8 Advanced Topics for Oracle Objects

Leaf-Level Attributes

An object type is like a tree structure, where the branches represent the attributes. Attributes that are objects sprout subbranches with their own attributes.

Ultimately, each branch ends at an attribute that is a built-in type; such as NUMBER, VARCHAR2, or REF, or a collection type, such as VARRAY or nested table. Each of these leaf-level attributes of the original object type is stored in a table column.

Leaf-level attributes that are not collection types are called the leaf-level scalar attributes of the object type.

The following topics relate to the discussion of object tables and relational tables in "How Objects are Stored in Tables".

How Row Objects Are Split Across Columns

In an object table, Oracle database stores the data for every leaf-level scalar or REF attribute in a separate column.

When you retrieve or change attributes of row objects in an object table, the database performs the corresponding operations on the columns of the table. Accessing the value of the row object itself invokes the default constructor for the type, using the columns of the object table as arguments and produces a copy of the object.

The database stores the system-generated object identifier in a hidden column. The database uses the object identifier to construct REFs to the object.

Hidden Columns for Tables with Column Objects

When a table (relational table) is defined with a column of an object type, the database adds hidden columns to the table for the leaf-level attributes of the object type. Each object-type column also has a corresponding hidden column to store the NULL information for the column objects (that is, the atomic nulls of the top-level and the nested objects).

Hidden Columns for Substitutable Columns and Object Tables

A substitutable column or object table has a hidden column not only for each attribute of the object type of the column but also for each attribute added in any subtype of the object type. These columns store the values of those attributes for any subtype instances inserted in the substitutable column.

Besides the type-discriminant column and the null-image column, the following are associated with a substitutable column of person_typ, created by Example 8-1

• A hidden column for each of the attributes of person_typ: idno, name, and phone

• Hidden columns for attributes of the subtypes of person_typ

Thus, the following might be associated with a substitutable column of person_typ: the attributes dept_id and major (for student_typ) and number_hours (for part_time_student_typ).

When you create a subtype, the database automatically adds hidden columns for new attributes in the subtype to tables containing a substitutable column of any of the ancestor types of the new subtype. These retrofit the tables to store data of the new type. If, for some reason, the columns cannot be added, creation of the subtype is rolled back.

When you drop a subtype using DROP TYPE with the VALIDATE option, the database automatically drops hidden columns for attributes unique to the subtype that do not contain data. Errors are raised if these columns contain data.

Example 8-1 creates types needed for subsequent examples in this chapter

-- drop any of these objects created for Ex.7-10
CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20),
  MAP MEMBER FUNCTION get_idno RETURN NUMBER )
  NOT FINAL;
/
CREATE TYPE BODY person_typ AS
  MAP MEMBER FUNCTION get_idno RETURN NUMBER IS
  BEGIN
    RETURN idno;
  END;
END;
/
CREATE TYPE student_typ UNDER person_typ (
    dept_id NUMBER,
    major VARCHAR2(30))
NOT FINAL;
/
CREATE TYPE part_time_student_typ UNDER student_typ (
  number_hours NUMBER);
/
CREATE TYPE employee_typ UNDER person_typ (
    emp_id NUMBER, 
    mgr VARCHAR2(30));
/
CREATE TABLE person_obj_table OF person_typ;  // an object table
INSERT INTO person_obj_table 
  VALUES (person_typ(12, 'Bob Jones', '650-555-0130'));
INSERT INTO person_obj_table 
  VALUES (student_typ(51, 'Joe Lane', '1-650-555-0140', 12, 'HISTORY'));
INSERT INTO person_obj_table 
  VALUES (part_time_student_typ(52, 'Kim Patel', '1-650-555-0135', 14,
          'PHYSICS', 20));

Substitutable columns are associated with hidden type-discriminant columns. The hidden columns contains an identifier, called a typeid, that identifies the most specific type of each object in the substitutable columns. Typically, a typeid (RAW) is one byte, though it can be as big as four bytes for a large hierarchy.

You can find the typeid of a specified object instance using the function SYS_TYPEID.

