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<chapter id="datatype">

<title id="datatype-title">Data Types</title>

</sect1>

<sect1 id="datatype-xml">

<title><acronym>XML</> Document Support</title>

<indexterm zone="datatype">

<primary>xml</primary>

</indexterm>

<para>

<acronym>XML</> (eXtensible Markup Language) support is not one

capability, but a variety of features supported by a database

system. These capabilities include storage, import/export,

validation, indexing, efficiency of modification, searching,

transforming, and <acronym>XML</> to <acronym>SQL</> mapping.

<productname>PostgreSQL</> supports some but not all of these

<acronym>XML</> capabilities. Future releases of

<productname>PostgreSQL</> will continue to improve <acronym>XML</>

support. For an overview of <acronym>XML</> use in databases, see <ulink

url="http://www.rpbourret.com/xml/XMLAndDatabases.htm"></>.

</para>

<variablelist>

<varlistentry>

<term>Storage</term>

<listitem>

<para>

PostgreSQL does not have a specialized <acronym>XML</> data type.

Users should store <acronym>XML</> documents in ordinary

<type>TEXT</> fields. If you need the document split apart into

its component parts so each element is stored separately, you must

use a middle-ware solution to do that, but once done, the data

becomes relational and has to be processed accordingly.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Import/Export</term>

<listitem>

<para>

There is no facility for mapping <acronym>XML</> to relational

tables. An external tool must be used for this. One simple way to

export <acronym>XML</> is to use <application>psql</> in

<acronym>HTML</> mode (<literal>\pset format html</>), and convert

the <acronym>XHTML</> output to XML using an external tool.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Validation</term>

<listitem>

<para>

<filename>/contrib/xml2</> has a function called

<function>xml_valid()</> that can be used in a <literal>CHECK</>

constraint to enforce that a field contains valid <acronym>XML</>.

It does not support validation against a specific <acronym>XML</>

schema. A server-side language with <acronym>XML</> capabilities

could be used to do schema-specific <acronym>XML</> checks.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Indexing</term>

<listitem>

<para>

<filename>/contrib/xml2</> functions can be used in expression

indexes to index specific <acronym>XML</> fields. To index the

full contents of <acronym>XML</> documents, the full-text indexing

tool <filename>/contrib/tsearch2</> can be used. Of course,

tsearch2 indexes have no <acronym>XML</> awareness so additional

<filename>/contrib/xml2</> checks should be added to queries.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Modification</term>

<listitem>

<para>

If an <command>UPDATE</> does not modify an <acronym>XML</> field,

the <acronym>XML</> data is shared between the old and new rows.

However, if the <command>UPDATE</> modifies an <acronym>XML</>

field, a full modified copy of the <acronym>XML</> field must be

created internally.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Searching</term>

<listitem>

<para>

XPath searches are implemented using <filename>/contrib/xml2</>.

It processes <acronym>XML</> text documents and returns results

based on the requested query.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Transforming</term>

<listitem>

<para>

<filename>/contrib/xml2</> supports <acronym>XSLT</> (XML

Stylesheet Language Transformation).

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>XML to SQL Mapping</term>

<listitem>

<para>

This involves converting <acronym>XML</> data to and from

relational structures. <productname>PostgreSQL</> has no internal

support for such mapping, and relies on external tools to do such

conversions.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Missing Features</term>

<listitem>

<para>

Missing features include XQuery, SQL/XML syntax (ISO/IEC

9075-14), and an <acronym>XML</> data type optimized for

<acronym>XML</> storage.

</para>

</listitem>

</varlistentry>

</variablelist>

</sect1>

</chapter>

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