Updating XML documents

Transcription

1 Grindei Manuela Lidia Updating XML documents XQuery Update XQuery is the a powerful functional language, which enables accessing different nodes of an XML document. However, updating could not be done in XQuery so far, which was a drawback for it. Now it is possible that this situation changes, by adding some imperative constructs allowing updates. This is the goal of a team called XML Query Working Group, that is currently working on it. XQuery Update is the first public working draft made by W3C, that extends XQuery with an update facility by providing expressions for making persistent data changes to the XQuery 1.0 and the XPath 2.0 Data Model instances. Four new operations are thus introduced: insertion of a node deletion of a node modification of a node by changing some of its properties while preserving its identity creation of a modified copy of a node with a new identity In this extended XQuery language, the expressions can be classified in updating and non-updating. An updating expression is an expression which can modify the state of an existing node, while a nonupdating expression is an expression that cannot modify the state of an existing node. For instance, insert, delete, replace, rename are updating expressions, while transform is non-updating. Some examples of operations using these expressions will be shown next. Example 1 Insert a year element after the publisher of the first book. insert <year>2005</year> after fn:doc("bib.xml")/books/book[1]/publisher Example 2 Delete the last author of the first book in a given bibliography. delete{fn:doc("bib.xml")/books/book[1]/author[last()]} Example 3 Replace the publisher of the first book with the publisher of the second book. replace {fn:doc("bib.xml")/books/book[1]/publisher} with fn:doc("bib.xml")/books/book[2]/publisher Example 4 Rename the first author element of the first book to principal-author. 1

2 rename {fn:doc("bib.xml")/books/book[1]/author[1]} to "principal-author" Example 5 Return a sequence containing of all employee elements that have Java as a skill, excluding their salary child-elements. for $e in //employee[skill = "Java"] return transform copy $je := $e do delete {$je/salary} return $je ORDPATH ORDPATH is a labeling method for the nodes of an XML tree, which is particularly flexible to insertions. The encoding is made as it follows: root receives label 1 the n-th child of a node labelled m receives the label m.(2 * n 1). Futhermore, the document order is exactly the lexicographical ORDPATH order. This provides certain advantages (e.g. the descendants of a node are consecutive in the database, if ORDPATH is the primary key, leading to efficient caching). Insertions at the rightmost side are very easy to be made: the label of the inserted node is p.(2*(n+1)-1), where p is the parent label and n the number of children of the parent node. It can be easily remarked that every label of such nodes should end in with an odd positive number. The following example illustrates these ideas. We can insert a new child on the left of all existing children, just by adding -2 to the last ordinal of the first child, using negative ordinal values if necessary. 2

3 For example, in this figure, if we want to introduce a new child for the node labelled 5 at the left of 5.1, then this child should be labelled The remaining case refers to inserting a new child between two siblings (careting in), which is done by creating a component with an even ordinal falling between their final ordinals, then following it with a new odd component. In the image below, a subtree is inserted between the siblings 5.5 and the 5.7. The new nodes will receive a 6 in the label, and then they will be labelled usually. The carets do not count for ancestry, as we will see next. Only two more important observations remain to be made: the ORDPATH of the parent of a node X has the rightmost component of the ORDPATH of X removed (always an odd number) and then all rightmost even ordinal components the descendants of X can be easily spotted, as the ones having the ORDPATH label greater than X's and smaller than the label of X with 1 added to the rightmost ordinal. Updates in MonetDB/XQuery In the following part, we will use pre/size/level representation, which is equivalent to the pre/post representation, since post = pre + size level. The following image shows an example of this it. 3

4 XML updates can be classified in value updates(node value changes and any change regarding attributes) and structural updates(insert or delete nodes in an XML document). Since the value updates can be easily made by usual table operations, we will focus next on the structural updates. The complexity of this problem is shown by the following case, where most of the nodes suffer modifications. One possible efficient method of handling updates is using a table pos/size/level, where pos is a densely increasing integer column(0, 1, 2,...). This table is divided into logical pages and each logical page may have unused tuples. New logical pages are appended only. The pre/size/level table is therefore a view on pos/size/level with all pages in logical order. The nodes remain logically consecutive after the insertions, but physically it is not necessary so. A small example is shown below. 4

5 This way of modelling the system has two main advantages: structural deletes leave the tuples of the deleted nodes in place, without any shifts in the pre numbers inserts cause at most page-wise table appends Conclusion- pros and cons of the two ways of managing updates Pre/Post Updates + fixed block sizes for all data structures are provided - it is a Monet-oriented concept, hardly appliable to arbitrary RDBMS (although claimed in the paper) ORDPATH + provides (almost) constant-time updates, as well as a clear structure - key sizes are variable References Don Chamberlin, Daniela Florescu, Jonathan Robie: XQuery Update Facility, in Peter A. Boncz, Stefan Manegold, Jan Rittinger: Updating the Pre/Post Plane in MonetDB/XQuery, in XIME-P Patrick E. O'Neil, Elizabeth J. O'Neil, et al.: ORDPATHs: Insert-Friendly XML Node Labels., in SIGMOD, pp