Example 8-2 retrieves typeids of object instances stored in the substitutable object table created in Example 8-1:

-- Requires Ex. 8-1
SELECT name, SYS_TYPEID(VALUE(p)) typeid 
  FROM person_obj_table p;

The catalog views USER_TYPES, DBA_TYPES, and ALL_TYPES contain a TYPEID column (not hidden) that gives the typeid value for each type. You can join on this column to get the type names corresponding to the typeids in a type-discriminant column.

Storage of REFs

When the database constructs a REF to a row object, the constructed REF is made up of the object identifier (OID), some metadata of the object table, and, optionally, the ROWID.

The size of a REF in a column of REF type depends on the storage requirements associated with the column, as follows:

• If the column is declared as a REF WITH ROWID, the database stores the ROWID in the REF column. The ROWID hint is ignored for object references in constrained REF columns.

• If a column is declared as a REF with a SCOPE clause, the column decreases due to the omission of the object table metadata and the ROWID. A scoped REF is 16 bytes long.

If the object identifier is primary-key based, the database may create one or more internal columns to store the values of the primary key, depending on how many columns comprise the primary key.

Internal Layout of Nested Tables

The rows of a nested table are stored in a separate storage table. Each nested table column has a single associated storage table. The storage table holds all the elements for all of the nested tables in that column. The storage table has a hidden NESTED_TABLE_ID column with a system-generated value that lets Oracle database map the nested table elements back to the appropriate row.

You can speed up queries that retrieve entire collections by making the storage table index-organized. Include the ORGANIZATION INDEX clause inside the STORE AS clause.

See "Nested Table Storage".

A nested table type can contain objects or scalars:

• If the elements are objects, the storage table is like an object table: the top-level attributes of the object type become the columns of the storage table. However, you cannot construct REFs to objects in a nested table because a nested table row has no object identifier column.

• If the elements are scalars, the storage table contains a single column called COLUMN_VALUE that contains the scalar values.

Internal Layout of VARRAYs

All the elements of a VARRAY are stored in a single column. Depending upon the size of the array, it may be stored inline or in a BLOB. See Storage Considerations for Varrays for details.

Indexing a Type-Discriminant Column

Using the SYS_TYPEID function, you can build an index on the hidden type-discriminant column of substitutable columns. The type-discriminant column contains typeids that identify the most specific type of every object instance stored in the substitutable column. The system uses this information to evaluate queries that filter by type using the IS OF predicate, but you can access the typeids for your own purposes using the SYS_TYPEID function.

Generally, a type-discriminant column contains only a small number of distinct typeids: at most, there can be only as many as there are types in the related type hierarchy. The low cardinality of this column makes it a good candidate for a bitmap index.

For example, the following statement creates a bitmap index on the type-discriminant column underlying the substitutable contact column of table contacts. The function SYS_TYPEID references the type-discriminant column:

-- Requires Ex. 8-1
CREATE TABLE contacts (
  contact         person_typ,
  contact_date    DATE );
INSERT INTO contacts VALUES (
  person_typ (65,'Vrinda Mills', '1-650-555-0125'),'24 Jun 2003' );
INSERT INTO contacts VALUES (
  person_typ (12, 'Bob Jones', '650-555-0130'),'24 Jun 2003' );
INSERT INTO contacts VALUES (
  student_typ(51, 'Joe Lane', '1-650-555-0140', 12, 'HISTORY'),'24 Jun 2003' );
INSERT INTO contacts VALUES ( part_time_student_typ(52, 'Kim Patel', '1-650-555-0135', 14, 'PHYSICS', 20),'24 Jun 2003' ); 
CREATE BITMAP INDEX typeid_idx ON contacts (SYS_TYPEID(contact));

Indexing Subtype Attributes of a Substitutable Column

You can build an index on attributes for any types that can be stored in a substitutable column. You can reference attributes of subtypes in the CREATE INDEX statement by filtering out types other than the desired subtype (and its subtypes) using the TREAT function; you then use dot notation to specify the desired attribute.

For example, the following statement creates an index on the major attribute of all students in the contacts table. The declared type of the contact column is person_typ, of which student_typ is a subtype, so the column may contain instances of person_typ, student_typ, and subtypes of either one:

-- Requires Ex.8-1- and 8-3
CREATE INDEX major1_idx ON contacts 
  (TREAT(contact AS student_typ).major);

The student_typ type first defined the major attribute: the person_typ supertype does not have it. Consequently, all the values in the hidden column for the major attribute are values for persons of type student_typ or parttimestudent_typ (a student_typ subtype). This means that the values of the hidden column are identical to the values returned by the TREAT expression, major values for all students, including student subtypes: both the hidden column and the TREAT expression list majors for students and nulls for non-students. The system exploits this fact and creates index major1_idx as an ordinary B-tree index on the hidden column.

Values in a hidden column are only identical to the values returned by the TREAT expression just described if the type named as the target of the TREAT function (student_typ) is the type that first defined the major attribute. If the target of the TREAT function is a subtype that merely inherited the attribute, as in the following example, the TREAT expression returns non-null major values for the subtype (part-time students) but not for its supertype (other students).

CREATE INDEX major2_idx ON contacts 
  (TREAT(contact AS part_time_student_typ).major);

Here, the values stored in the hidden column for major may be different from the results of the TREAT expression. Consequently, an ordinary B-tree index cannot be created on the underlying column. Therefore, the database treats the TREAT expression like any other function-based expression and tries to create the index as a function-based index on the result.

The following example, like the previous one, creates a function-based index on the major attribute of part-time students, but in this case, the hidden column for major is associated with a substitutable object table person_obj_table:

CREATE INDEX major3_idx ON person_obj_table p 
  (TREAT(VALUE(p) AS part_time_student_typ).major);

Type Evolution and Dependent Schema Objects

Dependent schema objects of a type are objects that directly or indirectly reference the type and are affected by a change to it.

A type can have these kinds of dependent schema objects: tables; types or subtypes; program units (PL/SQL blocks) such as procedures, functions, packages, and triggers; indextypes; views (including object views); function-based indexes; and operators.

How a dependent schema object is affected by a change to a type depends on the object and on the nature of the change.

• Dependent program units, views, operators, and indextypes are marked invalid when the type is modified. The next time one of these invalid schema objects is referenced, it is revalidated using the new type definition. If the object recompiles successfully, it becomes valid and can be used again.

• Dependent function-based indexes may be dropped or disabled, depending on the type change, and must be rebuilt.

• Dependent tables have one or more internal columns added for each attribute added to the type, depending on the attribute type. New attributes are added with NULL values. For each dropped attribute, the columns associated with that attribute are dropped. For each modified attribute, the length, precision, or scale of its associated column is changed accordingly.

These changes mainly involve updating the metadata of the tables and can be performed quickly. However, the data in those tables must be updated to the format of the new type version as well, as discussed in "Options for Updating Data".

Options for Updating Data

Depending on the amount of data, updating can be time-consuming, so the ALTER TYPE command has options to let you choose whether to convert all dependent table data immediately or to leave it in the old format to be converted piecemeal as it is updated in the course of business.

The CASCADE option for ALTER TYPE propagates a type change to dependent types and tables. See "ALTER TYPE Statement for Type Evolution". CASCADE itself has the following options that let you choose whether or not to convert table data to the new type format as part of the propagation:

• INCLUDING TABLE DATA: converts the data (default)

• NOT INCLUDING TABLE DATA : does not convert data

By default, the CASCADE option converts the data. In either case, table data is always returned in the format of the latest type version. If the table data is stored in the format of an earlier type version, the database converts the data to the format of the latest version before returning it, even though the format in which the data is actually stored is not changed until the data is rewritten.

You can retrieve the definition of the latest type from the system view USER_SOURCE. You can view definitions of all versions of a type in the USER_TYPE_VERSIONS view.

Effects of Structural Changes to Types

Structural changes to a type affect dependent data and require the data to be converted. This is not true for changes that are confined to method definitions or behavior (implementation) of the type.

These possible changes to a type are structural:

• Add or drop an attribute

• Modify the length, precision, or scale of an attribute

• Change the finality of a type from FINAL to NOT FINAL or the reverse

These changes result in new versions of the altered type and all its dependent types and require the system to add, drop, or modify internal columns of dependent tables as part of the process of converting to the new version.

When you make any of these kinds of changes to a type that has dependent types or tables, the effects of propagating the change are not confined only to metadata but also affect data storage arrangements and require data conversion.

Besides converting data, you may also need to make other changes. For example, if a new attribute is added to a type, and the type body invokes the constructor of the type, then each constructor in the type body must be modified to specify a value for the new attribute. Similarly, if a new method is added, then the type body must be replaced to add the implementation of the new method. The type body can be modified by using the CREATE OR REPLACE TYPE BODY statement.

Altering a Type by Adding and Dropping Attributes

Example 8-5 illustrates how to make a simple change to person_typ by adding one attribute and dropping another. The CASCADE keyword propagates the type change to dependent types and tables, but the phrase NOT INCLUDING TABLE DATA prevents conversion of the related data.

-- Drop person_typ and person_obj_table if they exist
CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20));
/
CREATE TABLE person_obj_table OF person_typ;

INSERT INTO person_obj_table 
  VALUES (person_typ(12, 'Bob Jones', '650-555-0130'));

SELECT value(p) FROM person_obj_table p;

You can add the email attribute and drop the phone attribute as follows:

ALTER TYPE person_typ
  ADD ATTRIBUTE (email VARCHAR2(80)), 
  DROP ATTRIBUTE phone CASCADE NOT INCLUDING TABLE DATA;

Then disconnect and reconnect to accommodate the type change:

connect oe/oe;
connect hr/hr;
ALTER SESSION SET PLSQL_WARNINGS = 'enable:all';
-- The data of table person_obj_table has not been converted yet, but
-- when the data is retrieved, Oracle returns the data based on
-- the latest type version. The new attribute is initialized to NULL.
SELECT value(p) FROM person_obj_table p;

During SELECT statements, even though column data may be converted to the latest type version, the converted data is not written back to the column. If you retrieve a particular user-defined type column in a table often, consider converting that data to the latest type version to eliminate redundant data conversions. Converting is especially beneficial if the column contains VARRAY attributes which typically take more time to convert than objects or nested table columns.

You can convert a column of data by issuing an UPDATE statement to set the column to itself, as indicated in the following code snippet, which is unrelated to previous code.

UPDATE dept_tab SET emp_array_col = emp_array_col;

You can convert all columns in a table by using ALTER TABLE with the UPGRADE INCLUDING DATA. For example:

ALTER TYPE person_typ ADD ATTRIBUTE (photo BLOB)
  CASCADE NOT INCLUDING TABLE DATA;
ALTER TABLE person_obj_table UPGRADE INCLUDING DATA;

The ALTER TABLE line converts only the table listed. The CASCADE option prevents conversion of other tables or dependents.

Altering a Type by Adding a Nested Table Attribute

This section describes the steps required to make a complex change to a type: the addition of a nested table attribute to an object type that is included in a nested table.

Example 8-6 provides the initial schema which is altered by Example 8-7.

-- Drop existing person_typ, department_type, people_typ objects or tables
CREATE TYPE person_typ AS OBJECT (
  idno           NUMBER,
  name           VARCHAR2(30),
  phone          VARCHAR2(20));
/
-- creating a nested table type
CREATE TYPE people_typ AS TABLE OF person_typ;/
CREATE TYPE department_typ AS OBJECT (
  manager    person_typ,
  employee   people_typ);  // a nested table/
CREATE TABLE department OF department_typ
  NESTED TABLE employee STORE AS employee_store_nt;

Example 8-7 starts by creating a new object tasks_typ and a nested table type to hold it, tasks_nttab.

The following steps, both in Example 8-7, and in other programs, are necessary to add the nested table tasks as an attribute to the object type person_typ, which is already included in the nested table people_typ.

1. Issue an ALTER TYPE..INVALIDATE statement to alter the type person_typ. This statement bypasses all type and table checks to save time and invalidates dependent objects. You cannot access table data until it is validated.

   The ALTER TYPE statement includes ADD ATTRIBUTE to add the nested table tasks.

   The UPGRADE.. STORE AS clause upgrades the affected nested table, and specifies name of the new storage table.

   Example 8-7 Altering an Object Type by Adding a Nested Table Attribute

   -- Requires Ex. 8-6
   CREATE TYPE tasks_typ AS OBJECT (
     priority       VARCHAR2(2),
     description    VARCHAR2(30));
   /
   
   CREATE TYPE tasks_nttab AS TABLE OF tasks_typ;
   /
   
   ALTER TYPE person_typ ADD ATTRIBUTE tasks tasks_nttab
     INVALIDATE;
   
   -- Propagate the change to employee_store_nt
   -- Specify a storage name for the new nested table
   ALTER TABLE employee_store_nt
     UPGRADE NESTED TABLE tasks STORE AS tasks_nt;
   

2. Use CREATE OR REPLACE TYPE BODY for person_typ to update the corresponding type body to make it current with the new type definition, if necessary.

