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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.

This is a backpatch for branches 10 and 11, taken from Tom's commit
12d46ac392d0 for 12, then edited to correctly describe behaviors that
are fixed in 12 but still broken in 10 and 11.

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
This commit is contained in:
Alvaro Herrera 2019-09-12 16:52:03 -03:00
parent 0a8cd4c296
commit 603a28b449
4 changed files with 492 additions and 88 deletions
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5
doc/src/sgml/datatype.sgml
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@ -4228,6 +4228,11 @@ a0ee-bc99-9c0b-4ef8-bb6d-6bb9-bd38-0a11
value is a full document or only a content fragment. value is a full document or only a content fragment.
</para> </para>
<para>
Limits and compatibility notes for the <type>xml</type> data type
can be found in <xref linkend="xml-limits-conformance">.
</para>
<sect2> <sect2>
<title>Creating XML Values</title> <title>Creating XML Values</title>
<para> <para>

381
doc/src/sgml/features.sgml
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@ -16,7 +16,8 @@
Language SQL</quote>. A revised version of the standard is released Language SQL</quote>. A revised version of the standard is released
from time to time; the most recent update appearing in 2011. 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 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 replaces the previous one, so claims of conformance to earlier
versions have no official merit. versions have no official merit.
<productname>PostgreSQL</productname> development aims for <productname>PostgreSQL</productname> development aims for
@ -155,4 +156,382 @@
</para> </para>
</sect1> </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. In this release,
an explicit cast is needed if an <function>xmltable</function> column
expression produces a boolean or double value; see
<xref linkend="functions-xml-limits-postgresql">.
</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>
Cast needed for <function>xmltable</function> column
of boolean or double type
</title>
<para>
An <function>xmltable</function> column expression evaluating to an XPath
boolean or number result will produce an <quote>unexpected XPath object
type</quote> error. The workaround is to rewrite the column expression to
be inside the XPath <function>string</function> function;
<productname>PostgreSQL</productname> will then assign the string value
successfully to an SQL output column of boolean or double type.
</para>
</sect3>
<sect3>
<title>
Column path result or SQL result column of XML type
</title>
<para>
In this release, a <function>xmltable</function> column expression
that evaluates to an XML node-set can be assigned to an SQL result
column of XML type, producing a concatenation of: for most types of
node in the node-set, a text node containing the XPath 1.0
<firstterm>string-value</firstterm> of the node, but for an element node,
a copy of the node itself. Such a node-set may be assigned to an SQL
column of non-XML type only if the node-set has a single node, with the
string-value of most node types replaced with an empty string, the
string-value of an element node replaced with a concatenation of only its
direct text-node children (excluding those of descendants), and the
string-value of a text or attribute node being as defined in XPath 1.0.
An XPath string value assigned to a result column of XML type must be
parsable as XML.
</para>
<para>
It is best not to develop code that relies on these behaviors, which have
little resemblance to the spec, and are changed in
<productname>PostgreSQL 12</productname>.
</para>
</sect3>
<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>
In this release, <productname>PostgreSQL</productname> will accept
<literal>BY REF</literal> in an
<function>XMLEXISTS</function> or <function>XMLTABLE</function>
construct, but will ignore it. 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> </appendix>

