On 03/27/19 19:27, Chapman Flack wrote:
> A column marked FOR ORDINALITY will be populated with row numbers
> matching the order in which the output rows appeared in the original
> input XML document.
>
> I've been skimming right over it all this time, and that right there is
> a glaring built-in reliance on the observable-but-disclaimed iteration
> order of a libxml2 node-set.
So, xml-functions-type-docfix-6.patch.
I changed that language to say "populated with row numbers, starting
with 1, in the order of nodes retrieved from the row_expression's
result node-set."
That's not such a terrible thing to have to say; in fact, it's the
*correct* description for the standard, XQuery-based, XMLTABLE (where
the language gives you control of the result sequence's order).
I followed that with a short note saying since XPath 1.0 doesn't
specify that order, relying on it is implementation-dependent, and
linked to the existing Appendix D discussion.
I would have like to link directly to the <listitem>, but of course
<xref> doesn't know what to call that, so I linked to the <sect3>
instead.
Regards,
-Chap
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 52c28e7..0aed14c 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -4219,6 +4219,12 @@ a0ee-bc99-9c0b-4ef8-bb6d-6bb9-bd38-0a11
value is a full document or only a content fragment.
</para>
+ <para>
+ Limits and compatibility notes for the <type>xml</type> data type
+ in <productname>PostgreSQL</productname> can be found in
+ <xref linkend="xml-limits-conformance"/>.
+ </para>
+
<sect2>
<title>Creating XML Values</title>
<para>
diff --git a/doc/src/sgml/features.sgml b/doc/src/sgml/features.sgml
index 6c22d69..e8015a9 100644
--- a/doc/src/sgml/features.sgml
+++ b/doc/src/sgml/features.sgml
@@ -16,7 +16,8 @@
Language SQL</quote>. A revised version of the standard is released
from time to time; the most recent update appearing in 2011.
The 2011 version is referred to as ISO/IEC 9075:2011, or simply as SQL:2011.
- The versions prior to that were SQL:2008, SQL:2003, SQL:1999, and SQL-92. Each version
+ The versions prior to that were SQL:2008, SQL:2006, SQL:2003, SQL:1999,
+ and SQL-92. Each version
replaces the previous one, so claims of conformance to earlier
versions have no official merit.
<productname>PostgreSQL</productname> development aims for
@@ -155,4 +156,329 @@
</para>
</sect1>
+ <sect1 id="xml-limits-conformance">
+ <title>XML Limits and Conformance to SQL/XML</title>
+
+ <indexterm>
+ <primary>SQL/XML</primary>
+ <secondary>limits and conformance</secondary>
+ </indexterm>
+
+ <para>
+ Significant revisions to the ISO/IEC 9075-14 XML-related specifications
+ (SQL/XML) were introduced with SQL:2006. The
+ <productname>PostgreSQL</productname> implementation of the XML data type
+ and related functions largely follows the earlier, 2003 edition, with some
+ borrowing from the later editions. In particular:
+ <itemizedlist>
+ <listitem>
+ <para>
+ Where the current standard provides a family of XML data types
+ to hold <quote>document</quote> or <quote>content</quote> in
+ untyped or XML Schema-typed variants, and a type
+ <type>XML(SEQUENCE)</type> to hold arbitrary pieces of XML content,
+ <productname>PostgreSQL</productname> provides the single
+ <type>xml</type> type, which can hold <quote>document</quote> or
+ <quote>content</quote>, and no equivalent of the <quote>sequence</quote>
+ type.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <productname>PostgreSQL</productname> provides two functions introduced
+ in SQL:2006, but in variants that use the language XPath 1.0, rather than
+ XML Query as specified for them in the standard.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ This section presents some of the resulting differences you may encounter.
+ </para>
+
+ <sect2 id="functions-xml-limits-xpath1">
+ <title>Queries restricted to XPath 1.0</title>
+
+ <para>
+ The <productname>PostgreSQL</productname>-specific functions
+ <function>xpath</function> and <function>xpath_exists</function> query
+ XML documents using the XPath language, and
+ <productname>PostgreSQL</productname> also provides XPath-only variants of
+ the standard functions <function>XMLEXISTS</function> and
+ <function>XMLTABLE</function>, which officially use
+ the XQuery language. For all of these functions,
+ <productname>PostgreSQL</productname> relies on the
+ <productname>libxml2</productname> library, which provides only XPath 1.0.