3. Upgrade the dependent tables to the latest type version and convert the data in the tables. This validates the table and allow for data access again.

   ALTER TABLE department UPGRADE INCLUDING DATA;
   

4. Alter dependent PL/SQL program units as needed to take account of changes to the type.

5. Use OTT or JPublisher to generate new header files for applications, depending on whether the application is written in C or Java.

   Adding a new attribute to a supertype also increases the number of attributes in all its subtypes because these inherit the new attribute. Inherited attributes always precede declared (locally defined) attributes, so adding a new attribute to a supertype causes the ordinal position of all declared attributes of any subtype to be incremented by one recursively. The mappings of the altered type must be updated to include the new attributes. Oracle Type Translator (OTT) and JPublisher do this. If you use another tool, you must be sure that the type headers are properly synchronized with the type definition in the server; otherwise, unpredictable behavior may result.

6. Modify application code as needed and rebuild the application.

Validating a Type That Has Been Altered

When the system executes an ALTER TYPE statement, it first validates the requested type change syntactically and semantically to make sure it is legal. The system performs the same validations as for a CREATE TYPE statement plus some additional ones. If the new spec of the target type or any of its dependent types fails the type validations, the ALTER TYPE statement aborts. No new type version is created, and all dependent objects remain unchanged.

If dependent tables exist, further checking ensures that restrictions relating to the tables and indexes are observed. For example, it ensures that an attribute being dropped is not used as a partitioning key. Again, if the ALTER TYPE statement fails the check of table-related restrictions, then the type change is aborted, and no new version of the type is created.

When a single ALTER TYPE statement adds multiple attributes, it is done in the order specified. Multiple type changes can be specified in the same ALTER TYPE statement, but no attribute name or method signature can be specified more than once in the statement. For example, adding and modifying the same attribute in a single statement is not allowed.

The following sections contain other notes on type changes including:

• Dropping an Attribute

• Modifying the Length, Precision, or Scale of an Attribute Type

• Dropping a Method

• Modifying the INSTANTIABLE Property

Dropping an Attribute

• Dropping all attributes from a root type is not allowed. Instead, you must drop the type. Because a subtype inherits all the attributes from its supertype, dropping all the attributes from a subtype does not reduce its attribute count to zero; therefore, dropping all attributes declared locally in a subtype is allowed.

• Only an attribute declared locally in the target type can be dropped. You cannot drop an inherited attribute from a subtype. Instead, drop the attribute from the type where it is locally declared.

• Dropping an attribute which is part of a table partitioning or sub-partitioning key in a table is not allowed.

• Dropping an attribute of a primary key OID of an object table or an index-organized table (IOT) is not allowed.

• When an attribute is dropped, the column corresponding to the dropped attribute is dropped.

• When an attribute is dropped, any indexes, statistics, constraints, and referential integrity constraints that reference it are removed.

Modifying the Length, Precision, or Scale of an Attribute Type

• You are not allowed to expand the length of an attribute referenced in a function-based index, clustered key or domain index on a dependent table.

• You are not allowed to decrease the length, precision, or scale of an attribute.

Dropping a Method

• You can only drop a method from the type in which the method is defined (or redefined): You cannot drop an inherited method from a subtype, and you cannot drop an redefined method from a supertype.

• If a method is not redefined, dropping it using the CASCADE option removes the method from the target type and all subtypes. However, if a method is redefined in a subtype, the CASCADE will fail and roll back. For the CASCADE to succeed, you must first drop each redefined method from the subtype that defines it and then drop the method from the supertype.

  You can consult the USER_DEPENDENCIES table to find all the schema objects, including types, that depend on a given type. You can also run the DBMS_UTILITY.GET_DEPENDENCY utility to find the dependencies of a type.

• You can use the INVALIDATE option to drop a method that has been redefined, but the redefined versions in the subtypes must still be dropped manually. The subtypes will remain in an invalid state until they are explicitly altered to drop the redefined versions. Until then, an attempt to recompile the subtypes for revalidation will produce the error Method does not override.

