Significant revisions to the XML-related specifications in ISO/IEC 9075-14 (SQL/XML) were introduced with SQL:2006. PostgreSQL's implementation of the XML data type and related functions largely follows the earlier 2003 edition, with some borrowing from later editions. In particular:
Where the current standard provides a family of XML data types
to hold “document” or “content” in
untyped or XML Schema-typed variants, and a type
XML(SEQUENCE) to hold arbitrary pieces of XML content,
PostgreSQL provides the single
xml type, which can hold “document” or
“content”. There is no equivalent of the
standard's “sequence” type.
PostgreSQL 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.
This section presents some of the resulting differences you may encounter.
The PostgreSQL-specific functions
xpath() and xpath_exists()
query XML documents using the XPath language.
PostgreSQL also provides XPath-only variants
of the standard functions XMLEXISTS and
XMLTABLE, which officially use
the XQuery language. For all of these functions,
PostgreSQL relies on the
libxml2 library, which provides only XPath 1.0.
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.
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 PostgreSQL-specific functions.
Features of XQuery beyond those of XPath include:
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.
XQuery has control constructs for iteration, sorting, and grouping.
XQuery allows declaration and use of local functions.
Recent XPath versions begin to offer capabilities overlapping with
these (such as functional-style for-each and
sort, anonymous functions, and
parse-xml to create a node from a string),
but such features were not available before XPath 3.0.
For developers familiar with XQuery and XPath 2.0 or later, XPath 1.0 presents a number of differences to contend with:
The fundamental type of an XQuery/XPath expression, the
sequence, 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.
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.
The libxml2 library does seem to always return node-sets to PostgreSQL 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.
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 boolean,
double, and string.
XPath 1.0 has no conditional operator. An XQuery/XPath expression
such as if ( hat ) then hat/@size else "no hat"
has no XPath 1.0 equivalent.
XPath 1.0 has no ordering comparison operator for strings. Both
"cat" < "dog" and
"cat" > "dog" are false, because each is a
numeric comparison of two NaNs. In contrast,
= and != do compare the strings
as strings.
XPath 1.0 blurs the distinction between
value comparisons and
general comparisons as XQuery/XPath define
them. Both sale/@hatsize = 7 and
sale/@customer = "alice" are existentially
quantified comparisons, true if there is
any sale with the given value for the
attribute, but sale/@taxable = false() is a
value comparison to the
effective boolean value of a whole node-set.
It is true only if no sale has
a taxable attribute at all.
In the XQuery/XPath data model, a document
node 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
root node, can only be in document form.
This is part of the reason an xml value passed as the
context item to any PostgreSQL
XPath-based function must be in document form.
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 changes and language changes 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.
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.
When PostgreSQL maps SQL data values to XML
(as in xmlelement), or XML to SQL (as in the output
columns of xmltable), except for a few cases
treated specially, PostgreSQL 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.
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 Section 9.8) explicitly to produce the standard mappings.
This section concerns limits that are not inherent in the libxml2 library, but apply to the current implementation in PostgreSQL.
BY VALUE Passing Mechanism Is Supported
The SQL standard defines two passing mechanisms
that apply when passing an XML argument from SQL to an XML function or
receiving a result: BY REF, in which a particular XML
value retains its node identity, and BY VALUE, 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.
To illustrate the difference, if
x is an XML value, these two queries in
an SQL:2006 environment would produce true and false, respectively:
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);
PostgreSQL will accept
BY VALUE or BY REF in an
XMLEXISTS or XMLTABLE
construct, but it ignores them. The xml data type holds
a character-string serialized representation, so there is no node
identity to preserve, and passing is always effectively BY
VALUE.
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.
XML(SEQUENCE) Type
The PostgreSQL xml data type
can only hold a value in DOCUMENT
or CONTENT 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 CONTENT. The upshot is that a
well-formed DOCUMENT is the only form of XML value
that PostgreSQL can supply as an XPath
context item.