+ </para>
+
+ <para>
+ 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 XPath 1.0
+ and XQuery or the later XPath versions; it was an earlier language and
+ differs in many respects.
+ </para>
+
+ <para>
+ 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
+ <productname>PostgreSQL</productname>-specific functions.
+ </para>
+
+ <sect3>
+ <title>Restriction of XQuery to XPath</title>
+
+ <para>
+ Features of XQuery beyond those of XPath include:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ XQuery expressions can construct and return new XML nodes, in addition
+ to all possible XPath values. XPath can introduce and return values of
+ the atomic types (numbers, strings, and so on) but can only return XML
+ nodes already present in documents supplied as input to the expression.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ XQuery has control constructs for iteration, sorting, and grouping.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ XQuery allows the declaration and use of local functions.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ Recent XPath versions begin to offer overlapping capabilities
+ (such as the functional-style <function>for-each</function> and
+ <function>sort</function>, anonymous functions, and
+ <function>parse-xml</function> to create a node from a string),
+ but these were not available before XPath 3.0.
+ </para>
+ </sect3>
+
+ <sect3 id="xml-xpath-1-specifics">
+ <title>Restriction of XPath to 1.0</title>
+
+ <para>
+ For developers familiar with XQuery and XPath 2.0 or later, or porting
+ queries from other systems, XPath 1.0 presents a number of differences to
+ contend with:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ The fundamental type of an XQuery/XPath expression, the
+ <type>sequence</type>, 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 (possibly empty, or with one XML node or more), or a single
+ atomic value.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ 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, can have no member appear more than once.
+ <note>
+ <para>
+ The <productname>libxml2</productname> library does seem to always
+ return node-sets to <productname>PostgreSQL</productname> with their
+ members in the same relative order they had in the input document. It
+ does not commit to this behavior, and an XPath 1.0 expression cannot
+ control it.
+ </para>
+ </note>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ 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 three atomic types, <type>boolean</type>,
+ <type>double</type>, and <type>string</type>.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ XPath 1.0 has no conditional operator. An XQuery/XPath expression
+ such as <userinput>if ( hat ) then hat/@size else "no hat"</userinput>
+ has no XPath 1.0 equivalent.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ XPath 1.0 has no ordering comparison operator for strings. Both
+ <userinput>"cat" < "dog"</userinput> and
+ <userinput>"cat" > "dog"</userinput> are false, because each is a
+ numeric comparison of two <literal>NaN</literal>s. In contrast,
+ <literal>=</literal> and <literal>!=</literal> do compare the strings
+ as strings.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ XPath 1.0 blurs the distinction between
+ <firstterm>value comparisons</firstterm> and
+ <firstterm>general comparisons</firstterm> as XQuery/XPath define them.
+ Both <userinput>sale/@hatsize = 7</userinput> and
+ <userinput>sale/@customer = "alice"</userinput> are existentially
+ quantified comparisons, true if there is any sale with the given value
+ for the attribute, but <userinput>sale/@taxable = false()</userinput>
+ is a value comparison to the
+ <firstterm>effective boolean value</firstterm> of a whole node-set,
+ and true only if no sale has a <literal>taxable</literal> attribute
+ at all.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ In the XQuery/XPath data model, a <firstterm>document node</firstterm>
+ can have either document form (exactly one top-level element, only
+ comments and processing instructions outside of it) or content form
+ (with those constraints relaxed). Its equivalent in XPath 1.0, the
+ <firstterm>root node</firstterm>, can only be in document form.
+ This is part of the reason an <type>xml</type> value passed as the
+ context item to any <productname>PostgreSQL</productname> XPath-based
+ function must be in document form.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ 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 function library
+ <ulink url='https://www.w3.org/TR/2010/REC-xpath-functions-20101214/#xpath1-compatibility'>changes</ulink>
+ and language
+ <ulink url='https://www.w3.org/TR/xpath20/#id-backwards-compatibility'>changes</ulink>
+ applied in that mode offer a more complete account of the
+ differences, if still not exhaustive; the compatibility mode cannot make
+ the later languages exactly equivalent to XPath 1.0.
+ </para>
+ </sect3>
+
+ <sect3 id="functions-xml-limits-casts">
+ <title>Mappings between SQL and XML data types and values</title>
+
+ <para>
+ 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.
