How does XML DB handle XML Schema Validation ?

Question

In order to validate an XML document against an XML Schema using Oracle XML DB the XML Schema must first be registered with XML DB using the method registerSchema provided by the package DBMS_XMLSCHEMA.
XDB provides two types of schema validation, 'Lax' Validation and 'Strict' Validation.
'Lax' validation takes place whenever a schema based document is converted from it's textual representation into the XML DB internal object model. Errors caught by this validation will be prefixed with 'ORA'.
'Stict' validation takes place when the XMLType methods schemaValidate() or isSchemaValid() are invoked.
The schemaValidate() method throws an exception with an error message indicating what is wrong it encounteres an invalid document. The error message associated with the exception will be prefixed with LSX.
The isSchemaValid() method returns true or false, depending on whether or not the document is valid. It cannot return any information about why the document is invalid.
The reason for having both the Lax and Strict validation models is that Strict validation is much more expensive in terms of CPU and memory usage than Lax validation.

Answer 1

Here are some examples of what is caught by Lax validation (ORA errors) and what is only caught by Strict validation (LSX errors).
SQL> begin
  2     dbms_xmlschema.registerSchema('test',xmltype(
  3  '<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xdb="http://xmlns.oracle.co
eFormDefault="unqualified">
  4     <xs:complexType name="RootType">
  5             <xs:sequence>
  6                     <xs:element name="Mandatory"/>
  7                     <xs:element name="Enumeration">
  8                             <xs:simpleType>
  9                                     <xs:restriction base="xs:string">
10                                             <xs:enumeration value="A"/>
11                                             <xs:enumeration value="B"/>
12                                             <xs:enumeration value="C"/>
13                                     </xs:restriction>
14                             </xs:simpleType>
15                     </xs:element>
16                     <xs:element name="MinLength">
17                             <xs:simpleType>
18                                     <xs:restriction base="xs:string">
19                                             <xs:minLength value="4"/>
20                                             <xs:maxLength value="20"/>
21                                     </xs:restriction>
22                             </xs:simpleType>
23                     </xs:element>
24                     <xs:element name="MaxLength">
25                             <xs:simpleType>
26                                     <xs:restriction base="xs:string">
27                                             <xs:minLength value="1"/>
28                                             <xs:maxLength value="4"/>
29                                     </xs:restriction>
30                             </xs:simpleType>
31                     </xs:element>
32                     <xs:element name="MaxOccurs" type="xs:string" maxOccurs="2"/>
33                     <xs:element name="MinOccurs" minOccurs="2" maxOccurs="2"/>
34                     <xs:element name="Optional" type="xs:string" minOccurs="0"/>
35             </xs:sequence>
36     </xs:complexType>
37     <xs:element name="Root" type="RootType" xdb:defaultTable="ROOT_TABLE"/>
38  </xs:schema>'));
39  end;
40  /
PL/SQL procedure successfully completed.
SQL> --
SQL> -- Valid Document
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
1 row created.
SQL> --
SQL> -- Undefined element 'Illegal'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Illegal>Hello World</Illegal>
  5     <Enumeration>A</Enumeration>
  6     <MinLength>ABCD</MinLength>
  7     <MaxLength>WXYZ</MaxLength>
  8     <MaxOccurs>1</MaxOccurs>
  9     <MaxOccurs>2</MaxOccurs>
10     <MinOccurs>1</MinOccurs>
11     <MinOccurs>2</MinOccurs>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30937: No schema definition for 'Illegal' (namespace '##local') in parent
'/Root'
SQL> --
SQL> -- Multiple occurences of 'Optional'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30936: Maximum number (1) of 'Optional' XML node elements exceeded
SQL> --
SQL> -- Missing element 'Manadatory'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Enumeration>A</Enumeration>
  4     <MinLength>ABCD</MinLength>
  5     <MaxLength>WXYZ</MaxLength>
  6     <MaxOccurs>1</MaxOccurs>
  7     <MaxOccurs>2</MaxOccurs>
  8     <MinOccurs>1</MinOccurs>
  9     <MinOccurs>2</MinOccurs>
10     <Optional>Goodbye</Optional>
11  </Root>'
12  ))
13  /
1 row created.
SQL> --
SQL> -- Invalid Enumeration Value
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>Z</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-31038: Invalid enumeration value: "Z"
SQL> --
SQL> -- MinLength Violation
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABC</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  --
15  -- MaxLength Violation
16  --
17  /
1 row created.
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>VWXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30951: Element or attribute at Xpath /Root/MaxLength exceeds maximum length
SQL> --
SQL> -- Missing element Optional - Valid Document
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11  </Root>'
12  ))
13  /
1 row created.
SQL> --
SQL> -- Too many instances of 'MaxOccurs'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MaxOccurs>3</MaxOccurs>
10     <MinOccurs>1</MinOccurs>
11     <MinOccurs>2</MinOccurs>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30936: Maximum number (2) of 'MaxOccurs' XML node elements exceeded
SQL> --
SQL> -- Too few instances of 'MinOccurs'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <Optional>Goodbye</Optional>
11  </Root>'
12  ))
13  /
1 row created.
SQL> create trigger validateSchema
  2  before insert on ROOT_TABLE
  3  for each row
  4  begin
  5     :new.object_value.schemaValidate();
  6  end;
  7  /
Trigger created.
SQL> --
SQL> -- Valid Document
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
1 row created.
SQL> --
SQL> -- Undefined element 'Illegal'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Illegal>Hello World</Illegal>
  5     <Enumeration>A</Enumeration>
  6     <MinLength>ABCD</MinLength>
  7     <MaxLength>WXYZ</MaxLength>
  8     <MaxOccurs>1</MaxOccurs>
  9     <MaxOccurs>2</MaxOccurs>
10     <MinOccurs>1</MinOccurs>
11     <MinOccurs>2</MinOccurs>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30937: No schema definition for 'Illegal' (namespace '##local') in parent
'/Root'
SQL> --
SQL> -- Multiple occurences of 'Optional'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30936: Maximum number (1) of 'Optional' XML node elements exceeded
SQL> --
SQL> -- Missing element 'Manadatory'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Enumeration>A</Enumeration>
  4     <MinLength>ABCD</MinLength>
  5     <MaxLength>WXYZ</MaxLength>
  6     <MaxOccurs>1</MaxOccurs>
  7     <MaxOccurs>2</MaxOccurs>
  8     <MinOccurs>1</MinOccurs>
  9     <MinOccurs>2</MinOccurs>
10     <Optional>Goodbye</Optional>
11  </Root>'
12  ))
13  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-31154: invalid XML document
ORA-19202: Error occurred in XML processing
LSX-00213: only 0 occurrences of particle "Mandatory", minimum is 1
ORA-06512: at "SYS.XMLTYPE", line 345
ORA-06512: at "XDBTEST.VALIDATESCHEMA", line 2
ORA-04088: error during execution of trigger 'XDBTEST.VALIDATESCHEMA'
SQL> --
SQL> -- Invalid Enumeration Value
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>Z</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-31038: Invalid enumeration value: "Z"
SQL> --
SQL> -- MinLength Violation
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABC</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  --
15  -- MaxLength Violation
16  --
17  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-31154: invalid XML document
ORA-19202: Error occurred in XML processing
LSX-00221: "ABC" is too short (minimum length is 4)
ORA-06512: at "SYS.XMLTYPE", line 345
ORA-06512: at "XDBTEST.VALIDATESCHEMA", line 2
ORA-04088: error during execution of trigger 'XDBTEST.VALIDATESCHEMA'
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>VWXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11     <Optional>Goodbye</Optional>
12  </Root>'
13  ))
14  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30951: Element or attribute at Xpath /Root/MaxLength exceeds maximum length
SQL> --
SQL> -- Missing element Optional - Valid Document
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <MinOccurs>2</MinOccurs>
11  </Root>'
12  ))
13  /
1 row created.
SQL> --
SQL> -- Too many instances of 'MaxOccurs'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MaxOccurs>3</MaxOccurs>
10     <MinOccurs>1</MinOccurs>
11     <MinOccurs>2</MinOccurs>
12     <Optional>Goodbye</Optional>
13  </Root>'
14  ))
15  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-30936: Maximum number (2) of 'MaxOccurs' XML node elements exceeded
SQL> --
SQL> -- Too few instances of 'MinOccurs'
SQL> --
SQL> insert into ROOT_TABLE values (xmltype(
  2  '<Root>
  3     <Mandatory>Hello World</Mandatory>
  4     <Enumeration>A</Enumeration>
  5     <MinLength>ABCD</MinLength>
  6     <MaxLength>WXYZ</MaxLength>
  7     <MaxOccurs>1</MaxOccurs>
  8     <MaxOccurs>2</MaxOccurs>
  9     <MinOccurs>1</MinOccurs>
10     <Optional>Goodbye</Optional>
11  </Root>'
12  ))
13  /
insert into ROOT_TABLE values (xmltype(
ERROR at line 1:
ORA-31154: invalid XML document
ORA-19202: Error occurred in XML processing
LSX-00213: only 1 occurrences of particle "MinOccurs", minimum is 2
ORA-06512: at "SYS.XMLTYPE", line 345
ORA-06512: at "XDBTEST.VALIDATESCHEMA", line 2
ORA-04088: error during execution of trigger 'XDBTEST.VALIDATESCHEMA'
SQL>