Difference between revisions of "XQuery Extensions"

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The expression returns <code>ok</code> if the effective boolean value of <code>$test</code> is true, and it returns <code>fails</code> otherwise.
 
The expression returns <code>ok</code> if the effective boolean value of <code>$test</code> is true, and it returns <code>fails</code> otherwise.
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==Elvis Operator==
 
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The Elvis operator is also available in other languages. It is sometimes called [https://en.wikipedia.org/wiki/Null_coalescing_operator null-coalescing operator]. In XQuery, the value of the first operand will be returned if it is a non-empty sequence. Otherwise, the value of the second operand will be returned.
 
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<syntaxhighlight lang="xquery">
 
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(: if/then/else :)
 
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if (exists($argument)) then $argument else 0
 
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(: elvis operator :)
 
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$argument ?: -1
 
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</syntaxhighlight>
 
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The behavior of the operator is equivalent to the {{Function|Utility|util:or}} function.
 
  
 
==If Without Else==
 
==If Without Else==
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</syntaxhighlight>
 
</syntaxhighlight>
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The behavior of the if expression is equivalent to the {{Function|Utility|util:if}} function.
 
  
 
=Functions=
 
=Functions=
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;Version 11:
 
;Version 11:
  
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* Removed: Elvis operator <code>?:</code>, in favor of the new <code>[https://qt4cg.org/specifications/xquery-40/xquery-40.html#id-otherwise otherwise]</code> expression.
 
* Updated: Renamed from {{Code|non-deterministic}} to {{Code|nondeterministic}}.
 
* Updated: Renamed from {{Code|non-deterministic}} to {{Code|nondeterministic}}.
  

Revision as of 15:18, 31 October 2023

This article is part of the XQuery Portal. It lists extensions and optimizations that are specific to the BaseX XQuery processor.

Expressions

Some of the extensions that have been added to BaseX may also be made available in other XQuery processors in the near future.

Ternary If

The ternary if operator provides a short syntax for conditions. It is also called conditional operator or ternary operator. In most languages, the syntax is a ? b : c. As ? and : have already been taken in XQuery, the syntax of Perl 6 is used:

<syntaxhighlight lang="xquery"> (: if/then/else :) if ($ok) then 1 else 0, (: ternary if :) $ok ?? 1 !! 0 </syntaxhighlight>

The expression returns ok if the effective boolean value of $test is true, and it returns fails otherwise.

If Without Else

In XQuery 3.1, both branches of the if expression need to be specified. In many cases, only one branch is required, so the else branch was made optional in BaseX. If the second branch is omitted, an empty sequence will be returned if the effective boolean value of the test expression is false. Some examples:

<syntaxhighlight lang="xquery"> if (doc-available($doc)) then doc($doc), if (file:exists($file)) then file:delete($file), if (permissions:valid($user)) then <html>Welcome!</html> </syntaxhighlight>

If conditions are nested, a trailing else branch will be associated with the innermost if:

<syntaxhighlight lang="xquery"> if ($a) then if($b) then '$a and $b is true' else 'only $a is true' </syntaxhighlight>

In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code:

<syntaxhighlight lang="xquery"> if ($a) then ( 

 if($b) then '$a and $b is true' else 'only $a is true'

) </syntaxhighlight>

Functions

Regular Expressions

In analogy with Saxon, you can specify the flag j to revert to Java’s default regex parser. For example, this allows you to use the word boundary option \b, which has not been included in the XQuery grammar for regular expressions:

Example: <syntaxhighlight lang="xquery"> (: yields "!Hi! !there!" :) replace('Hi there', '\b', '!', 'j') </syntaxhighlight>

Serialization

  • basexis used as the default serialization method: nodes are serialized as XML, atomic values are serialized as string, and items of binary type are output in their native byte representation. Function items (including maps and arrays) are output just like with the adaptive method.
  • With csv, you can output XML nodes as CSV data (see the CSV Module for more details).
  • With json, items are output as JSON as described in the official specification. If the root node is of type element(json), items are serialized as described for the direct format in the JSON Module.

For more information and some additional BaseX-specific parameters, see the article on Serialization.

Option Declarations

Database Options

Local database options can be set in the prolog of an XQuery main module. In the option declaration, options need to be bound to the Database Module namespace. All values will be reset after the evaluation of a query:

<syntaxhighlight lang="xquery"> declare option db:catalog 'etc/w3-catalog.xml'; doc('doc.xml') </syntaxhighlight>

XQuery Locks

If locks are declared in the query prolog of a module via the basex:lock option, access to functions of this module locks will be controlled by the central transaction management. See Transaction Management for further details.

Pragmas

BaseX Pragmas

Updated with Version 11: Renamed from non-deterministic to nondeterministic.

Many optimizations in BaseX will only be performed if an expression is deterministic (i. e., if it always yields the same output and does not have side effects). By flagging an expression as nondeterministic, optimizations and query rewritings can be suppressed:

<syntaxhighlight lang="xquery"> sum( (# basex:nondeterministic #) { 

 1 to 100000000

}) </syntaxhighlight>

This pragma can be helpful when debugging your code.