188
doc/src/sgml/func.sgml
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@ -9903,17 +9903,26 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
<sect1 id="functions-xml"> <sect1 id="functions-xml">
<title>XML Functions</title> <title>XML Functions</title>
<indexterm>
<primary>XML Functions</primary>
</indexterm>
<para> <para>
The functions and function-like expressions described in this 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> linkend="datatype-xml"> for information about the <type>xml</type>
type. The function-like expressions <function>xmlparse</function> type. The function-like expressions <function>xmlparse</function>
and <function>xmlserialize</function> for converting to and from and <function>xmlserialize</function> for converting to and from
type <type>xml</type> are not repeated here. Use of most of these type <type>xml</type> are documented there, not in this section.
functions requires the installation to have been built </para>
with <command>configure --with-libxml</>.
<para>
Use of most of these functions
requires <productname>PostgreSQL</productname> to have been built
with <command>configure --with-libxml</command>.
</para> </para>
<sect2 id="functions-producing-xml"> <sect2 id="functions-producing-xml">
@ -10107,8 +10116,8 @@ SELECT xmlelement(name foo, xmlattributes('xyz' as bar),
encoding, depending on the setting of the configuration parameter encoding, depending on the setting of the configuration parameter
<xref linkend="guc-xmlbinary">. The particular behavior for <xref linkend="guc-xmlbinary">. The particular behavior for
individual data types is expected to evolve in order to align the individual data types is expected to evolve in order to align the
SQL and PostgreSQL data types with the XML Schema specification, PostgreSQL mappings with those specified in SQL:2006 and later,
at which point a more precise description will appear. as discussed in <xref linkend="functions-xml-limits-casts">.
</para> </para>
</sect3> </sect3>
@ -10350,10 +10359,13 @@ SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
</synopsis> </synopsis>
<para> <para>
The function <function>xmlexists</function> returns true if the The function <function>xmlexists</function> evaluates an XPath 1.0
XPath expression in the first argument returns any nodes, and expression (the first argument), with the passed XML value as its context
false otherwise. (If either argument is null, the result is item. The function returns false if the result of that evaluation
null.) 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>
<para> <para>
@ -10369,14 +10381,14 @@ SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Tor
</para> </para>
<para> <para>
The <literal>BY REF</literal> clauses have no effect in The <literal>BY REF</literal> clauses
PostgreSQL, but are allowed for SQL conformance and compatibility are accepted in <productname>PostgreSQL</productname>, but are ignored,
with other implementations. Per SQL standard, the as discussed in <xref linkend="functions-xml-limits-postgresql">.
first <literal>BY REF</literal> is required, the second is In the SQL standard, the <function>xmlexists</function> function
optional. Also note that the SQL standard specifies evaluates an expression in the XML Query language,
the <function>xmlexists</function> construct to take an XQuery but <productname>PostgreSQL</productname> allows only an XPath 1.0
expression as first argument, but PostgreSQL currently only expression, as discussed in
supports XPath, which is a subset of XQuery. <xref linkend="functions-xml-limits-xpath1">.
</para> </para>
</sect3> </sect3>
@ -10482,12 +10494,12 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
</synopsis> </synopsis>
<para> <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</> value) expression <replaceable>xpath</replaceable> (a <type>text</type> value)
against the XML value against the XML value
<replaceable>xml</replaceable>. It returns an array of XML values <replaceable>xml</replaceable>. It returns an array of XML values
corresponding to the node set produced by the XPath expression. corresponding to the node-set produced by the XPath expression.
If the XPath expression returns a scalar value rather than a node set, If the XPath expression returns a scalar value rather than a node-set,
a single-element array is returned. a single-element array is returned.
</para> </para>
@ -10549,9 +10561,10 @@ SELECT xpath('//mydefns:b/text()', '<a xmlns="http://example.com"><b>test</b></a
<para> <para>
The function <function>xpath_exists</function> is a specialized form The function <function>xpath_exists</function> is a specialized form
of the <function>xpath</function> function. Instead of returning the of the <function>xpath</function> function. Instead of returning the
individual XML values that satisfy the XPath, this function returns a individual XML values that satisfy the XPath 1.0 expression, this function
Boolean indicating whether the query was satisfied or not. This returns a Boolean indicating whether the query was satisfied or not
function is equivalent to the standard <literal>XMLEXISTS</> predicate, (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. except that it also offers support for a namespace mapping argument.
</para> </para>
@ -10592,8 +10605,8 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
<para> <para>
The <function>xmltable</function> function produces a table based The <function>xmltable</function> function produces a table based
on the given XML value, an XPath filter to extract rows, and an on the given XML value, an XPath filter to extract rows, and a
optional set of column definitions. set of column definitions.
</para> </para>
<para> <para>
@ -10604,30 +10617,34 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
</para> </para>
<para> <para>
The required <replaceable>row_expression</> argument is an XPath The required <replaceable>row_expression</replaceable> argument is
expression that is evaluated against the supplied XML document to an XPath 1.0 expression that is evaluated, passing the
obtain an ordered sequence of XML nodes. This sequence is what <replaceable>document_expression</replaceable> as its context item, to
<function>xmltable</> transforms into output rows. 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>
<para> <para>
<replaceable>document_expression</> provides the XML document to <replaceable>document_expression</replaceable> provides the context
operate on. item for the <replaceable>row_expression</replaceable>. It must be a
The <literal>BY REF</literal> clauses have no effect in PostgreSQL, well-formed XML document; fragments/forests are not accepted.
but are allowed for SQL conformance and compatibility with other The <literal>BY REF</literal> clause
implementations. is accepted but ignored, as discussed in
The argument must be a well-formed XML document; fragments/forests <xref linkend="functions-xml-limits-postgresql">.
are not accepted. 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>
<para> <para>
The mandatory <literal>COLUMNS</literal> clause specifies the list The mandatory <literal>COLUMNS</literal> clause specifies the list
