PostgreSQL

9.14.1. Producing XML Content

9.14.2. XML Predicates

9.14.3. Processing XML

9.14.4. Mapping Tables to XML

The functions and function-like expressions described in this section operate on values of type xml. See Section 8.13 for information about the xml type. The function-like expressions xmlparse and xmlserialize for converting to and from type xml are documented there, not in this section.

Use of most of these functions requires PostgreSQL to have been built with configure --with-libxml.

A set of functions and function-like expressions are available for producing XML content from SQL data. As such, they are particularly suitable for formatting query results into XML documents for processing in client applications.

The function xmlcomment creates an XML value containing an XML comment with the specified text as content. The text cannot contain “[.quote]--” or end with a “[.quote]-” so that the resulting construct is a valid XML comment. If the argument is null, the result is null.

Example:

The function xmlconcat concatenates a list of individual XML values to create a single value containing an XML content fragment. Null values are omitted; the result is only null if there are no nonnull arguments.

Example:

XML declarations, if present, are combined as follows. If all argument values have the same XML version declaration, that version is used in the result, else no version is used. If all argument values have the standalone declaration value “[.quote]#yes”, then that value is used in the result. If all argument values have a standalone declaration value and at least one is [.quote]“no”#, then that is used in the result. Else the result will have no standalone declaration. If the result is determined to require a standalone declaration but no version declaration, a version declaration with version 1.0 will be used because XML requires an XML declaration to contain a version declaration. Encoding declarations are ignored and removed in all cases.

Example:

The xmlelement expression produces an XML element with the given name, attributes, and content.

Examples:

Element and attribute names that are not valid XML names are escaped by replacing the offending characters by the sequence _x`HHHH_, where HHHH` is the character’s Unicode codepoint in hexadecimal notation. For example:

An explicit attribute name need not be specified if the attribute value is a column reference, in which case the column’s name will be used as the attribute name by default. In other cases, the attribute must be given an explicit name. So this example is valid:

But these are not:

Element content, if specified, will be formatted according to its data type. If the content is itself of type xml, complex XML documents can be constructed. For example:

Content of other types will be formatted into valid XML character data. This means in particular that the characters <, >, and & will be converted to entities. Binary data (data type bytea) will be represented in base64 or hex encoding, depending on the setting of the configuration parameter xmlbinary. The particular behavior for individual data types is expected to evolve in order to align the PostgreSQL mappings with those specified in SQL:2006 and later, as discussed in Section D.3.1.3.

The xmlforest expression produces an XML forest (sequence) of elements using the given names and content.

Examples:

As seen in the second example, the element name can be omitted if the content value is a column reference, in which case the column name is used by default. Otherwise, a name must be specified.

Element names that are not valid XML names are escaped as shown for xmlelement above. Similarly, content data is escaped to make valid XML content, unless it is already of type xml.

Note that XML forests are not valid XML documents if they consist of more than one element, so it might be useful to wrap xmlforest expressions in xmlelement.

The xmlpi expression creates an XML processing instruction. The content, if present, must not contain the character sequence ?>.

Example:

The xmlroot expression alters the properties of the root node of an XML value. If a version is specified, it replaces the value in the root node’s version declaration; if a standalone setting is specified, it replaces the value in the root node’s standalone declaration.

The function xmlagg is, unlike the other functions described here, an aggregate function. It concatenates the input values to the aggregate function call, much like xmlconcat does, except that concatenation occurs across rows rather than across expressions in a single row. See Section 9.20 for additional information about aggregate functions.

Example:

To determine the order of the concatenation, an ORDER BY clause may be added to the aggregate call as described in Section 4.2.7. For example:

The following non-standard approach used to be recommended in previous versions, and may still be useful in specific cases:

The expressions described in this section check properties of xml values.

The expression IS DOCUMENT returns true if the argument XML value is a proper XML document, false if it is not (that is, it is a content fragment), or null if the argument is null. See Section 8.13 about the difference between documents and content fragments.

The expression IS NOT DOCUMENT returns false if the argument XML value is a proper XML document, true if it is not (that is, it is a content fragment), or null if the argument is null.

The function xmlexists 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.

Example:

The BY REF and BY VALUE clauses are accepted in PostgreSQL, but are ignored, as discussed in Section D.3.2. In the SQL standard, the xmlexists function evaluates an expression in the XML Query language, but PostgreSQL allows only an XPath 1.0 expression, as discussed in Section D.3.1.

These functions check whether a text string is well-formed XML, returning a Boolean result. xml_is_well_formed_document checks for a well-formed document, while xml_is_well_formed_content checks for well-formed content. xml_is_well_formed does the former if the xmloption configuration parameter is set to DOCUMENT, or the latter if it is set to CONTENT. This means that xml_is_well_formed is useful for seeing whether a simple cast to type xml will succeed, whereas the other two functions are useful for seeing whether the corresponding variants of XMLPARSE will succeed.

Examples:

The last example shows that the checks include whether namespaces are correctly matched.

To process values of data type xml, PostgreSQL offers the functions xpath and xpath_exists, which evaluate XPath 1.0 expressions, and the XMLTABLE table function.

The function xpath evaluates the XPath 1.0 expression `xpath (a `text value) against the XML value `xml`. 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, a single-element array is returned.

The second argument must be a well formed XML document. In particular, it must have a single root node element.

The optional third argument of the function is an array of namespace mappings. This array should be a two-dimensional text array with the length of the second axis being equal to 2 (i.e., it should be an array of arrays, each of which consists of exactly 2 elements). The first element of each array entry is the namespace name (alias), the second the namespace URI. It is not required that aliases provided in this array be the same as those being used in the XML document itself (in other words, both in the XML document and in the xpath function context, aliases are local).