+ </para>
+
+ <para>
+ When <productname>PostgreSQL</productname> maps SQL data values to XML
+ (as in <function>xmlelement</function>), or XML to SQL (as in the output
+ columns of <function>xmltable</function>), except for the few cases
+ treated specially, <productname>PostgreSQL</productname> 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.
+ </para>
+
+ <para>
+ Where interoperability with other systems is a concern, for some data
+ types, it may be necessary to use available data type formatting functions
+ (such as those in <xref linkend="functions-formatting"/>) explicitly in
+ queries to produce the standard mappings.
+ </para>
+ </sect3>
+ </sect2>
+
+ <sect2 id="functions-xml-limits-postgresql">
+ <title>
+ Incidental limits of the implementation
+ </title>
+
+ <para>
+ This section concerns limits that are not inherent in the
+ <productname>libxml2</productname> library, but apply to the current
+ implementation in <productname>PostgreSQL</productname>.
+ </para>
+
+ <sect3>
+ <title>Only <literal>BY VALUE</literal> passing mechanism supported</title>
+
+ <para>
+ The SQL standard defines two <firstterm>passing mechanisms</firstterm>
+ that apply when passing an XML argument from SQL to an XML function or
+ receiving a result: <literal>BY REF</literal>, in which a particular XML
+ value retains its node identity, and <literal>BY VALUE</literal>, 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.
+ </para>
+
+ <para>
+ To illustrate the difference, if
+ <replaceable>x</replaceable> is an XML value, these two queries in
+ an SQL:2006 environment would produce true and false, respectively:
+
+ <screen><![CDATA[
+SELECT XMLQUERY('$a is $b' PASSING BY REF x AS a, x AS b NULL ON EMPTY);
+SELECT XMLQUERY('$a is $b' PASSING BY VALUE x AS a, x AS b NULL ON EMPTY);
+]]></screen>
+ </para>
+
+ <para>
+ <productname>PostgreSQL</productname> will accept either
+ <literal>BY VALUE</literal> or <literal>BY REF</literal> in an
+ <function>XMLEXISTS</function> or <function>XMLTABLE</function> construct,
+ but ignores them; the <type>xml</type> data type holds a character-string
+ serialized representation, so there is no node identity to preserve,
+ and passing is always <literal>BY VALUE</literal>.
+ </para>
+ </sect3>
+
+ <sect3>
+ <title>Cannot pass named parameters to queries</title>
+
+ <para>
+ 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.
+ </para>
+ </sect3>
+
+ <sect3>
+ <title>No <type>XML(SEQUENCE)</type> type</title>
+
+ <para>
+ The <productname>PostgreSQL</productname> <type>xml</type> can only hold
+ a value in <literal>DOCUMENT</literal> or <literal>CONTENT</literal> form.
+ An XQuery/XPath expression context item must be a single XML node
+ or atomic value, while XPath 1.0 further restricts it to only an XML node,
+ and has no node type allowing <literal>CONTENT</literal>. The upshot is
+ that a well-formed <literal>DOCUMENT</literal> is the only form of XML
+ value that <productname>PostgreSQL</productname> can supply as an XPath
+ context item.
+ </para>
+ </sect3>
+ </sect2>
+ </sect1>
+
</appendix>
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 1a01473..7952f02 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -10249,8 +10249,13 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
<sect1 id="functions-xml">
+
<title>XML Functions</title>
+ <indexterm>
+ <primary>XML Functions</primary>
+ </indexterm>
+
<para>
The functions and function-like expressions described in this
section operate on values of type <type>xml</type>. Check <xref
@@ -10453,8 +10458,8 @@ SELECT xmlelement(name foo, xmlattributes('xyz' as bar),
encoding, depending on the setting of the configuration parameter
<xref linkend="guc-xmlbinary"/>. The particular behavior for
individual data types is expected to evolve in order to align the
- SQL and PostgreSQL data types with the XML Schema specification,
- at which point a more precise description will appear.
+ PostgreSQL mappings with those specified in SQL:2006 and later,
+ as discussed in <xref linkend="functions-xml-limits-casts"/>.
</para>
</sect3>
@@ -10696,10 +10701,12 @@ SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
</synopsis>
<para>
- The function <function>xmlexists</function> returns true if the
- XPath expression in the first argument returns any nodes, and
- false otherwise. (If either argument is null, the result is
- null.)
