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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>D.3. XML Limits and Conformance to SQL/XML</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="unsupported-features-sql-standard.html" title="D.2. Unsupported Features" /><link rel="next" href="release.html" title="Appendix E. Release Notes" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">D.3. XML Limits and Conformance to SQL/XML</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="unsupported-features-sql-standard.html" title="D.2. Unsupported Features">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="features.html" title="Appendix D. SQL Conformance">Up</a></td><th width="60%" align="center">Appendix D. SQL Conformance</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="release.html" title="Appendix E. Release Notes">Next</a></td></tr></table><hr /></div><div class="sect1" id="XML-LIMITS-CONFORMANCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">D.3. XML Limits and Conformance to SQL/XML</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="xml-limits-conformance.html#FUNCTIONS-XML-LIMITS-XPATH1">D.3.1. Queries Are Restricted to XPath 1.0</a></span></dt><dt><span class="sect2"><a href="xml-limits-conformance.html#FUNCTIONS-XML-LIMITS-POSTGRESQL">D.3.2. Incidental Limits of the Implementation</a></span></dt></dl></div><a id="id-1.11.5.13.2" class="indexterm"></a><p>
Significant revisions to the XML-related specifications in ISO/IEC 9075-14
(SQL/XML) were introduced with SQL:2006.
<span class="productname">PostgreSQL</span>'s implementation of the XML data
type and related functions largely follows the earlier 2003 edition,
with some borrowing from later editions. In particular:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Where the current standard provides a family of XML data types
to hold <span class="quote">“<span class="quote">document</span>”</span> or <span class="quote">“<span class="quote">content</span>”</span> in
untyped or XML Schema-typed variants, and a type
<code class="type">XML(SEQUENCE)</code> to hold arbitrary pieces of XML content,
<span class="productname">PostgreSQL</span> provides the single
<code class="type">xml</code> type, which can hold <span class="quote">“<span class="quote">document</span>”</span> or
<span class="quote">“<span class="quote">content</span>”</span>. There is no equivalent of the
standard's <span class="quote">“<span class="quote">sequence</span>”</span> type.
</p></li><li class="listitem"><p>
<span class="productname">PostgreSQL</span> provides two functions
introduced in SQL:2006, but in variants that use the XPath 1.0
language, rather than XML Query as specified for them in the
standard.
</p></li></ul></div><p>
</p><p>
This section presents some of the resulting differences you may encounter.
</p><div class="sect2" id="FUNCTIONS-XML-LIMITS-XPATH1"><div class="titlepage"><div><div><h3 class="title">D.3.1. Queries Are Restricted to XPath 1.0</h3></div></div></div><p>
The <span class="productname">PostgreSQL</span>-specific functions
<code class="function">xpath()</code> and <code class="function">xpath_exists()</code>
query XML documents using the XPath language.
<span class="productname">PostgreSQL</span> also provides XPath-only variants
of the standard functions <code class="function">XMLEXISTS</code> and
<code class="function">XMLTABLE</code>, which officially use
the XQuery language. For all of these functions,
<span class="productname">PostgreSQL</span> relies on the
<span class="application">libxml2</span> library, which provides only XPath 1.0.
</p><p>
There is a strong connection between the XQuery language and XPath
versions 2.0 and later: any expression that is syntactically valid and
executes successfully in both produces the same result (with a minor
exception for expressions containing numeric character references or
predefined entity references, which XQuery replaces with the
corresponding character while XPath leaves them alone). But there is
no such connection between these languages and XPath 1.0; it was an
earlier language and differs in many respects.
</p><p>
There are two categories of limitation to keep in mind: the restriction
from XQuery to XPath for the functions specified in the SQL standard, and
the restriction of XPath to version 1.0 for both the standard and the
<span class="productname">PostgreSQL</span>-specific functions.
</p><div class="sect3" id="id-1.11.5.13.5.5"><div class="titlepage"><div><div><h4 class="title">D.3.1.1. Restriction of XQuery to XPath</h4></div></div></div><p>
Features of XQuery beyond those of XPath include:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
XQuery expressions can construct and return new XML nodes, in
addition to all possible XPath values. XPath can create and return
values of the atomic types (numbers, strings, and so on) but can
only return XML nodes that were already present in documents
supplied as input to the expression.
</p></li><li class="listitem"><p>
XQuery has control constructs for iteration, sorting, and grouping.
</p></li><li class="listitem"><p>
XQuery allows declaration and use of local functions.
