XQuery is generic, widely adopted language for querying and manipulating XML data. Many of currently available native XML databases are using XQuery as its primary query language. The XQuery specification requires each XML node to belong to exactly one XML tree. In case of the XML subtree is appended into a new XML structure, the whole subtree has to be copied. This may lead into excessive and unnecessary data copying and duplication. In this paper, we present a new XML node copying scheme that defers the node data copy operation unless necessary. We will show that this schemes significantly reduces the XML node copy operations required during the query processing.
1 Introduction . . .
Every node belongs to exactly one tree, and every tree has exactly one root node.
. . .
If a XML node is added into a new XML tree, the naive realization of this requirement would create a new node (by copying the original one) and the copy would be placed into the new XML tree. Consider the query at figure 1 is to be evaluated and its output is to be serialized to an output file.
A whole XML subtree that matches fn:doc("doc.xml")//authors is never used. This may lead into excessive node copying and higher memory consumptions depending on the subtree size.
In this paper we will describe an efficient node-copying scheme that avoids unnecessary copying while preserving XQuery semantics. We will also discuss its correctness and benchmark results. 1 let $authors = element authors { fn:doc("doc.xml")//authors } 2 let $titles = element titles { fn:doc("doc.xml")//titles } 3 return element result { $titles }
The paper is organized as follows: section 2 give an overall description of the node-copying scheme mentioned above. Section 3 discusses experimental results based on running XMark benchmarks. Section 4 provides a brief overview of related work. Section 5 concludes by summarizing presented work. 2
Deferred Node-copying The basic idea is simple: share existing XML nodes between node hierarchies and defer node-copy operation unless absolutely inevitable. In our implementation the XML node can belong into multiple node hierarchies, although the XQuery specification requirement mentioned in section 1 is preserved.
The deferred node copying scheme has been developed to meet two main goals: – separate query processing logic from underlying physical data model and – reduce memory consumption by preventing unnecessary data copying
The first requirement has software engineering origin. XQuery processors should be able to operate over various data models, not necessarily XML-based. Moreover, good separation of query processor from physical data model provides possibility to use one XQuery implementation in multiple environments – as a standalone XQuery tools or within a database management machine.
The latter goal came from practical needs. In case of large documents and complex queries, naive implementation of an XQuery may consume – in edge cases – twice more memory than actually needed. 2.1
XDM Adaptor XDM specification defines a sequence to be an instance of data model. Each sequence consists of zero or more items. An item is either a node or atomic value. The specification also defines a bunch of node properties such as dm:node-name or dm:parent.
To meet our first goal we separates node from its physical data storage though an XDM adaptor which operates on so called node ids. Node id is an unique identifier of an XML node within particular physical storage. The structure of the node id is not defined – in fact node id could be anything: reference to a DOM node in memory, pointer to a database file or simple integer.
Usage of XDM adaptor give us easy and straightforward way how to access different physical data models. XDM adaptor abstracts any kind of data source and may use any kind of optimization (such as extensive caching) to access data effectively. However the physical data storage and access strategies are hidden to the rest of the XQuery processor. 2.2
Node States In order to defer copy operation, a new node property called node state is introduced. Each node is in exactly one state from following three states: Accessed State. Nodes that come from external data source are in accessed state.
Constructed State. Nodes that are constructed during query processing are in accessed state.
Hybrid State. Nodes which belongs to multiple node hierarchies are in a hybrid state. 2.3
Actions During the query processing, the state of the node may change. The state diagram of the node is shown at figure 2. There are three kinds of actions: Copy Action. The copy action is performed whenever the XML subtree is appended into a new XML hierarchy. The original subtree should be duplicated in order to meet the requirement XML node to belong into just one node hierarchy.
Change Action. The change action models any change in a data model such as setting a new parent. “Child Read” Action. The “child read” action represents the situation when the XQuery processor accesses child nodes of given node.
Copy
Copy
Child read
Change/child read Hybrid
Accessed error
Constructed
Copy
Change
Change/child read
Consider a document doc.xml (it’s content is shown at figure 3) and query 1 (figure 4). During execution of the query, following actions are performed: 1 <?xml version="1.0"?> 2 <root> 3 <elem>elem1</elem> 4 <elem>elem2</elem> 5 </root> 1 element myroot { 2 attribute attr { ’value’ }, 3 fn:doc("doc.xml")/elem[0] 4 } 1. The myroot element is created in a constructed state. Then change actions are issued on that node: setting the node name “myroot”, adding attribute “attr” and appending a text node. 2. Afterwards, the doc.xml is read and two child read actions are performed in order to evaluate XPath expression. 3. Finally, the first elem (accessed) node from doc.xml is to be added into the myroot (constructed) node – the elem node and all its descendants should be copied. 2.4
Transitions Accessed Node Transitions. When a copy action of accessed node is triggered, the node state is changed from accessed to hybrid and no physical data copy is made. Changes to accessed nodes are not permitted – any change will immediately lead into an error.