+ The function <function>xmlexists</function> evaluates an XPath 1.0
+ expression (the first argument), with the passed value as its context item.
+ The function returns false if the result of that evaluation yields an
+ empty node-set, true if it yields any other value. The function returns
+ null if an argument is null. A nonnull value passed as the context item
+ must be an XML document, not a content fragment or any non-XML value.
</para>
<para>
@@ -10716,24 +10723,12 @@ SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY VALUE '<towns><town>T
<para>
The <literal>BY REF</literal> or <literal>BY VALUE</literal> clauses
- have no effect in <productname>PostgreSQL</productname>, but are allowed
- for compatibility with other implementations. Per the <acronym>SQL</acronym>
- standard, the one that precedes any argument is required, and indicates
- the default for arguments that follow, and one may follow any argument to
- override the default.
- <productname>PostgreSQL</productname> ignores <literal>BY REF</literal>
- and passes by value always.
- </para>
-
- <para>
- In the <acronym>SQL</acronym> standard, an <function>xmlexists</function>
- construct evaluates an expression in the XQuery language, allows passing
- values for named parameters in the expression as well as for the context
- item, and does not require the passed values to be documents, or even of
- XML type.
- In <productname>PostgreSQL</productname>, this construct currently only
- evaluates an XPath 1.0 expression, and allows passing only one value,
- which must be an XML document, to be the context item.
+ are accepted in <productname>PostgreSQL</productname>, but ignored, as
+ discussed in <xref linkend="functions-xml-limits-postgresql"/>.
+ In the SQL standard, an <function>xmlexists</function> construct first
+ appears in SQL:2006 and evaluates an expression in the XML Query language,
+ but this implementation allows only an XPath 1.0 expression, as discussed
+ in <xref linkend="functions-xml-limits-xpath1"/>.
</para>
</sect3>
@@ -10839,12 +10834,12 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
</synopsis>
<para>
- The function <function>xpath</function> evaluates the XPath
+ The function <function>xpath</function> evaluates the XPath 1.0
expression <replaceable>xpath</replaceable> (a <type>text</type> value)
against the XML value
<replaceable>xml</replaceable>. It returns an array of XML values
- corresponding to the node set produced by the XPath expression.
- If the XPath expression returns a scalar value rather than a node set,
+ corresponding to the node-set produced by the XPath expression.
+ If the XPath expression returns a scalar value rather than a node-set,
a single-element array is returned.
</para>
@@ -10906,9 +10901,10 @@ SELECT xpath('//mydefns:b/text()', '<a xmlns="http://example.com";><b>test</b></a
<para>
The function <function>xpath_exists</function> is a specialized form
of the <function>xpath</function> function. Instead of returning the
- individual XML values that satisfy the XPath, this function returns a
- Boolean indicating whether the query was satisfied or not. This
- function is equivalent to the standard <literal>XMLEXISTS</literal> predicate,
+ individual XML values that satisfy the XPath 1.0 expression, this function
+ returns a Boolean indicating whether the query was satisfied or not
+ (specifically, whether it produced any value other than an empty node-set).
+ This function is equivalent to the <literal>XMLEXISTS</literal> predicate,
except that it also offers support for a namespace mapping argument.
</para>
@@ -10949,8 +10945,8 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com";>test</m
<para>
The <function>xmltable</function> function produces a table based
- on the given XML value, an XPath filter to extract rows, and an
- optional set of column definitions.
+ on the given XML value, an XPath filter to extract rows, and a
+ set of column definitions.
</para>
<para>
@@ -10961,30 +10957,33 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com";>test</m
</para>
<para>
- The required <replaceable>row_expression</replaceable> argument is an XPath
- expression that is evaluated against the supplied XML document to
- obtain an ordered sequence of XML nodes. This sequence is what
- <function>xmltable</function> transforms into output rows.
+ The required <replaceable>row_expression</replaceable> argument is
+ an XPath 1.0 expression that is evaluated, passing the
+ <replaceable>document_expression</replaceable> as its context item, to
+ obtain a set of XML nodes. These nodes are what
+ <function>xmltable</function> transforms into output rows. No rows
+ will be produced if the <replaceable>document_expression</replaceable>
+ is null, or the <replaceable>row_expression</replaceable> produces an
+ empty node-set, or any value other than a node-set.
</para>
<para>
- <replaceable>document_expression</replaceable> provides the XML document to
- operate on.
