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diff --git a/doc/src/sgml/html/xml-limits-conformance.html b/doc/src/sgml/html/xml-limits-conformance.html new file mode 100644 index 0000000..c01d537 --- /dev/null +++ b/doc/src/sgml/html/xml-limits-conformance.html @@ -0,0 +1,204 @@ +<?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 V1.79.1" /><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 xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 13.4 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></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 xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></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. 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