XQuery Extensions

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. As  and   have already been taken in XQuery, the syntax of Perl 6 is used:

 $test ?? 'ok' !! 'fails'

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

Elvis Operator
The Elvis operator is also available in other languages. It is sometimes called 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.

 let $number := 123 return ( (: if/then/else :)  if (exists($number)) then $number else 0,  (: elvis operator :)  $number ?: 0 )

The behavior of the operator is equivalent to the $$ function.

If Without Else
In XQuery 3.1, both branches of the  expression need to be specified. In many cases, only one branch is required, so the  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:

 if (doc-available($doc)) then doc($doc), if (file:exists($file)) then file:delete($file), if (permissions:valid($user)) then Welcome!

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

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

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

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

The behavior of the if expression is equivalent to the $$ function.

=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:  (: yields "!Hi! !there!" :) replace('Hi there', '\b', '!', 'j')

=Serialization=


 * is 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:

 declare option db:chop 'false'; doc('doc.xml')

XQuery Locks
If XQuery Locks are defined in the query prolog of a module, access to functions of this module locks will be controlled by the central transaction management.

If the following XQuery code is called by two clients in parallel, the queries will be evaluated one after another:

 declare option basex:write-lock 'CONFIGLOCK'; file:write('config.xml', )

=Pragmas=

BaseX Pragmas
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 non-deterministic, optimizations and query rewritings can be suppressed:

 sum( (# basex:non-deterministic #) { 1 to 100000000 })

This pragma can be helpful when debugging your code.

In analogy with option declarations and function annotations, XQuery Locks can also set via pragmas:

 (# basex:write-lock CONFIGLOCK #) { file:write('config.xml', ) }

Database Pragmas
All local options can be assigned via pragmas. Some examples:


 * Enforce query to be rewritten for index access. This can e. g. be helpful if the name of a database is not static (see Enforce Rewritings for more examples):

 (# db:enforceindex #) { for $db in ('persons1', 'persons2', 'persons3') return db:open($db)//name[text = 'John'] }


 * Temporarily disable node copying in node constructors (see for more details). The following query will be evaluated faster, and take much less memory, than without pragma, because the database nodes will not be fully copied, but only attached to the new xml parent element:

 file:write( 'wrapped-db-nodes.xml',  (# db:copynode false #) {    { db:open('huge') }   } )


 * Specify an XML catalog for URI rewriting. 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:

 declare function local:square($a) { $a * $a }; for $i in 1 to 3 return local:square($i)

Query after function inlining:

 for $i in 1 to 3 return let $a := $i return $a * $a

Query after further optimizations:

 for $i in 1 to 3 return $i * $i

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 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:

 (: disable function inlining; the full stack trace will be shown... :) declare %basex:inline(0) function local:e { error }; local:e

Result:

<pre class="brush:xml"> Stopped at query.xq, 1/53: [FOER0000] Halted on error.

Stack Trace: - query.xq, 2/9

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

Example: <pre class="brush:xquery"> declare %basex:lazy variable $january := doc('does-not-exist'); if(month-from-date(current-date) = 1) then $january else

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

XQuery Locks
In analogy with option declarations and pragmas, XQuery Locks can also set via annotations:

<pre class="brush:xquery"> declare %basex:write-lock('CONFIGLOCK') function local:write { file:write('config.xml', ) };

=Non-Determinism=

In XQuery, deterministic functions are “guaranteed to produce ·identical· results from repeated calls within a single ·execution scope· if the explicit and implicit arguments are identical”. In BaseX, many extension functions are non-deterministic or side-effecting. If an expression is internally flagged as non-deterministic, various optimizations that might change their execution order will not be applied.

<pre class="brush:xquery"> (: QUERY A... :) let $n := 456 for $i in 1 to 2 return $n

(: ...will be optimized to :) for $i in 1 to 2 return 456

(: QUERY B will not be rewritten :) let $n := random:integer for $i in 1 to 2 return $n

In some cases, functions may contain non-deterministic code, but the query compiler may not be able to detect this statically. See the following example:

<pre class="brush:xquery"> for $read in (file:read-text#1, file:read-binary#1) let $ignored := non-deterministic $read('input.file') return

Two non-deterministic functions will be bound to, and the result of the function call will be bound to. As the variable is not referenced in the subsequent code, the let clause would usually be discarded by the compiler. In the given query, however, execution will be enforced because of the BaseX-specific non-deterministic keyword.

=Namespaces=

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

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

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:

=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:

<pre class="brush:xquery"> 'Hello World!'


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

<pre class="brush:xquery"> module namespace hello = 'http://basex.org/examples/hello';

declare function hello:world { 'Hello World!' };

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 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.