- The argument must be a well-formed XML document; fragments/forests
- are not accepted.
+ <replaceable>document_expression</replaceable> provides the context item
+ for the <replaceable>row_expression</replaceable>. It must be a well-formed
+ XML document; fragments/forests are not accepted.
The <literal>BY REF</literal> or <literal>BY VALUE</literal> clauses are
- accepted, as described for the <function>xmlexists</function> predicate,
- but ignored; PostgreSQL currently passes XML by value always.
+ accepted but ignored, as discussed in
+ <xref linkend="functions-xml-limits-postgresql"/>.
+ In the SQL standard, an <function>xmltable</function> construct first
+ appears in SQL:2006 and evaluates expressions in the XML Query language,
+ but this implementation allows only XPath 1.0 expressions, as discussed
+ in <xref linkend="functions-xml-limits-xpath1"/>.
</para>
<para>
The mandatory <literal>COLUMNS</literal> clause specifies the list
of columns in the output table.
- If the <literal>COLUMNS</literal> clause is omitted, the rows in the result
- set contain a single column of type <literal>xml</literal> containing the
- data matched by <replaceable>row_expression</replaceable>.
- If <literal>COLUMNS</literal> is specified, each entry describes a
- single column.
+ Each entry describes a single column.
See the syntax summary above for the format.
The column name and type are required; the path, default and
nullability clauses are optional.
@@ -10992,48 +10991,93 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com";>test</m
<para>
A column marked <literal>FOR ORDINALITY</literal> will be populated
- with row numbers matching the order in which the
- output rows appeared in the original input XML document.
+ with row numbers, starting with 1, in the order of nodes retrieved from
+ the <replaceable>row_expression</replaceable>'s result node-set.
At most one column may be marked <literal>FOR ORDINALITY</literal>.
</para>
+ <note>
+ <para>
+ XPath 1.0 does not specify an order for nodes in a node-set, so code that
+ relies on a particular order here will be implementation-dependent.
+ Details for this implementation can be found in
+ <xref linkend="xml-xpath-1-specifics"/>.
+ </para>
+ </note>
+
<para>
- The <literal>column_expression</literal> for a column is an XPath expression
- that is evaluated for each row, relative to the result of the
- <replaceable>row_expression</replaceable>, to find the value of the column.
+ The <literal>column_expression</literal> for a column is an XPath 1.0
+ expression
+ that is evaluated for each row, with the current node from the
+ <replaceable>row_expression</replaceable> result as its context item,
+ to find the value of the column.
If no <literal>column_expression</literal> is given, then the column name
is used as an implicit path.
</para>
<para>
- If a column's XPath expression returns multiple elements, an error
- is raised.
- If the expression matches an empty tag, the result is an
- empty string (not <literal>NULL</literal>).
- Any <literal>xsi:nil</literal> attributes are ignored.
+ If a column's XPath expression returns a non-XML value (limited to
+ string, boolean, or double in XPath 1.0) and the column has a
+ PostgreSQL type other than <type>xml</type>, the column will be set
+ as if by assigning the value's string representation to the PostgreSQL
+ type (adjusting the <quote>string representation</quote> of a boolean to
+ <literal>1</literal> or <literal>0</literal> if the target column type
+ category is numeric, otherwise <literal>true</literal> or
+ <literal>false</literal>).
</para>
<para>
- The text body of the XML matched by the <replaceable>column_expression</replaceable>
- is used as the column value. Multiple <literal>text()</literal> nodes
- within an element are concatenated in order. Any child elements,
- processing instructions, and comments are ignored, but the text contents
- of child elements are concatenated to the result.
+ If the column's expression returns a non-empty set of XML nodes
+ and the target column's type is <type>xml</type>, the column will
+ be assigned the expression result exactly, if it is of document or
+ content form.
+ <footnote>
+ <para>
+ A result containing more than one element node at the top level, or
+ non-whitespace text outside of an element, is an example of content form.
+ An XPath result can be of neither form, for example if it returns an
+ attribute node selected from the element that contains it. Such a result
+ will be put into content form with each such disallowed node replaced by
+ its string value, as defined for the XPath 1.0
+ <function>string</function> function.
+ </para>
+ </footnote>
+ </para>
+
+ <para>
+ A non-XML result assigned to an <type>xml</type> output column produces
+ content, a single text node with the string value of the result.
