8.13. XML Type
The xml data type can be used to store XML data. Its advantage over storing XML data in a text field is that it checks the input values for well-formedness, and there are support functions to perform type-safe operations on it; see Section 9.14. Use of this data type requires the installation to have been built with configure --with-libxml.
The xml type can store well-formed “documents”, as defined by the XML standard, as well as “content” fragments, which are defined by reference to the more permissive “document node” of the XQuery and XPath data model. Roughly, this means that content fragments can have more than one top-level element or character node. The expression xmlvalue IS DOCUMENTxml value is a full document or only a content fragment.
Limits and compatibility notes for the xml data type can be found in Section D.3.
8.13.1. Creating XML Values
To produce a value of type xml from character data, use the function xmlparse:
XMLPARSE ( { DOCUMENT | CONTENT } value)
Examples:
XMLPARSE (DOCUMENT '<?xml version="1.0"?><book><title>Manual</title><chapter>...</chapter></book>') XMLPARSE (CONTENT 'abc<foo>bar</foo><bar>foo</bar>')
While this is the only way to convert character strings into XML values according to the SQL standard, the Postgres Pro-specific syntaxes:
xml '<foo>bar</foo>' '<foo>bar</foo>'::xml
can also be used.
The xml type does not validate input values against a document type declaration (DTD), even when the input value specifies a DTD. There is also currently no built-in support for validating against other XML schema languages such as XML Schema.
The inverse operation, producing a character string value from xml, uses the function xmlserialize:
XMLSERIALIZE ( { DOCUMENT | CONTENT } value AS type )
type can be character, character varying, or text (or an alias for one of those). Again, according to the SQL standard, this is the only way to convert between type xml and character types, but Postgres Pro also allows you to simply cast the value.
When a character string value is cast to or from type xml without going through XMLPARSE or XMLSERIALIZE, respectively, the choice of DOCUMENT versus CONTENT is determined by the “XML option” session configuration parameter, which can be set using the standard command:
SET XML OPTION { DOCUMENT | CONTENT };
or the more Postgres Pro-like syntax
SET xmloption TO { DOCUMENT | CONTENT };
The default is CONTENT, so all forms of XML data are allowed.
8.13.2. Encoding Handling
Care must be taken when dealing with multiple character encodings on the client, server, and in the XML data passed through them. When using the text mode to pass queries to the server and query results to the client (which is the normal mode), Postgres Pro converts all character data passed between the client and the server and vice versa to the character encoding of the respective end; see Section 22.3. This includes string representations of XML values, such as in the above examples. This would ordinarily mean that encoding declarations contained in XML data can become invalid as the character data is converted to other encodings while traveling between client and server, because the embedded encoding declaration is not changed. To cope with this behavior, encoding declarations contained in character strings presented for input to the xml type are ignored, and content is assumed to be in the current server encoding. Consequently, for correct processing, character strings of XML data must be sent from the client in the current client encoding. It is the responsibility of the client to either convert documents to the current client encoding before sending them to the server, or to adjust the client encoding appropriately. On output, values of type xml will not have an encoding declaration, and clients should assume all data is in the current client encoding.
When using binary mode to pass query parameters to the server and query results back to the client, no encoding conversion is performed, so the situation is different. In this case, an encoding declaration in the XML data will be observed, and if it is absent, the data will be assumed to be in UTF-8 (as required by the XML standard; note that Postgres Pro does not support UTF-16). On output, data will have an encoding declaration specifying the client encoding, unless the client encoding is UTF-8, in which case it will be omitted.
Needless to say, processing XML data with Postgres Pro will be less error-prone and more efficient if the XML data encoding, client encoding, and server encoding are the same. Since XML data is internally processed in UTF-8, computations will be most efficient if the server encoding is also UTF-8.
Caution
Some XML-related functions may not work at all on non-ASCII data when the server encoding is not UTF-8. This is known to be an issue for xmltable() and xpath() in particular.
8.13.3. Accessing XML Values
The xml data type is unusual in that it does not provide any comparison operators. This is because there is no well-defined and universally useful comparison algorithm for XML data. One consequence of this is that you cannot retrieve rows by comparing an xml column against a search value. XML values should therefore typically be accompanied by a separate key field such as an ID. An alternative solution for comparing XML values is to convert them to character strings first, but note that character string comparison has little to do with a useful XML comparison method.
Since there are no comparison operators for the xml data type, it is not possible to create an index directly on a column of this type. If speedy searches in XML data are desired, possible workarounds include casting the expression to a character string type and indexing that, or indexing an XPath expression. Of course, the actual query would have to be adjusted to search by the indexed expression.
The text-search functionality in Postgres Pro can also be used to speed up full-document searches of XML data. The necessary preprocessing support is, however, not yet available in the Postgres Pro distribution.