From: Chapman Flack <chap@anastigmatix.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Markus Winand <markus.winand@winand.at>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Pavel Stehule <pavel.stehule@gmail.com>
Subject: Re: The two "XML Fixes" patches still in need of review
Date: Sat, 23 Mar 2019 21:21:57 -0400
Message-ID: <5C96DBB5.2080103@anastigmatix.net> (raw)
In-Reply-To: <5C96B141.5050802@anastigmatix.net>
References: <5C81F8C0.6090901@anastigmatix.net>
<12427.1553375114@sss.pgh.pa.us>
<5C96B141.5050802@anastigmatix.net>
On 03/23/19 18:20, Chapman Flack wrote:
> On 03/23/19 17:05, Tom Lane wrote:
>> I've now pushed a somewhat-adjusted version of the XML-content fix
>> patch. The documentation patch needs some small rebasing to apply
>> after that one instead of before it.
>
> Will do.
>
>> Perhaps it'd make sense under the XML section in datatype.sgml,
>> but I think I might lean to making it a new section in Appendix D
>> (SQL Conformance).
>
> Sounds like the option (4) I proposed back in [1]. I suppose it won't
> be much trouble to move.
PFA xml-functions-type-docfix-4.patch rebased, and with the limits/
compatibility section moved to Appendix D.
Regards,
-Chap
Attachments:
[text/x-patch] xml-functions-type-docfix-4.patch (30.5K, ../5C96DBB5.2080103@anastigmatix.net/2-xml-functions-type-docfix-4.patch)
download | inline diff:diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgmlindex 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.sgmlindex 6c22d69..5d65eae 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 +156,325 @@
</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 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>libxml</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>+ <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 nodeset (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 nodeset has no guaranteed order and,+ like any set, can have no member appear more than once. (The+ <productname>libxml</productname> library does seem to always return+ nodesets 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>+ </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+ nodesets 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 nodeset,+ 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>libxml</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.sgmlindex 1a01473..c46240f 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 nodeset, 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 nodeset produced by the XPath expression.+ If the XPath expression returns a scalar value rather than a nodeset,
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 nodeset).+ 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 nodeset, or any value other than a nodeset.)
</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.
@@ -10998,42 +10997,78 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
</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.+ 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 "string representation" 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>+ 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 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.+ 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 nodeset+ (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 +11080,14 @@ 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.txtindex 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: chap@anastigmatix.net, tgl@sss.pgh.pa.us, pgsql-hackers@lists.postgresql.org, markus.winand@winand.at, alvherre@2ndquadrant.com, michael@paquier.xyz, pavel.stehule@gmail.com
Subject: Re: The two "XML Fixes" patches still in need of review
In-Reply-To: <5C96DBB5.2080103@anastigmatix.net>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox