XQuery Update

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This article is part of the XQuery Portal. It summarizes the update features of BaseX.

BaseX offers a complete implementation of the XQuery Update Facility (XQUF). This article aims to provide a very quick and basic introduction to the XQUF. First, some examples for update expressions are given. After that, a few problems are addressed that frequently arise due to the nature of the language. These are stated in the Concepts paragraph.

Contents

[edit] Features

[edit] Updating Expressions

There are five new expressions to modify data. While insert, delete, rename and replace are basically self-explanatory, the transform expression is different, as modified nodes are copied in advance and the original databases remain untouched.

An expression consists of a target node (the node we want to alter) and additional information like insertion nodes, a QName, etc. which depends on the type of expression. Optional modifiers are available for some of them. You can find a few examples and additional information below.

[edit] insert

insert node (attribute { 'a' } { 5 }, 'text', <e/>) into /n

Insert enables you to insert a sequence of nodes into a single target node. Several modifiers are available to specify the exact insert location: insert into as first/as last, insert before/after and insert into.

Note: in most cases, as last and after will be evaluated faster than as first and before!

[edit] delete

delete node //node

The example query deletes all <node> elements in your database. Note that, in contrast to other updating expressions, the delete expression allows multiple nodes as a target.

[edit] replace

replace node /n with <a/>

The target element is replaced by the DOM node <a/>. You can also replace the value of a node or its descendants by using the modifier value of.

replace value of node /n with 'newValue'

All descendants of /n are deleted and the given text is inserted as the only child. Note that the result of the insert sequence is either a single text node or an empty sequence. If the insert sequence is empty, all descendants of the target are deleted. Consequently, replacing the value of a node leaves the target with either a single text node or no descendants at all.

[edit] rename

for $n in //node
return rename node $n as 'renamedNode' 

All node elements are renamed. An iterative approach helps to modify multiple nodes within a single statement. Nodes on the descendant- or attribute-axis of the target are not affected. This has to be done explicitly as well.

[edit] Non-Updating Expressions

[edit] transform

copy $c := doc('example.xml')//node[@id = 1]
modify rename node $c as 'copyOfNode'
return $c

The node element with @id=1 is copied and subsequently assigned a new QName using the rename expression. Note that the transform expression is the only expression which returns an actual XDM instance as a result. You can therefore use it to modify results and especially DOM nodes. This is an issue beginners are often confronted with. More on this topic can be found in the XQUF Concepts section.

The following example demonstrates a common use case:

Query:

copy $c :=
  <entry>
    <title>Transform expression example</title>
    <author>BaseX Team</author>
  </entry>
modify (
  replace value of node $c/author with 'BaseX',
  replace value of node $c/title with concat('Copy of: ', $c/title),
  insert node <author>Joey</author> into $c
)
return $c

Result:

<entry>
  <title>Copy of: Transform expression example</title>
  <author>BaseX</author>
  <author>Joey</author>
</entry>

The <entry> element (here it is passed to the expression as a DOM node) can also be replaced by a database node, e.g.:

copy $c := (db:open('example')//entry)[1]
...

In this case, the original database node remains untouched as well, as all updates are performed on the node copy.

Here is an example where we return an entire document, parts modified and all:

copy $c := doc("zaokeng.kml")
modify (
  for $d in $c//*:Point
  return insert node (
    <extrude>1</extrude>,
    <altitudeMode>relativeToGround</altitudeMode>
  )  before $d/*:coordinates
)
return $c

[edit] update

for $item in db:open('data')//item
return $item update delete node text()

The update expression is a convenience operator for writing simple transform expressions. Similar to the XQuery 3.0 map operator, the value of the first expression is bound as context item, and the second expression performs updates on this item. The updated item is returned as result.

Please note that update is not part of the official XQuery Update Facility yet. It is currently being discussed in the W3 Bug Tracker; your feedback is welcome.

[edit] Functions

[edit] fn:put

fn:put() is also part of the XQUF and enables the user to serialize XDM instances to secondary storage. It is executed at the end of a snapshot. Serialized documents therefore reflect all changes made effective during a query.

[edit] Database Functions

Some additional, updating database functions exist in order to perform updates on document and database level.

[edit] Concepts

There are a few specialties around XQuery Update that you should know about. In addition to the simple expression, the XQUF adds the updating expression as a new type of expression. An updating expression returns only a Pending Update List (PUL) as a result which is subsequently applied to addressed databases and DOM nodes. A simple expression cannot perform any permanent changes and returns an empty or non-empty sequence.

[edit] Pending Update List

The most important thing to keep in mind when using XQuery Update is the Pending Update List (PUL). Updating statements are not executed immediately, but are first collected as update primitives within a set-like structure. At the end of a query, after some consistency checks and optimizations, the update primitives will be applied in the following order:

If an inconsistency is found, an error message is returned and all accessed databases remain untouched (atomicity). For the user, this means that updates are only visible after the end of a snapshot.

It may be surprising to see db:create in the lower part of this list. This means that newly created database cannot be accessed by the same query, which can be explained by the semantics of updating queries: all expressions can only be evaluated on databases that already exist while the query is evaluated. As a consequence, db:create is mainly useful in the context of Command Scripts, or Web Applications, in which a redirect to another page can be triggered after having created a database.

[edit] Example

The query…

insert node <b/> into /doc,
for $n in /doc/child::node()
return rename node $n as 'justRenamed'

…applied on the document…

<doc> <a/> </doc>

…results in the following document:

<doc> <justRenamed/><b/> </doc>

Despite explicitly renaming all child nodes of <doc/>, the former <a/> element is the only one to be renamed. The <b/> element is inserted within the same snapshot and is therefore not yet visible to the user.

[edit] Returning Results

By default, it is not possible to mix different types of expressions in a query result. The outermost expression of a query must either be a collection of updating or non-updating expressions. But there are two ways out:

db:output("Update successful."), insert node <c/> into doc('factbook')/mondial

If you want to modify nodes in main memory, you can use the transform expression.

[edit] Function Declaration

To use updating expressions within a function, the %updating annotation has to be added to the function declaration. A correct declaration of a function that contains updating expressions (or one that calls updating functions) looks like this:

declare %updating function { ... }

[edit] Effects

[edit] Original Files

In BaseX, all updates are performed on database nodes or in main memory. By default, update operations never affect the original input file (since Version 8.0, the info string "Updates are not written back" appears in the query info to indicate this). The following solutions exist to write XML documents and binary resources to disk:

[edit] Indexes

Index structures are discarded after update operations when UPDINDEX is turned off (which is the default). More details are found in the article on Indexing.

[edit] Error Messages

Along with the Update Facility, a number of new error codes and messages have been added to the specification and BaseX. All errors are listed in the XQuery Errors overview.

[edit] Changelog

Version 8.0
Version 7.8
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