Constructed Node Transitions. Copy operations on constructed nodes behaves exactly as on accessed nodes. Changes to constructed nodes are permitted. Hybrid Node Transitions. Transitions based on actions on hybrid nodes are bit more interesting: Copy Action. Copy action on hybrid nodes is a no-op. As a result, the same node is returned with its state unchanged.
Change Action. Whenever any of node properties (dm:parent, dm:name etc.) is to be changed the node state is changed to constructed and all node properties are copied. See the query at figure 6. When processing expression at line 5, two things happen (in that order): 1. The text node “elem1” (a result of $doc/elem[0]/text() expression) is added to the myroot element. States of nodes after this addition are depicted at left side of figure 7. 2. Afterwards, the text node value “elem1“ has to be changed to the “elem1 is the first” because of the specification requirements. Obviously, the hybrid text node must be copied. The XML data accessible though $doc must remain unchanged.
Child Read Action. While appending a XML tree into a new structure, the state of a root node of the appended tree is changed to hybrid, the reference from the new structure is added to the hybrid. The rest of the appended tree (children of the root node) are unchanged – they don’t know, that their parent has changed its state to hybrid. This cause serious problems while executing XPath commands. To overcome this issue, we convert hybrid node to a constructed one during child read action. Such a behavior is illustrated at figure 8.
Data are physically copied only when hybrid node is either being changed or its children are being read.
} 1 let $doc:= doc("doc.xml") 2 return 3 element myroot { 4 element myelem { 5 { $doc/elem[0]/text() } is the first } 6 } 7 Serialization of Result Set. Once the query is processed, serialization of result set may not lead into XML node copying. Because query is already processed, no node kind transitions must be performed during serialization and thus no node copies must be created. Obviously, if the application wants work with the result set as with nodes in memory and wants to perform some modification on it, the result set must be copied.
Discussion
Although deferred node-copying scheme does not require the XML nodes to
belong to exactly one node hierarchy it preserves original XQuery semantics.
Our claim is based on the results from the XQuery Test Suite [
The axes tests and element constructors tests from Minimal Conformance Expressions section of XQTS Catalogue cover the node identity semantics and were used to test the correctness of deferred node-copying scheme. Our proofof-concept implementation successfully passes all the mentioned test cases. 3.2
Presented deferred node-copying scheme has been developed in order to increase
XQuery processor performance by reducing number of copy operations. A natural
question is whether this scheme has substantial effect in real-world applications.
The table 3 shows number of copy operations for selected XMark [
Number of saved copies is dependent on a query characteristics. There are no new nodes created in a Q1 command and that is why there is no difference in results. There are text nodes appended to elements in a Q2 command. The text nodes does not need to be copied at all, only transformed to the hybrid state.
There is a subtree appended to each result item during the Q13 execution. Without the optimization, each element of a tree has to be copied, but with the optimization turned on, only a few of nodes are copied. 1 Plus one nonstandard query marked INC. Its code is element a {doc("file:///auctions.xml") }. We include it as an illustration of extreme case.
DNC
IC
Q. #
DNC
IC Nh Nc Nh
Nh Nc Nh
Nc Q1 Q3 Q5 Q7 Q9 Q11 Q13 Q15 Q17 Q19 Nh – number of hybrid nodes created Nc – number of physically copied nodes DNC – evaluated using deferred node-copying scheme IC – evaluated using immediate copy as specified by the XQuery specification eXist XQuery Processor. eXist2 is an open-source XML-native database with XQuery as its primary query language. As far as we know, eXist XQuery implementation unconditionally copies nodes whenever the node is to be added into a different node hierarchy. Our approach is different since we avoid unnecessary copy operations.
Saxon XQuery Processor. Saxon3 is well-known, widely adopted XML tool set including XSLT 2.0, XPath 2.0 and XQuery 1.0 processor. Saxon’s XQuery processor introduces concept of virtual nodes – a light-weigh node shallow copies that shares as many properties as possible with their origin.
Similarly to our approach, for a given virtual node some of standard XDM properties may be overridden – namely the parent property. When the Saxon 2 http://exist.sourceforge.net/ 3 http://saxon.sourceforge.net/ XQuery processor iterates over virtual node’s children, those are converted to virtual nodes.
However, presented deferred node copying scheme differs from virtual nodes approach in several aspects: 1. Creating virtual copies requires a new object to be allocated in the memory.
Deferred node copying scheme shares the same object. 2. Creation of virtual copies is a part of XQuery processing logic and must be explicitly expressed, whereas our approach separates copying logic of an XDM model from the query evaluation logic. 5
Conclusion and Future Work This paper presents a deferred XML node-copying scheme for XQuery processors that significantly reduces number of source nodes copy operations required during query processing. This scheme defers the copy operation unless absolutely inevitable. Whether the node is actually copied depends on a node state, a new property which is maintained for each node in addition to standard XDM properties. Correctness of this approach has been successfully tested by XQuery Test Suite.
The main benefits of deferred node-copying scheme are: (i) efficiency, (ii) easy to implement, (iii) independent on physical data model and (iv) independent on XQuery processing logic.
As a future plan, we plan to extend this scheme for use with various XML
indexing approaches, Ctree [