  Unlike CASCADE, INVALIDATE bypasses all the type and table checks and simply invalidates all schema objects dependent on the type. The objects are revalidated the next time they are accessed. This option is faster than using CASCADE, but you must be certain that no problems occur when revalidating dependent types and tables. Table data cannot be accessed while a table is invalid; if a table cannot be validated, its data remains inaccessible.

  See "If a Type Change Validation Fails".

Modifying the INSTANTIABLE Property

• Altering an object type from INSTANTIABLE to NOT INSTANTIABLE is allowed only if the type has no table dependents.

• Altering an object type from NOT INSTANTIABLE to INSTANTIABLE is allowed anytime. This change does not affect tables.

Modifying the FINAL Property

• Altering an object type from NOT FINAL to FINAL is only allowed if the target type has no subtypes.

• When you alter an object type from FINAL to NOT FINAL or vice versa, you must use CASCADE to convert data in dependent columns and tables immediately. You may not use the CASCADE option NOT INCLUDING TABLE DATA to defer converting data.

  • From NOT FINAL to FINAL, you must use CASCADE INCLUDING TABLE DATA.

  • From FINAL to NOT FINAL, you may use either CASCADE INCLUDING TABLE DATA or CASCADE CONVERT TO SUBSTITUTABLE.

    When you alter a type from FINAL to NOT FINAL, select the CASCADE option based on whether or not you want to insert new subtypes of the altered types into existing columns and tables.

  By default, altering a type from FINAL to NOT FINAL enables you to create new substitutable tables and columns of that type, but it does not automatically make existing columns (or object tables) of that type substitutable. In fact, just the opposite happens: existing columns and tables of the type are marked NOT SUBSTITUTABLE AT ALL LEVELS. If any embedded attribute of these columns is substitutable, an error is generated. New subtypes of the altered type cannot be inserted into these preexisting columns and tables.

  To alter an object type to NOT FINAL in a way that makes existing columns and tables of the type substitutable (assuming that they are not marked NOT SUBSTITUTABLE), use the CASCADE option CONVERT TO SUBSTITUTABLE.

  Example 8-8 shows the use of CASCADE with the option CONVERT TO SUBSTITUTABLE:

  Example 8-8 Converting a Type from FINAL to NOT FINAL

  CREATE TYPE shape AS OBJECT (
      name VARCHAR2(30),
      area NUMBER)
      FINAL;/
  ALTER TYPE shape NOT FINAL CASCADE CONVERT TO SUBSTITUTABLE;
  

  This CASCADE option marks each existing column as SUBSTITUTABLE AT ALL LEVELS and causes a new, hidden column to be added for the TypeId of instances stored in the column. The column can then store subtype instances of the altered type.

If a Type Change Validation Fails

The INVALIDATE option of the ALTER TYPE statement lets you alter a type without propagating the type change to dependent objects. In this case, the system does not validate the dependent types and tables, that is, does not ensure that all the ramifications of the type change are legal. Instead, the system marks all dependent schema objects invalid. These objects, including types and tables, are revalidated the next time they are referenced. If a type cannot be revalidated, it remains invalid, and any tables referencing it become inaccessible until the problem is corrected.

A table may fail validation for reasons such as: the addition of a new attribute to a type increased the number of columns in the table beyond the maximum of 1000, or an attribute used as a partitioning or clustering key of a table was dropped from a type.

To force a revalidation of a type, users can issue the ALTER TYPE COMPILE statement. To force a revalidation of an invalid table, users can issue the ALTER TABLE UPGRADE statement and specify whether or not the data is to be converted to the latest type version.

If a table cannot be converted to the latest type version, then INSERT, UPDATE and DELETE statements on the table are not allowed, and the table data becomes inaccessible. The following DDLs can be executed on the table, but all other statements which reference an invalid table are not allowed until the table is successfully validated:

• DROP TABLE

• TRUNCATE TABLE

All PL/SQL programs containing variables defined using %ROWTYPE of a table or %TYPE of a column or attribute from a table are compiled based on the latest type version. If the table fails the revalidation, then compiling any program units that reference that table also fails.