</p></li></ul></div><p>
</p><p>
Recent XPath versions begin to offer capabilities overlapping with
these (such as functional-style <code class="function">for-each</code> and
<code class="function">sort</code>, anonymous functions, and
<code class="function">parse-xml</code> to create a node from a string),
but such features were not available before XPath 3.0.
</p></div><div class="sect3" id="XML-XPATH-1-SPECIFICS"><div class="titlepage"><div><div><h4 class="title">D.3.1.2. Restriction of XPath to 1.0</h4></div></div></div><p>
For developers familiar with XQuery and XPath 2.0 or later, XPath 1.0
presents a number of differences to contend with:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
The fundamental type of an XQuery/XPath expression, the
<code class="type">sequence</code>, which can contain XML nodes, atomic values,
or both, does not exist in XPath 1.0. A 1.0 expression can only
produce a node-set (containing zero or more XML nodes), or a single
atomic value.
</p></li><li class="listitem"><p>
Unlike an XQuery/XPath sequence, which can contain any desired
items in any desired order, an XPath 1.0 node-set has no
guaranteed order and, like any set, does not allow multiple
appearances of the same item.
</p><div class="note"><h3 class="title">Note</h3><p>
The <span class="application">libxml2</span> library does seem to
always return node-sets to <span class="productname">PostgreSQL</span>
with their members in the same relative order they had in the
input document. Its documentation does not commit to this
behavior, and an XPath 1.0 expression cannot control it.
</p></div><p>
</p></li><li class="listitem"><p>
While XQuery/XPath provides all of the types defined in XML Schema
and many operators and functions over those types, XPath 1.0 has only
node-sets and the three atomic types <code class="type">boolean</code>,
<code class="type">double</code>, and <code class="type">string</code>.
</p></li><li class="listitem"><p>
XPath 1.0 has no conditional operator. An XQuery/XPath expression
such as <code class="literal">if ( hat ) then hat/@size else "no hat"</code>
has no XPath 1.0 equivalent.
</p></li><li class="listitem"><p>
XPath 1.0 has no ordering comparison operator for strings. Both
<code class="literal">"cat" < "dog"</code> and
<code class="literal">"cat" > "dog"</code> are false, because each is a
numeric comparison of two <code class="literal">NaN</code>s. In contrast,
<code class="literal">=</code> and <code class="literal">!=</code> do compare the strings
as strings.
</p></li><li class="listitem"><p>
XPath 1.0 blurs the distinction between
<em class="firstterm">value comparisons</em> and
<em class="firstterm">general comparisons</em> as XQuery/XPath define
them. Both <code class="literal">sale/@hatsize = 7</code> and
<code class="literal">sale/@customer = "alice"</code> are existentially
quantified comparisons, true if there is
any <code class="literal">sale</code> with the given value for the
attribute, but <code class="literal">sale/@taxable = false()</code> is a
value comparison to the
<em class="firstterm">effective boolean value</em> of a whole node-set.
It is true only if no <code class="literal">sale</code> has
a <code class="literal">taxable</code> attribute at all.
</p></li><li class="listitem"><p>
In the XQuery/XPath data model, a <em class="firstterm">document
node</em> can have either document form (i.e., exactly one
top-level element, with only comments and processing instructions
outside of it) or content form (with those constraints
relaxed). Its equivalent in XPath 1.0, the
<em class="firstterm">root node</em>, can only be in document form.
This is part of the reason an <code class="type">xml</code> value passed as the
context item to any <span class="productname">PostgreSQL</span>
XPath-based function must be in document form.
</p></li></ul></div><p>
</p><p>
The differences highlighted here are not all of them. In XQuery and
the 2.0 and later versions of XPath, there is an XPath 1.0 compatibility
mode, and the W3C lists of
<a class="ulink" href="https://www.w3.org/TR/2010/REC-xpath-functions-20101214/#xpath1-compatibility" target="_top">function library changes</a>
and
<a class="ulink" href="https://www.w3.org/TR/xpath20/#id-backwards-compatibility" target="_top">language changes</a>
applied in that mode offer a more complete (but still not exhaustive)
account of the differences. The compatibility mode cannot make the
later languages exactly equivalent to XPath 1.0.
</p></div><div class="sect3" id="FUNCTIONS-XML-LIMITS-CASTS"><div class="titlepage"><div><div><h4 class="title">D.3.1.3. Mappings between SQL and XML Data Types and Values</h4></div></div></div><p>
In SQL:2006 and later, both directions of conversion between standard SQL
data types and the XML Schema types are specified precisely. However, the
rules are expressed using the types and semantics of XQuery/XPath, and
have no direct application to the different data model of XPath 1.0.