Example:

To deal with default (anonymous) namespaces, do something like this:

The function xpath_exists is a specialized form of the xpath function. Instead of returning the 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 XMLEXISTS predicate, except that it also offers support for a namespace mapping argument.

Example:

The xmltable function produces a table based on the given XML value, an XPath filter to extract rows, and a set of column definitions.

The optional XMLNAMESPACES clause is a comma-separated list of namespaces. It specifies the XML namespaces used in the document and their aliases. A default namespace specification is not currently supported.

The required `row_expression argument is an XPath 1.0 expression that is evaluated, passing the document_expression as its context item, to obtain a set of XML nodes. These nodes are what `xmltable transforms into output rows. No rows will be produced if the `document_expression is null, nor if the row_expression` produces an empty node-set or any value other than a node-set.

`document_expression provides the context item for the row_expression. It must be a well-formed XML document; fragments/forests are not accepted. The `BY REF and BY VALUE clauses are accepted but ignored, as discussed in Section D.3.2. In the SQL standard, the xmltable function evaluates expressions in the XML Query language, but PostgreSQL allows only XPath 1.0 expressions, as discussed in Section D.3.1.

The mandatory COLUMNS clause specifies the list of columns in the output table. 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.

A column marked FOR ORDINALITY will be populated with row numbers, starting with 1, in the order of nodes retrieved from the `row_expression's result node-set. At most one column may be marked `FOR ORDINALITY.

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 Section D.3.1.2.

The `column_expression for a column is an XPath 1.0 expression that is evaluated for each row, with the current node from the row_expression result as its context item, to find the value of the column. If no column_expression` 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 xml, 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 1 or 0 if the output column’s type category is numeric, otherwise true or false.)

If a column’s XPath expression returns a non-empty set of XML nodes and the column’s PostgreSQL type is xml, the column will be assigned the expression result exactly, if it is of document or content form. ^[8]

A non-XML result assigned to an xml 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 string function) to the PostgreSQL type.

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 NULL). Any xsi:nil attributes are ignored. Note that the whitespace-only text() node between two non-text elements is preserved, and that leading whitespace on a text() node is not flattened. The XPath 1.0 string function may be consulted for the rules defining the string value of other XML node types and non-XML values.

The conversion rules presented here are not exactly those of the SQL standard, as discussed in Section D.3.1.3.

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 NULL, unless a `default_expression` is specified; then the value resulting from evaluating that expression is used.

Columns may be marked NOT NULL. If the `column_expression for a `NOT NULL column does not match anything and there is no DEFAULT or the `default_expression` also evaluates to null, an error is reported.

A `default_expression, rather than being evaluated immediately when `xmltable 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. This means that you can usefully use volatile functions like nextval in `default_expression`.

Examples:

The following example shows concatenation of multiple text() nodes, usage of the column name as XPath filter, and the treatment of whitespace, XML comments and processing instructions:

The following example illustrates how the XMLNAMESPACES clause can be used to specify a list of namespaces used in the XML document as well as in the XPath expressions:

The following functions map the contents of relational tables to XML values. They can be thought of as XML export functionality:

The return type of each function is xml.

table_to_xml maps the content of the named table, passed as parameter `tbl. The `regclass type accepts strings identifying tables using the usual notation, including optional schema qualifications and double quotes. query_to_xml executes the query whose text is passed as parameter `query and maps the result set. `cursor_to_xml fetches the indicated number of rows from the cursor specified by the parameter `cursor`. This variant is recommended if large tables have to be mapped, because the result value is built up in memory by each function.

If `tableforest` is false, then the resulting XML document looks like this:

If `tableforest` is true, the result is an XML content fragment that looks like this:

If no table name is available, that is, when mapping a query or a cursor, the string table is used in the first format, row in the second format.

The choice between these formats is up to the user. The first format is a proper XML document, which will be important in many applications. The second format tends to be more useful in the cursor_to_xml function if the result values are to be reassembled into one document later on. The functions for producing XML content discussed above, in particular xmlelement, can be used to alter the results to taste.

The data values are mapped in the same way as described for the function xmlelement above.

The parameter `nulls` determines whether null values should be included in the output. If true, null values in columns are represented as:

where xsi is the XML namespace prefix for XML Schema Instance. An appropriate namespace declaration will be added to the result value. If false, columns containing null values are simply omitted from the output.

The parameter `targetns` specifies the desired XML namespace of the result. If no particular namespace is wanted, an empty string should be passed.

The following functions return XML Schema documents describing the mappings performed by the corresponding functions above:

It is essential that the same parameters are passed in order to obtain matching XML data mappings and XML Schema documents.

The following functions produce XML data mappings and the corresponding XML Schema in one document (or forest), linked together. They can be useful where self-contained and self-describing results are wanted:

In addition, the following functions are available to produce analogous mappings of entire schemas or the entire current database:

Note that these potentially produce a lot of data, which needs to be built up in memory. When requesting content mappings of large schemas or databases, it might be worthwhile to consider mapping the tables separately instead, possibly even through a cursor.

The result of a schema content mapping looks like this:

where the format of a table mapping depends on the `tableforest` parameter as explained above.

The result of a database content mapping looks like this:

where the schema mapping is as above.

As an example of using the output produced by these functions, Example 9.1 shows an XSLT stylesheet that converts the output of table_to_xml_and_xmlschema to an HTML document containing a tabular rendition of the table data. In a similar manner, the results from these functions can be converted into other XML-based formats.

Example 9.1. XSLT Stylesheet for Converting SQL/XML Output to HTML

^[8] 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 string function.

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