ALTER TYPE Statement for Type Evolution

Table 8-1 lists some of the important options in the ALTER TYPE and ALTER TYPE...CASCADE statements for altering the attribute or method definition of a type.

ALTER TABLE Statement for Type Evolution

You can use ALTER TABLE to convert table data to the latest version of referenced types. For an example, see "Altering a Type by Adding a Nested Table Attribute". See Table 8-1 for a discussion of the INCLUDING DATA option.

The Attribute-Value Constructor

The system-defined constructor, also known as the attribute-value constructor, requires you to pass the constructor a value for each attribute of the type. The constructor then sets the attributes of the new object instance to those values, as shown in Example 8-9.

CREATE TYPE shape AS OBJECT (
    name VARCHAR2(30),
    area NUMBER);
/
CREATE TABLE building_blocks of shape;

-- attribute-value constructor: Sets instance attributes to the specified values
INSERT INTO building_blocks
  VALUES (
    NEW shape('my_shape', 4));

The keyword NEW preceding a call to a constructor is optional but recommended.

Constructors and Type Evolution

The attribute-value constructor saves you the trouble of defining your own constructors for a type. However, you must supply a value for every attribute declared in the type or the constructor call fails to compile.

This requirement can create a problem if you evolve the type later on, especially because the attribute-value constructor is implicit and not visible in the code, unlike a user-defined constructor. When you change the attributes of a type, the attribute-value constructor of the type changes, too. If you add an attribute, the updated attribute-value constructor expects a value for the new attribute; otherwise, any attribute-value constructor calls in your existing code fail.

See "Type Evolution".

Advantages of User-Defined Constructors

User-defined constructors do not need to explicitly set a value for every attribute of a type, unlike attribute-value constructors. A user-defined constructor can have any number of arguments, of any type, and these do not need to map directly to type attributes. When you define a constructor, you can initialize the attributes to any appropriate values. For any attributes which you do not supply values, the system initialized to NULL.

If you evolve a type—for example, by adding an attribute—calls to user-defined constructors for the type do not need to be changed. User-defined constructors are not automatically modified when the type evolves, so their signatures remain the same. You may, however, need to change the definition of the constructor if you do not want the new attribute to be initialized to NULL.

Defining and Implementing User-Defined Constructors

You define user-defined constructors in the type body, like an ordinary method. You introduce the declaration and the definition with the phrase CONSTRUCTOR FUNCTION and end with the clause RETURN SELF AS RESULT.

A constructor for a type must have the same name as the type. Example 8-10 defines two constructor functions for the shape type. As the example shows, you can overload user-defined constructors by defining multiple versions with different signatures.

CREATE TYPE shape AS OBJECT (
    name VARCHAR2(30),
    area NUMBER,
    CONSTRUCTOR FUNCTION shape(SELF IN OUT NOCOPY shape, name VARCHAR2)
                               RETURN SELF AS RESULT,
    CONSTRUCTOR FUNCTION shape(SELF IN OUT NOCOPY shape, name VARCHAR2, 
                               area NUMBER) RETURN SELF AS RESULT
) NOT FINAL;
/

CREATE TYPE BODY shape AS
    CONSTRUCTOR FUNCTION shape(SELF IN OUT NOCOPY shape, name VARCHAR2) 
                               RETURN SELF AS RESULT IS
    BEGIN
        SELF.name := name;
        SELF.area := 0;
        RETURN;
    END;
    CONSTRUCTOR FUNCTION shape(SELF IN OUT NOCOPY shape, name VARCHAR2, 
                                area NUMBER) RETURN SELF AS RESULT IS
    BEGIN
        SELF.name := name;
        SELF.area := area;
        RETURN;
    END;
END;
/

Overloading and Hiding Constructors

You can overload user-defined constructors, like other type methods.

User-defined constructors are not inherited, so a user-defined constructor defined in a supertype cannot be hidden in a subtype. However, a user-defined constructor does hide, and thus supersede, the attribute-value constructor for its type if the signature of the user-defined constructor exactly matches the signature of the attribute-value constructor. For the signatures to match, the names and types of the parameters (after the implicit SELF parameter) of the user-defined constructor must be the same as the names and types of the attributes of the type. The mode of the parameters (after the implicit SELF parameter) of the user-defined constructor must be IN.

If an attribute-value constructor is not hidden by a user-defined constructor that has the same name and signature, the attribute-value constructor can still be called.