In analogy with option declarations and function annotations, XQuery locks can also set via pragmas. See Transaction Management for details and examples.

<syntaxhighlight lang="xquery"> (# basex:write-lock CONFIGLOCK #) { 

 file:write('config.xml', <config/>)

} </syntaxhighlight>

Database Pragmas

Local database options can also be assigned via pragmas:

<syntaxhighlight lang="xquery"> (# db:enforceindex #) { 

 for $db in ('persons1', 'persons2', 'persons3')
 return db:get($db)//name[text() = 'John']

} </syntaxhighlight>

  • Node copying in node constructors can be disabled (see COPYNODE for more details). The following query will consume much less memory than without pragma as the database nodes will not be fully duplicated, but only attached to the xml parent element:

<syntaxhighlight lang="xquery"> file:write( 

 'wrapped-db-nodes.xml',
 (# db:copynode false #) {
   <xml>{ db:get('huge') }</xml>
 }

) </syntaxhighlight>

  • An XML catalog can be specified for URI rewritings. See the Catalog Resolver section for an example.

Annotations

Function Inlining

%basex:inline([limit]) controls if functions will be inlined.

If XQuery functions are inlined, the function call will be replaced by a FLWOR expression, in which the function variables are bound to let clauses, and in which the function body is returned. This optimization triggers further query rewritings that will speed up your query. An example:

Query:

<syntaxhighlight lang="xquery"> declare function local:square($a) { $a * $a }; for $i in 1 to 3 return local:square($i) </syntaxhighlight>

Query after function inlining:

<syntaxhighlight lang="xquery"> for $i in 1 to 3 return 

 let $a := $i
 return $a * $a

</syntaxhighlight>

Query after further optimizations:

<syntaxhighlight lang="xquery"> for $i in 1 to 3 return $i * $i </syntaxhighlight>

By default, XQuery functions will be inlined if the query body is not too large and does not exceed a fixed number of expressions, which can be adjusted via the INLINELIMIT option.

The annotation can be used to overwrite this global limit: Function inlining can be enforced if no argument is specified. Inlining will be disabled if 0 is specified.

Example:

<syntaxhighlight lang="xquery"> (: disable function inlining; the full stack trace will be shown... :) declare %basex:inline(0) function local:e() { error() }; local:e() </syntaxhighlight>

Result:

<syntaxhighlight lang="xml"> Stopped at query.xq, 1/53: [FOER0000] Halted on error().

Stack Trace: - query.xq, 2/9 </syntaxhighlight>

Lazy Evaluation

%basex:lazy enforces lazy evaluation of a global variable. An example:

Example: <syntaxhighlight lang="xquery"> declare %basex:lazy variable $january := doc('does-not-exist.xml'); if(month-from-date(current-date()) = 1) then $january else () </syntaxhighlight>

The annotation ensures that an error is only raised if the condition yields true. Without the annotation, the error is always raised if the referenced document is not found.

XQuery Locks

In analogy with option declarations and pragmas, locks can also set via annotations. See Transaction Management for details and examples.

Namespaces

In XQuery, some namespaces are statically bound to prefixes. The following query requires no additional namespaces declarations in the query prolog:

<syntaxhighlight lang="xquery"> <xml:abc xmlns:prefix='uri' local:fn='x'/>, fn:exists(1) </syntaxhighlight>

In BaseX, various other namespaces are predefined. Apart from the namespaces that are listed on the Module Library page, the following namespaces are statically bound:

Description Prefix Namespace URI
BaseX Annotations, Pragmas, … basex http://basex.org
RESTXQ: Input Options input http://basex.org/modules/input
EXPath Packages pkg http://expath.org/ns/pkg
XQuery Errors err http://www.w3.org/2005/xqt-errors
Serialization output http://www.w3.org/2010/xslt-xquery-serialization

Suffixes

In BaseX, files with the suffixes .xq, .xqm, .xqy, .xql, .xqu and .xquery are treated as XQuery files. In XQuery, there are main and library modules:

  • Main modules have an expression as query body. Here is a minimum example:

<syntaxhighlight lang="xquery"> 'Hello World!' </syntaxhighlight>

  • Library modules start with a module namespace declaration and have no query body:

<syntaxhighlight lang="xquery"> module namespace hello = 'http://basex.org/examples/hello';

declare function hello:world() { 

 'Hello World!'

}; </syntaxhighlight>

We recommend .xq as suffix for for main modules, and .xqm for library modules. However, the actual module type will dynamically be detected when a file is opened and parsed.

Miscellaneous

Various other extensions are described in the articles on XQuery Full Text and XQuery Update.

Changelog

Version 11
  • Removed: Elvis operator ?:, in favor of the new otherwise expression.
  • Updated: Renamed from non-deterministic to nondeterministic.
Version 9.1
  • Added: New Expressions: Ternary if, elvis Operator, if without else
  • Added: XQuery Locks via pragmas and function annotations.
  • Added: Regular Expressions, j flag for using Java’s default regex parser.
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