D.3. XML Limits and Conformance to SQL/XML#
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 singlexmltype, 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.
PostgreSQL does not support the
RETURNING CONTENTorRETURNING SEQUENCEclauses, functions which are defined to have these in the specification are implicitly returning content.
This section presents some of the resulting differences you may encounter.
D.3.1. Queries Are Restricted to XPath 1.0#
ThePostgreSQL-specific functionsxpath() andxpath_exists() query XML documents using the XPath language.PostgreSQL also provides XPath-only variants of the standard functionsXMLEXISTS andXMLTABLE, which officially use the XQuery language. For all of these functions,PostgreSQL relies on thelibxml2 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 thePostgreSQL-specific functions.
D.3.1.1. Restriction of XQuery to XPath#
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-stylefor-each andsort, anonymous functions, andparse-xml to create a node from a string), but such features were not available before XPath 3.0.
D.3.1.2. Restriction of XPath to 1.0#
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.
Note
Thelibxml2 library does seem to always return node-sets toPostgreSQL 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, andstring.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 twoNaNs. In contrast,=and!=do compare the strings as strings.XPath 1.0 blurs the distinction betweenvalue comparisons andgeneral comparisons as XQuery/XPath define them. Both
sale/@hatsize = 7andsale/@customer = "alice"are existentially quantified comparisons, true if there is anysalewith the given value for the attribute, butsale/@taxable = false()is a value comparison to theeffective boolean value of a whole node-set. It is true only if nosalehas ataxableattribute at all.In the XQuery/XPath data model, adocument 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, theroot node, can only be in document form. This is part of the reason an
xmlvalue passed as the context item to anyPostgreSQL 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 offunction library changes andlanguage 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.
D.3.1.3. Mappings between SQL and XML Data Types and Values#
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.
WhenPostgreSQL maps SQL data values to XML (as inxmlelement), or XML to SQL (as in the output columns ofxmltable), 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 inSection 9.8) explicitly to produce the standard mappings.
D.3.2. Incidental Limits of the Implementation#
This section concerns limits that are not inherent in thelibxml2 library, but apply to the current implementation inPostgreSQL.
D.3.2.1. OnlyBY VALUE Passing Mechanism Is Supported#
The SQL standard defines twopassing 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, andBY 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, ifx is an XML value, these two queries in an SQL:2006 environment would produce true and false, respectively:
SELECT XMLQUERY('$a is $b' PASSING BY REFx AS a,x AS b NULL ON EMPTY);SELECT XMLQUERY('$a is $b' PASSING BY VALUEx AS a,x AS b NULL ON EMPTY);PostgreSQL will acceptBY VALUE orBY REF in anXMLEXISTS orXMLTABLE construct, but it ignores them. Thexml data type holds a character-string serialized representation, so there is no node identity to preserve, and passing is always effectivelyBY VALUE.
D.3.2.2. Cannot Pass Named Parameters to Queries#
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.
D.3.2.3. NoXML(SEQUENCE) Type#
ThePostgreSQLxml data type can only hold a value inDOCUMENT orCONTENT 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 allowingCONTENT. The upshot is that a well-formedDOCUMENT is the only form of XML value thatPostgreSQL can supply as an XPath context item.