Difference between revisions of "XQuery Extensions"
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The [https://en.wikipedia.org/wiki/%3F: ternary if] operator provides a short syntax for conditions. It is also called '''conditional operator''' or '''ternary operator'''. In most languages, the syntax is <code>a ? b : c</code>. As <code>?</code> and <code>:</code> have already been taken in XQuery, the syntax of Perl 6 is used: | The [https://en.wikipedia.org/wiki/%3F: ternary if] operator provides a short syntax for conditions. It is also called '''conditional operator''' or '''ternary operator'''. In most languages, the syntax is <code>a ? b : c</code>. As <code>?</code> and <code>:</code> have already been taken in XQuery, the syntax of Perl 6 is used: | ||
| − | < | + | <pre lang='xquery'> |
(: if/then/else :) | (: if/then/else :) | ||
if ($ok) then 1 else 0, | if ($ok) then 1 else 0, | ||
| Line 22: | Line 22: | ||
In XQuery 3.1, both branches of the <code>if</code> expression need to be specified. In many cases, only one branch is required, so the <code>else</code> 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: | In XQuery 3.1, both branches of the <code>if</code> expression need to be specified. In many cases, only one branch is required, so the <code>else</code> 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: | ||
| − | < | + | <pre lang='xquery'> |
if (doc-available($doc)) then doc($doc), | if (doc-available($doc)) then doc($doc), | ||
if (file:exists($file)) then file:delete($file), | if (file:exists($file)) then file:delete($file), | ||
| Line 30: | Line 30: | ||
If conditions are nested, a trailing else branch will be associated with the innermost <code>if</code>: | If conditions are nested, a trailing else branch will be associated with the innermost <code>if</code>: | ||
| − | < | + | <pre lang='xquery'> |
if ($a) then if($b) then '$a and $b is true' else 'only $a is true' | if ($a) then if($b) then '$a and $b is true' else 'only $a is true' | ||
</syntaxhighlight> | </syntaxhighlight> | ||
| Line 36: | Line 36: | ||
In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code: | In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code: | ||
| − | < | + | <pre lang='xquery'> |
if ($a) then ( | if ($a) then ( | ||
if($b) then '$a and $b is true' else 'only $a is true' | if($b) then '$a and $b is true' else 'only $a is true' | ||
| Line 49: | Line 49: | ||
'''Example:''' | '''Example:''' | ||
| − | < | + | <pre lang='xquery'> |
(: yields "!Hi! !there!" :) | (: yields "!Hi! !there!" :) | ||
replace('Hi there', '\b', '!', 'j') | replace('Hi there', '\b', '!', 'j') | ||
| Line 68: | Line 68: | ||
[[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: | [[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: | ||
| − | < | + | <pre lang='xquery'> |
declare option db:catalog 'etc/w3-catalog.xml'; | declare option db:catalog 'etc/w3-catalog.xml'; | ||
doc('doc.xml') | doc('doc.xml') | ||
| Line 85: | Line 85: | ||
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: | 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: | ||
| − | < | + | <pre lang='xquery'> |
sum( (# basex:nondeterministic #) { | sum( (# basex:nondeterministic #) { | ||
1 to 100000000 | 1 to 100000000 | ||
| Line 95: | Line 95: | ||
In analogy with option declarations and function annotations, XQuery locks can also set via pragmas. See [[Transaction Management#Options|Transaction Management]] for details and examples. | In analogy with option declarations and function annotations, XQuery locks can also set via pragmas. See [[Transaction Management#Options|Transaction Management]] for details and examples. | ||
| − | < | + | <pre lang='xquery'> |
(# basex:write-lock CONFIGLOCK #) { | (# basex:write-lock CONFIGLOCK #) { | ||
file:write('config.xml', <config/>) | file:write('config.xml', <config/>) | ||
| Line 107: | Line 107: | ||
* [[Indexes|Index access rewritings]] can be enforced. This is helpful if the name of a database is not static (see [[Indexes#Enforce Rewritings|Enforce Rewritings]] for more details): | * [[Indexes|Index access rewritings]] can be enforced. This is helpful if the name of a database is not static (see [[Indexes#Enforce Rewritings|Enforce Rewritings]] for more details): | ||
| − | < | + | <pre lang='xquery'> |
(# db:enforceindex #) { | (# db:enforceindex #) { | ||
for $db in ('persons1', 'persons2', 'persons3') | for $db in ('persons1', 'persons2', 'persons3') | ||
| Line 116: | Line 116: | ||
* Node copying in node constructors can be disabled (see {{Option|COPYNODE}} for more details). The following query will consume much less memory than without pragma as the database nodes will not be fully duplicated, but only attached to the {{Code|xml}} parent element: | * Node copying in node constructors can be disabled (see {{Option|COPYNODE}} for more details). The following query will consume much less memory than without pragma as the database nodes will not be fully duplicated, but only attached to the {{Code|xml}} parent element: | ||