of columns in the output table. of columns in the output table.
If the <literal>COLUMNS</> clause is omitted, the rows in the result Each entry describes a single column.
set contain a single column of type <literal>xml</> containing the
data matched by <replaceable>row_expression</>.
If <literal>COLUMNS</literal> is specified, each entry describes a
single column.
See the syntax summary above for the format. See the syntax summary above for the format.
The column name and type are required; the path, default and The column name and type are required; the path, default and
nullability clauses are optional. nullability clauses are optional.
@ -10635,48 +10652,57 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
<para> <para>
A column marked <literal>FOR ORDINALITY</literal> will be populated A column marked <literal>FOR ORDINALITY</literal> will be populated
with row numbers matching the order in which the with row numbers, starting with 1, in the order of nodes retrieved from
output rows appeared in the original input XML document. the <replaceable>row_expression</replaceable>'s result node-set.
At most one column may be marked <literal>FOR ORDINALITY</literal>. At most one column may be marked <literal>FOR ORDINALITY</literal>.
</para> </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> <para>
The <literal>column_expression</> for a column is an XPath expression The <replaceable>column_expression</replaceable> for a column is an
that is evaluated for each row, relative to the result of the XPath 1.0 expression that is evaluated for each row, with the current
<replaceable>row_expression</>, to find the value of the column. node from the <replaceable>row_expression</replaceable> result as its
If no <literal>column_expression</> is given, then the column name context item, to find the value of the column. If
is used as an implicit path. no <replaceable>column_expression</replaceable> is given, then the
column name is used as an implicit path.
</para> </para>
<para> <para>
If a column's XPath expression returns multiple elements, an error If a column's XPath expression returns a non-XML value (limited to
is raised. string, boolean, or double in XPath 1.0) and the column has a
If the expression matches an empty tag, the result is an PostgreSQL type other than <type>xml</type>, the column will be set
empty string (not <literal>NULL</>). as if by assigning the value's string representation to the PostgreSQL
Any <literal>xsi:nil</> attributes are ignored. type. In this release, an XPath boolean or double result must be explicitly
cast to string (that is, the XPath 1.0 <function>string</function> function
wrapped around the original column expression);
<productname>PostgreSQL</productname> can then successfully assign the
string to an SQL result column of boolean or double type.
These conversion rules differ from those of the SQL
standard, as discussed in <xref linkend="functions-xml-limits-casts">.
</para> </para>
<para> <para>
The text body of the XML matched by the <replaceable>column_expression</> In this release, SQL result columns of <type>xml</type> type, or
is used as the column value. Multiple <literal>text()</literal> nodes column XPath expressions evaluating to an XML type, regardless of the
within an element are concatenated in order. Any child elements, output column SQL type, are handled as described in
processing instructions, and comments are ignored, but the text contents <xref linkend="functions-xml-limits-postgresql">; the behavior
of child elements are concatenated to the result. changes significantly in <productname>PostgreSQL 12</productname>.
Note that the whitespace-only <literal>text()</> node between two non-text
elements is preserved, and that leading whitespace on a <literal>text()</>
node is not flattened.
</para> </para>
<para> <para>
If the path expression does not match for a given row but If the path expression returns an empty node-set
<replaceable>default_expression</> is specified, the value resulting (typically, when it does not match)
from evaluating that expression is used. for a given row, the column will be set to <literal>NULL</literal>, unless
If no <literal>DEFAULT</> clause is given for the column, a <replaceable>default_expression</replaceable> is specified; then the
the field will be set to <literal>NULL</>. value resulting from evaluating that expression is used.
It is possible for a <replaceable>default_expression</> 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.
</para> </para>
<para> <para>
@ -10688,20 +10714,14 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
</para> </para>
<para> <para>
Unlike regular PostgreSQL functions, <replaceable>column_expression</> A <replaceable>default_expression</replaceable>, rather than being
and <replaceable>default_expression</> are not evaluated to a simple evaluated immediately when <function>xmltable</function> is called,
value before calling the function. is evaluated each time a default is needed for the column.
<replaceable>column_expression</> is normally evaluated If the expression qualifies as stable or immutable, the repeat
exactly once per input row, and <replaceable>default_expression</>
is evaluated each time a default is needed for a field.
If the expression qualifies as stable or immutable the repeat
evaluation may be skipped. evaluation may be skipped.
Effectively <function>xmltable</> behaves more like a subquery than a
function call.
This means that you can usefully use volatile functions like This means that you can usefully use volatile functions like
<function>nextval</> in <replaceable>default_expression</>, and <function>nextval</function> in
<replaceable>column_expression</> may depend on other parts of the <replaceable>default_expression</replaceable>.
XML document.
</para> </para>
<para> <para>

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

@ -593,7 +593,7 @@ X085 Predefined namespace prefixes NO
X086 XML namespace declarations in XMLTable NO X086 XML namespace declarations in XMLTable NO
X090 XML document predicate YES X090 XML document predicate YES
X091 XML content predicate NO 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 X100 Host language support for XML: CONTENT option NO
X101 Host language support for XML: DOCUMENT option NO X101 Host language support for XML: DOCUMENT option NO
X110 Host language support for XML: VARCHAR mapping 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 X283 XMLValidate with SEQUENCE option NO
X284 XMLValidate: NAMESPACE without ELEMENT clause NO X284 XMLValidate: NAMESPACE without ELEMENT clause NO
X286 XMLValidate: NO NAMESPACE with 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 X301 XMLTable: derived column list option YES
X302 XMLTable: ordinality column option YES X302 XMLTable: ordinality column option YES
X303 XMLTable: column default 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 X305 XMLTable: initializing an XQuery variable NO
X400 Name and identifier mapping YES X400 Name and identifier mapping YES
X410 Alter column data type: XML type YES X410 Alter column data type: XML type YES