Oracle constraints in XML data

Oracle introduced pretty cool XML support in 9i, it's even cooler in 10g and I don't understand why people don't use it more often. I keep seeing XML data stored in CLOB's all the time. Why not store it as proper XML, it's possible to index, query and even update individual XML elements, attributes or nodes. Fast, simple, easy, no need for full text indexes. Performance of xpath queries is pretty good if indexed correctly, 25 000 XML documents 10k each is still in the sub second range when hitting an index.

One thing that can be quite nice to have in the XML store is constraints to avoid duplicate data, indexes on XML data are pretty much plain standard pseudo-column indexes (or "functional indexes" as some refer to them as).
We just use the basic extract() or extractValue() functions in Oracles XML feature set.

Here's an example on how to to create unique constraints (indexes) on XML elements (or attributes):

SQL> create table x (a xmltype);

Table created.

SQL> insert into x values('<type><name>dog</name></type>');

1 row created.

SQL> insert into x values('<type><name>cat</name></type>');

1 row created.

SQL> create unique index xui on x(extractValue(a, '/type/name'));

Index created.

SQL> insert into x values('<type><name>cat</name></type>');
insert into x values('<type><name>cat</name></type>')
*
ERROR at line 1:
ORA-00001: unique constraint (HLINDEN.XUI) violated


SQL> insert into x values('<type><name>fish</name></type>');

1 row created.

-- Lets try the constraint on an attribute.
-- attributes are handeled just like elements but need a @ sign prefix

SQL> drop index xui;

Index dropped.

SQL> truncate table x;

Table truncated.

SQL> insert into x(a) values('<type id="1"><name>sally</name></type>');

1 row created.

SQL> insert into x(a) values('<type id="2"><name>bob</name></type>');

1 row created.

SQL> create unique index xui on x(extractValue(a, '/type/@id'));

Index created.

SQL> insert into x(a) values('<type id="2"><name>carol</name></type>');
insert into x(a) values('<type id="2"><name>carol</name></type>')
*
ERROR at line 1:
ORA-00001: unique constraint (HLINDEN.XUI) violated


SQL> insert into x(a) values('<type id="3"><name>carol</name></type>');

1 row created.

-- Ok, lets see if we can have duplicate names.

SQL> insert into x(a) values('<type id="4"><name>carol</name></type>');

1 row created.

SQL>

And some docs to read.