicitly by calling the fn:error function, which only raises an error and never returns a value. This function is defined in [XQuery 1.0 and XPath 2.0 Functions and Operators]. For example, the following function call raises a dynamic error, providing a QName that identifies the error, a descriptive string, and a diagnostic value (assuming that the prefix app is bound to a namespace containing application-defined error codes):

fn:error(xs:QName("app:err057"), "Unexpected value", fn:string($v))

2.3.4 Errors and Optimization

Because different implementations may choose to evaluate or optimize an expression in different ways, certain aspects of the detection and reporting of dynamic errors are implementation-dependent, as described in this section.

An implementation is always free to evaluate the operands of an operator in any order.

In some cases, a processor can determine the result of an expression without accessing all the data that would be implied by the formal expression semantics. For example, the formal description of filter expressions suggests that $s[1] should be evaluated by examining all the items in sequence $s, and selecting all those that satisfy the predicate position()=1. In practice, many implementations will recognize that they can evaluate this expression by taking the first item in the sequence and then exiting. If $s is defined by an expression such as //book[author eq 'Berners-Lee'], then this strategy may avoid a complete scan of a large document and may therefore greatly improve performance. However, a consequence of this strategy is that a dynamic error or type error that would be detected if the expression semantics were followed literally might not be detected at all if the evaluation exits early. In this example, such an error might occur if there is a book element in the input data with more than one author subelement.

The extent to which a processor may optimize its access to data, at the cost of not detecting errors, is defined by the following rules.

Consider an expression Q that has an operand (sub-expression) E. In general the value of E is a sequence. At an intermediate stage during evaluation of the sequence, some of its items will be known and others will be unknown. If, at such an intermediate stage of evaluation, a processor is able to establish that there are only two possible outcomes of evaluating Q, namely the value V or an error, then the processor may deliver the result V without evaluating further items in the operand E. For this purpose, two values are considered to represent the same outcome if their items are pairwise the same, where nodes are the same if they have the same identity, and values are the same if they are equal and have exactly the same type.

There is an exception to this rule: If a processor evaluates an operand E (wholly or in part), then it is required to establish that the actual value of the operand E does not violate any constraints on its cardinality. For example, the expression $e eq 0 results in a type error if the value of $e contains two or more items. A processor is not allowed to decide, after evaluating the first item in the value of $e and finding it equal to zero, that the only possible outcomes are the value true or a type error caused by the cardinality violation. It must establish that the value of $e contains no more than one item.

These rules apply to all the operands of an expression considered in combination: thus if an expression has two operands E1 and E2, it may be evaluated using any samples of the respective sequences that satisfy the above rules.

The rules cascade: if A is an operand of B and B is an operand of C, then the processor needs to evaluate only a sufficient sample of B to determine the value of C, and needs to evaluate only a sufficient sample of A to determine this sample of B.

The effect of these rules is that the processor is free to stop examining further items in a sequence as soon as it can establish that further items would not affect the result except possibly by causing an error. For example, the processor may return true as the result of the expression S1 = S2 as soon as it finds a pair of equal values from the two sequences.

Another consequence of these rules is that where none of the items in a sequence contributes to the result of an expression, the processor is not obliged to evaluate any part of the sequence. Again, however, the processor cannot dispense with a required cardinality check: if an empty sequence is not permitted in the relevant context, then the processor must ensure that the operand is not an empty sequence.

Examples:

• If an implementation can find (for example, by using an index) that at least one item returned by $expr1 in the following example has the value 47, it is allowed to return true as the result of the some expression, without searching for another item returned by $expr1 that would raise an error if it were evaluated.

  some $x in $expr1 satisfies $x = 47
  

• In the following example, if an implementation can find (for example, by using an index) the product element-nodes that have an id child with the value 47, it is allowed to return these nodes as the result of the path expression, without searching for another product node that would raise an error because it has an id child whose value is not an integer.

  //product[id = 47]
  

For a variety of reasons, including optimization, implementations are free to rewrite expressions into equivalent expressions. Other than the raising or not raising of errors, the result of evaluating an equivalent expression must be the same as the result of evaluating the original expression. Expression rewrite is illustrated by the following examples.

• Consider the expression //part[color eq "Red"]. An implementation might choose to rewrite this expression as //part[color = "Red"][color eq "Red"]. The implementation might then process the expression as follows: First process the "=" predicate by probing an index on parts by color to quickly find all the parts that have a Red color; then process the "eq" predicate by checking each of these parts to make sure it has only a single color. The result would be as follows:

  • Parts that have exactly one color that is Red are returned.

  • If some part has color Red together with some other color, an error is raised.

  • The existence of some part that has no color Red but has multiple non-Red colors does not trigger an error.

• The expression in the following example cannot raise a casting error if it is evaluated exactly as written (i.e., left to right). Since neither predicate depends on the context position, an implementation might choose to reorder the predicates to achieve better performance (for example, by taking advantage of an index). This reordering could cause the expression to raise an error.

  $N[@x castable as xs:date][xs:date(@x) gt xs:date("2000-01-01")]
  

  To avoid unexpected errors caused by expression rewrite, tests that are designed to prevent dynamic errors should be expressed using conditional or typeswitch expressions. Conditional and typeswitch expressions raise only dynamic errors that occur in the branch that is actually selected. Thus, unlike the previous example, the following example cannot raise a dynamic error if @x is not castable into an xs:date:

  $N[if (@x castable as xs:date)
     then xs:date(@x) gt xs:date("2000-01-01")
     else false()]
  

2.4.1 Document Order

An ordering called document order is defined among all the nodes accessible during processing of a given query, which may consist of one or more trees (documents or fragments). Document order is defined in [XQuery/XPath Data Model (XDM)], and its definition is repeated here for convenience. [Definition: The node ordering that is the reverse of document order is called reverse document order.]

Document order is a total ordering, although the relative order of some nodes is