| − | < | + | <pre lang='xquery'> |
file:write( | file:write( | ||
'wrapped-db-nodes.xml', | 'wrapped-db-nodes.xml', | ||
| Line 137: | Line 137: | ||
'''Query:''' | '''Query:''' | ||
| − | < | + | <pre lang='xquery'> |
declare function local:square($a) { $a * $a }; | declare function local:square($a) { $a * $a }; | ||
for $i in 1 to 3 | for $i in 1 to 3 | ||
| Line 145: | Line 145: | ||
'''Query after function inlining:''' | '''Query after function inlining:''' | ||
| − | < | + | <pre lang='xquery'> |
for $i in 1 to 3 | for $i in 1 to 3 | ||
return | return | ||
| Line 154: | Line 154: | ||
'''Query after further optimizations:''' | '''Query after further optimizations:''' | ||
| − | < | + | <pre lang='xquery'> |
for $i in 1 to 3 | for $i in 1 to 3 | ||
return $i * $i | return $i * $i | ||
| Line 165: | Line 165: | ||
'''Example:''' | '''Example:''' | ||
| − | < | + | <pre lang='xquery'> |
(: disable function inlining; the full stack trace will be shown... :) | (: disable function inlining; the full stack trace will be shown... :) | ||
declare %basex:inline(0) function local:e() { error() }; | declare %basex:inline(0) function local:e() { error() }; | ||
| Line 186: | Line 186: | ||
'''Example:''' | '''Example:''' | ||
| − | < | + | <pre lang='xquery'> |
declare %basex:lazy variable $january := doc('does-not-exist.xml'); | declare %basex:lazy variable $january := doc('does-not-exist.xml'); | ||
if(month-from-date(current-date()) = 1) then $january else () | if(month-from-date(current-date()) = 1) then $january else () | ||
| Line 201: | Line 201: | ||
In XQuery, some namespaces are statically bound to prefixes. The following query requires no additional namespaces declarations in the query prolog: | In XQuery, some namespaces are statically bound to prefixes. The following query requires no additional namespaces declarations in the query prolog: | ||
| − | < | + | <pre lang='xquery'> |
<xml:abc xmlns:prefix='uri' local:fn='x'/>, | <xml:abc xmlns:prefix='uri' local:fn='x'/>, | ||
fn:exists(1) | fn:exists(1) | ||
| Line 241: | Line 241: | ||
* Main modules have an expression as query body. Here is a minimum example: | * Main modules have an expression as query body. Here is a minimum example: | ||
| − | < | + | <pre lang='xquery'> |
'Hello World!' | 'Hello World!' | ||
</syntaxhighlight> | </syntaxhighlight> | ||
| Line 247: | Line 247: | ||
* Library modules start with a module namespace declaration and have no query body: | * Library modules start with a module namespace declaration and have no query body: | ||
| − | < | + | <pre lang='xquery'> |
module namespace hello = 'http://basex.org/examples/hello'; | module namespace hello = 'http://basex.org/examples/hello'; | ||
Revision as of 18:33, 1 December 2023
This article is part of the XQuery Portal. It lists extensions and optimizations that are specific to the BaseX XQuery processor.
Contents
Expressions
Removed with Version 11: Elvis operator ?:, in favor of the new otherwise expression.
Ternary If
The ternary if operator provides a short syntax for conditions. It is also called conditional operator or ternary operator. In most languages, the syntax is a ? b : c. As ? and : have already been taken in XQuery, the syntax of Perl 6 is used:
(: if/then/else :)
if ($ok) then 1 else 0,
(: ternary if :)
$ok ?? 1 !! 0
</syntaxhighlight>
The expression returns ok if the effective boolean value of $test is true, and it returns fails otherwise.
If Without Else
In XQuery 3.1, both branches of the if expression need to be specified. In many cases, only one branch is required, so the else branch was made optional in BaseX. If the second branch is omitted, an empty sequence will be returned if the effective boolean value of the test expression is false. Some examples:
if (doc-available($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:
if ($a) then if($b) then '$a and $b is true' else 'only $a is true'
</syntaxhighlight>
In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code:
if ($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:
(: 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:
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:
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.
(# 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):
(# 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:
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:
declare function local:square($a) { $a * $a };
for $i in 1 to 3
return local:square($i)
</syntaxhighlight>
Query after function inlining:
for $i in 1 to 3
return
let $a := $i
return $a * $a
</syntaxhighlight>
Query after further optimizations:
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:
(: 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:
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:
<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:
'Hello World!'
</syntaxhighlight>
* Library modules start with a module namespace declaration and have no query body:
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.