+ An XML result assigned to a column of any other type may not have more than
+ one node, or an error is raised. If there is exactly one node, the column
+ will be set as if by assigning the node's string
+ value (as defined for the XPath 1.0 <function>string</function> function)
+ to the PostgreSQL type.
+ </para>
+
+ <para>
+ The string value of an XML element is the concatenation, in document order,
+ of all text nodes contained in that element and its descendants. The string
+ value of an element with no descendant text nodes is an
+ empty string (not <literal>NULL</literal>).
+ Any <literal>xsi:nil</literal> attributes are ignored.
Note that the whitespace-only <literal>text()</literal> node between two non-text
elements is preserved, and that leading whitespace on a <literal>text()</literal>
node is not flattened.
+ The XPath 1.0 <function>string</function> function may be consulted for the
+ rules defining the string value of other XML node types and non-XML values.
+ </para>
+
+ <para>
+ The conversion rules presented here are not exactly those of the SQL
+ standard, as discussed in <xref linkend="functions-xml-limits-casts"/>.
</para>
<para>
- If the path expression does not match for a given row but
- <replaceable>default_expression</replaceable> is specified, the value resulting
- from evaluating that expression is used.
- If no <literal>DEFAULT</literal> clause is given for the column,
- the field will be set to <literal>NULL</literal>.
- It is possible for a <replaceable>default_expression</replaceable> to reference
- the value of output columns that appear prior to it in the column list,
- so the default of one column may be based on the value of another
- column.
+ If the path expression returns an empty node-set
+ (typically, when it does not match)
+ for a given row, the column will be set to <literal>NULL</literal>, unless
+ a <replaceable>default_expression</replaceable> is specified; then the
+ value resulting from evaluating that expression is used.
</para>
<para>
@@ -11045,20 +11089,14 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com";>test</m
</para>
<para>
- Unlike regular PostgreSQL functions, <replaceable>column_expression</replaceable>
- and <replaceable>default_expression</replaceable> are not evaluated to a simple
- value before calling the function.
- <replaceable>column_expression</replaceable> is normally evaluated
- exactly once per input row, and <replaceable>default_expression</replaceable>
- is evaluated each time a default is needed for a field.
- If the expression qualifies as stable or immutable the repeat
+ A <replaceable>default_expression</replaceable>, rather than being
+ evaluated immediately when <function>xmltable</function> is called,
+ is evaluated each time a default is needed for the column.
+ If the expression qualifies as stable or immutable, the repeat
evaluation may be skipped.
- Effectively <function>xmltable</function> behaves more like a subquery than a
- function call.
This means that you can usefully use volatile functions like
- <function>nextval</function> in <replaceable>default_expression</replaceable>, and
- <replaceable>column_expression</replaceable> may depend on other parts of the
- XML document.
+ <function>nextval</function> in
+ <replaceable>default_expression</replaceable>.
</para>
<para>
diff --git a/src/backend/catalog/sql_features.txt b/src/backend/catalog/sql_features.txt
index bade0fe..32908c1 100644
--- a/src/backend/catalog/sql_features.txt
+++ b/src/backend/catalog/sql_features.txt
@@ -593,7 +593,7 @@ X085 Predefined namespace prefixes NO
X086 XML namespace declarations in XMLTable NO
X090 XML document predicate YES
X091 XML content predicate NO
-X096 XMLExists NO XPath only
+X096 XMLExists NO XPath 1.0 only
X100 Host language support for XML: CONTENT option NO
X101 Host language support for XML: DOCUMENT option NO
X110 Host language support for XML: VARCHAR mapping NO
@@ -661,11 +661,11 @@ X282 XMLValidate with CONTENT option NO
X283 XMLValidate with SEQUENCE option NO
X284 XMLValidate: NAMESPACE without ELEMENT clause NO
X286 XMLValidate: NO NAMESPACE with ELEMENT clause NO
-X300 XMLTable NO XPath only
+X300 XMLTable NO XPath 1.0 only
X301 XMLTable: derived column list option YES
X302 XMLTable: ordinality column option YES
X303 XMLTable: column default option YES
-X304 XMLTable: passing a context item YES
+X304 XMLTable: passing a context item YES must be XML DOCUMENT
X305 XMLTable: initializing an XQuery variable NO
X400 Name and identifier mapping YES
X410 Alter column data type: XML type YES