</p><p>
When <span class="productname">PostgreSQL</span> maps SQL data values to XML
(as in <code class="function">xmlelement</code>), or XML to SQL (as in the output
columns of <code class="function">xmltable</code>), except for a few cases
treated specially, <span class="productname">PostgreSQL</span> simply assumes
that the XML data type's XPath 1.0 string form will be valid as the
text-input form of the SQL datatype, and conversely. This rule has the
virtue of simplicity while producing, for many data types, results similar
to the mappings specified in the standard.
</p><p>
Where interoperability with other systems is a concern, for some data
types, it may be necessary to use data type formatting functions (such
as those in <a class="xref" href="functions-formatting.html" title="9.8. Data Type Formatting Functions">Section 9.8</a>) explicitly to
produce the standard mappings.
</p></div></div><div class="sect2" id="FUNCTIONS-XML-LIMITS-POSTGRESQL"><div class="titlepage"><div><div><h3 class="title">D.3.2. Incidental Limits of the Implementation</h3></div></div></div><p>
This section concerns limits that are not inherent in the
<span class="application">libxml2</span> library, but apply to the current
implementation in <span class="productname">PostgreSQL</span>.
</p><div class="sect3" id="id-1.11.5.13.6.3"><div class="titlepage"><div><div><h4 class="title">D.3.2.1. Only <code class="literal">BY VALUE</code> Passing Mechanism Is Supported</h4></div></div></div><p>
The SQL standard defines two <em class="firstterm">passing mechanisms</em>
that apply when passing an XML argument from SQL to an XML function or
receiving a result: <code class="literal">BY REF</code>, in which a particular XML
value retains its node identity, and <code class="literal">BY VALUE</code>, in which
the content of the XML is passed but node identity is not preserved. A
mechanism can be specified before a list of parameters, as the default
mechanism for all of them, or after any parameter, to override the
default.
</p><p>
To illustrate the difference, if
<em class="replaceable"><code>x</code></em> is an XML value, these two queries in
an SQL:2006 environment would produce true and false, respectively:
</p><pre class="programlisting">
SELECT XMLQUERY('$a is $b' PASSING BY REF <em class="replaceable"><code>x</code></em> AS a, <em class="replaceable"><code>x</code></em> AS b NULL ON EMPTY);
SELECT XMLQUERY('$a is $b' PASSING BY VALUE <em class="replaceable"><code>x</code></em> AS a, <em class="replaceable"><code>x</code></em> AS b NULL ON EMPTY);
</pre><p>
</p><p>
<span class="productname">PostgreSQL</span> will accept
<code class="literal">BY VALUE</code> or <code class="literal">BY REF</code> in an
<code class="function">XMLEXISTS</code> or <code class="function">XMLTABLE</code>
construct, but it ignores them. The <code class="type">xml</code> data type holds
a character-string serialized representation, so there is no node
identity to preserve, and passing is always effectively <code class="literal">BY
VALUE</code>.
</p></div><div class="sect3" id="id-1.11.5.13.6.4"><div class="titlepage"><div><div><h4 class="title">D.3.2.2. Cannot Pass Named Parameters to Queries</h4></div></div></div><p>
The XPath-based functions support passing one parameter to serve as the
XPath expression's context item, but do not support passing additional
values to be available to the expression as named parameters.
</p></div><div class="sect3" id="id-1.11.5.13.6.5"><div class="titlepage"><div><div><h4 class="title">D.3.2.3. No <code class="type">XML(SEQUENCE)</code> Type</h4></div></div></div><p>
The <span class="productname">PostgreSQL</span> <code class="type">xml</code> data type
can only hold a value in <code class="literal">DOCUMENT</code>
or <code class="literal">CONTENT</code> form. An XQuery/XPath expression
context item must be a single XML node or atomic value, but XPath 1.0
further restricts it to be only an XML node, and has no node type
allowing <code class="literal">CONTENT</code>. The upshot is that a
well-formed <code class="literal">DOCUMENT</code> is the only form of XML value
that <span class="productname">PostgreSQL</span> can supply as an XPath
context item.
</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="unsupported-features-sql-standard.html" title="D.2. Unsupported Features">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="features.html" title="Appendix D. SQL Conformance">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="release.html" title="Appendix E. Release Notes">Next</a></td></tr><tr><td width="40%" align="left" valign="top">D.2. Unsupported Features </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Appendix E. Release Notes</td></tr></table></div></body></html>
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