#XDM Data Model

PhoenixmlDb uses the XQuery Data Model (XDM) as its foundational data representation. Understanding XDM helps you write effective queries and understand how your documents are stored.

#Overview

XDM represents XML documents as trees of nodes. Every document, element, attribute, and text content is a node with specific properties and relationships.

#Node Types

XDM defines seven node types:

#Document Node

The root of an XML document tree.

<!-- Document node contains the entire document -->
<?xml version="1.0"?>
<root>
    <item>Content</item>
</root>

// Access in XQuery
let $doc := doc('container/document.xml')
return $doc  (: Returns document node :)

#Element Node

Represents an XML element with its tag name, namespace, and content.

<book xmlns="http://example.com/books" id="123">
    <title>XQuery Fundamentals</title>
</book>

Properties:

• Node name: QName (namespace URI + local name)

• String value: Concatenated text content of descendants

• Typed value: Based on schema type annotation

#Attribute Node

Represents an XML attribute.

<product id="123" category="electronics"/>

(: Access attributes :)
$product/@id           (: Returns attribute node :)
$product/@id/string()  (: Returns "123" :)
data($product/@id)     (: Returns typed value :)

#Text Node

Contains character data between elements.

<message>Hello, World!</message>

Note: Adjacent text nodes are always merged. CDATA sections become text nodes.

#Comment Node

Represents XML comments.

<!-- This is a comment -->

$doc//comment()  (: Selects all comment nodes :)

#Processing Instruction Node

Represents XML processing instructions.

<?xml-stylesheet type="text/xsl" href="style.xsl"?>

$doc//processing-instruction()  (: All PIs :)
$doc//processing-instruction('xml-stylesheet')  (: Specific PI :)

#Namespace Node

Represents namespace declarations (inherited by descendants).

<root xmlns="http://default.ns" xmlns:custom="http://custom.ns">

#Node Identity and Equality

#Node Identity

Every node has a unique identity. Two nodes are identical only if they are the exact same node in the tree.

let $x := <item>1</item>
let $y := $x
return $x is $y  (: true - same node :)
let $a := <item>1</item>
let $b := <item>1</item>
return $a is $b  (: false - different nodes :)

#Value Equality

Nodes can have equal values without being identical.

let $a := <item>1</item>
let $b := <item>1</item>
return $a = $b   (: true - equal values :)
return $a eq $b  (: true - equal values :)

#Node Relationships

#Parent-Child

         Document
             │
           <root>
           /    \
      <item>    <item>
         │         │
       "A"       "B"

$node/parent::*     (: Parent element :)
$node/..            (: Parent (shorthand) :)
$node/child::*      (: Child elements :)
$node/*             (: Child elements (shorthand) :)

#Ancestor-Descendant

$node/ancestor::*           (: All ancestors :)
$node/ancestor-or-self::*   (: Self and ancestors :)
$node/descendant::*         (: All descendants :)
$node//item                 (: Descendant items (shorthand) :)

#Sibling

$node/preceding-sibling::*  (: Siblings before :)
$node/following-sibling::*  (: Siblings after :)

#Document Order

Nodes are ordered in document order:

1. Document node first

2. Element before its children

3. Attributes before children (order among attributes is implementation-defined)

4. Children in order of appearance

(: Sort nodes in document order :)
for $node in ($c, $a, $b)
order by $node
return $node

#Atomic Values

XDM also includes atomic values (not nodes):

#Sequences

In XDM, everything is a sequence. A single value is a sequence of one item.

(: Empty sequence :)
()
(: Single item (sequence of one) :)
42
<item/>
(: Multiple items :)
(1, 2, 3)
(<a/>, <b/>, <c/>)
(: Mixed sequence :)
(1, "two", <three/>)
(: No nested sequences - automatically flattened :)
(1, (2, 3), 4)  (: Same as (1, 2, 3, 4) :)

#Type System

#Sequence Types

(: Type declarations :)
declare variable $x as xs:integer := 42;
declare variable $items as element()* := $doc//item;
(: Occurrence indicators :)
xs:integer      (: Exactly one integer :)
xs:integer?     (: Zero or one integer :)
xs:integer*     (: Zero or more integers :)
xs:integer+     (: One or more integers :)

#Type Checking

(: Instance of :)
$x instance of xs:integer
(: Cast :)
xs:integer($string)
$value cast as xs:date
(: Treat :)
$node treat as element()

#Storage in PhoenixmlDb

PhoenixmlDb stores XDM nodes efficiently:

1. Node Shredding: Documents are decomposed into individual nodes

2. Namespace Interning: Namespace URIs are stored once and referenced by ID

3. Structural Indexes: Parent-child relationships are indexed for fast navigation

4. Typed Storage: Atomic values are stored in their native types

Document Storage:
┌─────────────────────────────────────┐
│ Node Store (one entry per node)     │
│ ┌─────┬─────┬─────────┬──────────┐  │
│ │ ID  │Type │ Name    │ Value    │  │
│ ├─────┼─────┼─────────┼──────────┤  │
│ │ 1   │ Doc │ -       │ -        │  │
│ │ 2   │ Elem│ "root"  │ -        │  │
│ │ 3   │ Attr│ "id"    │ "123"    │  │
│ │ 4   │ Elem│ "item"  │ -        │  │
│ │ 5   │ Text│ -       │ "Hello"  │  │
│ └─────┴─────┴─────────┴──────────┘  │
└─────────────────────────────────────┘
Structural Index:
┌──────────┬──────────┬──────────┐
│ Parent   │ Child    │ Position │
├──────────┼──────────┼──────────┤
│ 1        │ 2        │ 1        │
│ 2        │ 3        │ 1        │
│ 2        │ 4        │ 2        │
│ 4        │ 5        │ 1        │
└──────────┴──────────┴──────────┘

#Best Practices

1. Understand node vs value: Use data() or /text() when you need string values

2. Use typed comparisons: Compare typed values for correct ordering

3. Leverage document order: Use positional predicates effectively

4. Mind the sequences: Functions behave differently with sequences vs single items