Note that, if you evolve a type—for example, by adding an attribute—the signature of the attribute-value constructor of the type changes accordingly. This can cause a formerly hidden attribute-value constructor to become usable again.

Calling User-Defined Constructors

You call a user-defined constructor like any other function and you can use it anywhere you can use an ordinary function.

The SELF argument is passed in implicitly and may not be passed in explicitly. In other words, usages like the following are not allowed:

NEW constructor(instance, argument_list)

A user-defined constructor cannot occur in the DEFAULT clause of a CREATE or ALTER TABLE statement, but an attribute-value constructor can. The arguments to the attribute-value constructor must not contain references to PL/SQL functions or to other columns, including the pseudocolumns LEVEL, PRIOR, and ROWNUM, or to date constants that are not fully specified. The same is true for check constraint expressions: an attribute-value constructor can be used as part of check constraint expressions while creating or altering a table, but a user-defined constructor cannot.

Parentheses are required in SQL even for constructor calls that have no arguments. In PL/SQL, parentheses are optional when invoking a zero-argument constructor. They do, however, make it more obvious that the constructor call is a function call. The following PL/SQL example omits parentheses in the constructor call to create a new shape:

shape s := NEW my_schema.shape;

The NEW keyword and the schema name are optional.

Example 8-11 creates a subtype under the type created in Example 8-10 and shows examples for calling the user-defined constructors.

-- Requires Ex. 8-10
CREATE TYPE rectangle UNDER shape (
    len NUMBER,
    wth NUMBER,
    CONSTRUCTOR FUNCTION rectangle(SELF IN OUT NOCOPY rectangle,
        name VARCHAR2, len NUMBER, wth NUMBER) RETURN SELF as RESULT,
    CONSTRUCTOR FUNCTION rectangle(SELF IN OUT NOCOPY rectangle,
        name VARCHAR2, side NUMBER) RETURN SELF as RESULT);
/
SHOW ERRORS
CREATE TYPE BODY rectangle IS 
    CONSTRUCTOR FUNCTION rectangle(SELF IN OUT NOCOPY rectangle,
        name VARCHAR2, len NUMBER, wth NUMBER) RETURN  SELF AS RESULT IS
    BEGIN 
        SELF.name := name;
        SELF.area := len*wth;
        SELF.len := len;
        SELF.wth := wth;
        RETURN ;
    END;
    CONSTRUCTOR FUNCTION rectangle(SELF IN OUT NOCOPY rectangle,
        name VARCHAR2, side NUMBER) RETURN  SELF AS RESULT IS
    BEGIN 
        SELF.name := name;
        SELF.area := side * side;
        SELF.len := side;
        SELF.wth := side;
        RETURN ;
    END;
END;
/

CREATE TABLE shape_table OF shape;
INSERT INTO shape_table VALUES(shape('shape1')); 
INSERT INTO shape_table VALUES(shape('shape2', 20)); 
INSERT INTO shape_table VALUES(rectangle('rectangle', 2, 5)); 
INSERT INTO shape_table VALUES(rectangle('quadrangle', 12, 3));
INSERT INTO shape_table VALUES(rectangle('square', 12));

SELECT VALUE(s) FROM shape_table s;

Constructors for SQLJ Object Types

A SQLJ object type is a SQL object type mapped to a Java class. A SQLJ object type has an attribute-value constructor. It can also have user-defined constructors that are mapped to constructors in the referenced Java class.

CREATE TYPE address AS OBJECT 
   EXTERNAL NAME 'university.address' LANGUAGE JAVA
   USING SQLData(
     street   VARCHAR2(100) EXTERNAL NAME 'street',
     city     VARCHAR2(50)  EXTERNAL NAME 'city',
     state    VARCHAR2(50)  EXTERNAL NAME 'state',
     zipcode  NUMBER        EXTERNAL NAME 'zipcode',
    CONSTRUCTOR FUNCTION address (SELF IN OUT NOCOPY address, street VARCHAR2,
                                  city VARCHAR2, state VARCHAR2, zipcode NUMBER)
      RETURN SELF AS RESULT AS LANGUAGE JAVA
      NAME  'university.address (java.lang.String, java.lang.String,
                      java.lang.String, int) return address');
/