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Improve documentation about our XML functionality.

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Add a section explaining how our XML features depart from current
versions of the SQL standard.  Update and clarify the descriptions
of some XML functions.

Chapman Flack, reviewed by Ryan Lambert

Discussion: https://postgr.es/m/5BD1284C.1010305@anastigmatix.net
Discussion: https://postgr.es/m/5C81F8C0.6090901@anastigmatix.net
Discussion: https://postgr.es/m/CAN-V+g-6JqUQEQZ55Q3toXEN6d5Ez5uvzL4VR+8KtvJKj31taw@mail.gmail.com
pull/40/head
Tom Lane 7 years ago
parent b2b819019f
commit 12d46ac392
4 changed files with 475 additions and 94 deletions
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  1. 5
      doc/src/sgml/datatype.sgml
  2. 334
      doc/src/sgml/features.sgml
  3. 224
      doc/src/sgml/func.sgml
  4. 6
      src/backend/catalog/sql_features.txt

5
doc/src/sgml/datatype.sgml
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@ -4219,6 +4219,11 @@ 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
can be found in <xref linkend="xml-limits-conformance"/>.
</para>
<sect2>
<title>Creating XML Values</title>
<para>

334
doc/src/sgml/features.sgml
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@ -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 +156,335 @@
</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 XML-related specifications in ISO/IEC 9075-14
(SQL/XML) were introduced with SQL:2006.
<productname>PostgreSQL</productname>'s implementation of the XML data
type and related functions largely follows the earlier 2003 edition,
with some borrowing from 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>. There is no equivalent of the
standard's <quote>sequence</quote> type.
</para>
</listitem>
<listitem>
<para>
<productname>PostgreSQL</productname> 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.
</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 are 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.
<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
<application>libxml2</application> 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 these languages and XPath 1.0; 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 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.
</para>
</listitem>
<listitem>
<para>
XQuery has control constructs for iteration, sorting, and grouping.
</para>
</listitem>
<listitem>
<para>
XQuery allows declaration and use of local functions.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Recent XPath versions begin to offer capabilities overlapping with
these (such as 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 such features 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, 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 (containing zero or more XML nodes), 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, does not allow multiple
appearances of the same item.
<note>
<para>
The <application>libxml2</application> 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. Its documentation 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 the 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 <literal>if ( hat ) then hat/@size else "no hat"</literal>
has no XPath 1.0 equivalent.
</para>
</listitem>
<listitem>
<para>
XPath 1.0 has no ordering comparison operator for strings. Both
<literal>"cat" &lt; "dog"</literal> and
<literal>"cat" &gt; "dog"</literal> 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 <literal>sale/@hatsize = 7</literal> and
<literal>sale/@customer = "alice"</literal> are existentially
quantified comparisons, true if there is
any <literal>sale</literal> with the given value for the
attribute, but <literal>sale/@taxable = false()</literal> is a
value comparison to the
<firstterm>effective boolean value</firstterm> of a whole node-set.
It is true only if no <literal>sale</literal> 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 (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
<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
<ulink url='https://www.w3.org/TR/2010/REC-xpath-functions-20101214/#xpath1-compatibility'>function library changes</ulink>
and
<ulink url='https://www.w3.org/TR/xpath20/#id-backwards-compatibility'>language changes</ulink>
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.
</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 a 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 data type formatting functions (such
as those in <xref linkend="functions-formatting"/>) explicitly 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
<application>libxml2</application> library, but apply to the current
implementation in <productname>PostgreSQL</productname>.
</para>
<sect3>
<title>Only <literal>BY VALUE</literal> passing mechanism is 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:
<programlisting>
SELECT XMLQUERY('$a is $b' PASSING BY REF <replaceable>x</replaceable> AS a, <replaceable>x</replaceable> AS b NULL ON EMPTY);
SELECT XMLQUERY('$a is $b' PASSING BY VALUE <replaceable>x</replaceable> AS a, <replaceable>x</replaceable> AS b NULL ON EMPTY);
</programlisting>
</para>
<para>
<productname>PostgreSQL</productname> will accept
<literal>BY VALUE</literal> or <literal>BY REF</literal> in an
<function>XMLEXISTS</function> or <function>XMLTABLE</function>
construct, but it 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 effectively <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> data 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, but XPath 1.0
further restricts it to be 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>

224
doc/src/sgml/func.sgml
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@ -10249,16 +10249,25 @@ 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
section operate on values of type <type>xml</type>. See <xref
linkend="datatype-xml"/> for information about the <type>xml</type>
type. The function-like expressions <function>xmlparse</function>
and <function>xmlserialize</function> for converting to and from
type <type>xml</type> are not repeated here. Use of most of these
functions requires the installation to have been built
type <type>xml</type> are documented there, not in this section.
</para>
<para>
Use of most of these functions
requires <productname>PostgreSQL</productname> to have been built
with <command>configure --with-libxml</command>.
</para>
@ -10453,8 +10462,8 @@ 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 +10705,13 @@ 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 XML 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 any 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>
@ -10715,25 +10727,14 @@ SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY VALUE '<towns><town>T
</para>
<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.
The <literal>BY REF</literal> and <literal>BY VALUE</literal> clauses
are accepted in <productname>PostgreSQL</productname>, but are ignored,
as discussed in <xref linkend="functions-xml-limits-postgresql"/>.
In the SQL standard, the <function>xmlexists</function> function
evaluates an expression in the XML Query language,
but <productname>PostgreSQL</productname> allows only an XPath 1.0
expression, as discussed in
<xref linkend="functions-xml-limits-xpath1"/>.
</para>
</sect3>
@ -10839,12 +10840,12 @@ 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 +10907,10 @@ 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 +10951,8 @@ 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 +10963,34 @@ 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, nor if 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.
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.
<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> and <literal>BY VALUE</literal> clauses
are accepted but ignored, as discussed in
<xref linkend="functions-xml-limits-postgresql"/>.
In the SQL standard, the <function>xmltable</function> function
evaluates expressions in the XML Query language,
but <productname>PostgreSQL</productname> 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 +10998,92 @@ 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 of the results will be
implementation-dependent. Details can be found in
<xref linkend="xml-xpath-1-specifics"/>.
</para>
</note>
<para>
The <replaceable>column_expression</replaceable> 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 <replaceable>column_expression</replaceable> is given, then the
column name is used as an implicit path.
</para>
<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.
If no <literal>column_expression</literal> is given, then the column name
is used as an implicit path.
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. (If the value is a boolean, its string representation is taken
to be <literal>1</literal> or <literal>0</literal> if the output
column's type category is numeric, otherwise <literal>true</literal> or
<literal>false</literal>.)
</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-empty set of XML nodes
and the column's PostgreSQL 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>
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.
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 +11095,14 @@ 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>

6
src/backend/catalog/sql_features.txt
Unescape View File

@ -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

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