XQuery Extensions
This article is part of the XQuery Portal. It lists extensions and optimizations that are specific to the BaseX XQuery processor.
Contents
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"> $test ?? 'ok' !! 'fails' </syntaxhighlight>
The expression returns ok
if the effective boolean value of $test
is true, and it returns fails
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.
<syntaxhighlight lang="xquery"> let $number := 123 return (
(: if/then/else :) if (exists($number)) then $number else 0, (: elvis operator :) $number ?: 0
) </syntaxhighlight>
The behavior of the operator is equivalent to the util:or
function.
If Without Else
In XQuery 3.1, both branches of the if
expression need to be specified. In many cases, only one branch is required, so the else
branch was made optional in BaseX. If the second branch is omitted, an empty sequence will be returned if the effective boolean value of the test expression is false. Some examples:
<syntaxhighlight lang="xquery"> if (doc-available($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 $b is true' </syntaxhighlight>
In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code:
<syntaxhighlight lang="xquery"> if ($a) then (
if($b) then '$a and $b is true' else 'only $b is true'
) </syntaxhighlight>
The behavior of the if expression is equivalent to the util:if
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: <syntaxhighlight lang="xquery"> (: yields "!Hi! !there!" :) replace('Hi there', '\b', '!', 'j') </syntaxhighlight>
Serialization
basex
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 typeelement(json)
, items are serialized as described for thedirect
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:chop 'false'; 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
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:
<syntaxhighlight lang="xquery"> sum( (# basex:non-deterministic #) {
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:
- Index access rewritings can be enforced. This is helpful if the name of a database is not static (see Enforce Rewritings for more details):
<syntaxhighlight lang="xquery"> (# db:enforceindex #) {
for $db in ('persons1', 'persons2', 'persons3') return db:open($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 thexml
parent element:
<syntaxhighlight lang="xquery"> file:write(
'wrapped-db-nodes.xml', (# db:copynode false #) { <xml>{ db:open('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'); if(month-from-date(current-date()) = 1) then $january else () </syntaxhighlight>
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, locks can also set via annotations. See Transaction Management for details and examples.
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.
<syntaxhighlight lang="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 </syntaxhighlight>
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:
<syntaxhighlight lang="xquery"> for $read in (file:read-text#1, file:read-binary#1) let $ignored := non-deterministic $read('input.file') return () </syntaxhighlight>
Two non-deterministic functions will be bound to $read
, and the result of the function call will be bound to $ignored
. 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:
<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 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.