Thread

Polymorphism in the PostgreSQL object model

Ganesh Prasad <ganesh.prasad@reply2.com.au> — 2001-07-10T05:45:51Z

Let's try to use polymorphism in PostgreSQL. This could be quite a useful
feature in many situations.

We create two tables, one inheriting from the other, then define functions
with the same name
on both of them, but with different internal logic. When we call the
function on all records of 
the parent class, we want the subclass's variant to be called for the
records corresponding to 
the subclass.

However, the syntax of function definitions for a table expects the table
name to be passed to the
function as an argument. Therefore, it does not seem possible to
transparently call different
functions depending on the class of a record.

Is there a way to do this? Is the syntax described in this example wrong?

Create a class "employee" with key "id" and attributes "name" and "salary".
Define a function 
"getTax()" that calculates tax as 20% of salary. Insert a record into it.

Now create a subclass of "employee" called "manager" with one extra
attribute "dept", and insert 
a record into it. Define a function "getTax()" for "manager" that calculates
tax as 25% of salary.
(Let managers pay more tax ;-)

If we call "getTax()" on all records of "employee", we want regular
employees to be shown taxed at 20%,
but managers taxed at 25%. That is polymorphic behaviour.

However, we are only able to invoke the employee version of getTax() on the
employee table. Application 
of the manager version is only possible (explicitly) on the manager table.

STEPS:

1. Run the script "setup.sql". This creates the two tables, populates them
with a record each, and 
creates the "getTax()" functions.

2. Run the script "test1.sql". This invokes the "getTax()" method on the
"employee" table for 
all records. Only the employee version is called.

     gettax 
    --------
      20000
      20000
    (2 rows)

This is not what we want. We want "test1.sql" to return the following result
(i.e. applying
the manager variant of getTax() to the manager record):

     gettax 
    --------
      20000
      25000 <-- Manager variant of getTax() should be used here
    (2 rows)

3. Run the script "test2.sql". This explicitly invokes the "getTax()" method
on the "manager" table 
for all records corresponding to the child class (manager). This calls the
manager variant.

     gettax 
    --------
      25000
    (1 row)

This is correct, but we shouldn't have to call this variant explicitly.
Polymorphism should
cause it to be called implicitly.

------ Start of scripts --------

setup.sql:
----------

drop function getTax( t_employee );
drop function getTax( t_manager );
drop table t_manager;
drop table t_employee;

/*
Employees have an id (key), a name and a salary.
*/
create table t_employee
(
id	int4 primary key,
name	varchar(50) not null,
salary	float8
);

/*
Managers are employees who manage a department.
*/
create table t_manager
(
dept	char(2)
)
inherits (t_employee);

/*
An ordinary employee.
*/
insert into t_employee
values
(
1,
'Joe Bloggs',
100000.0
);

/*
A manager.
*/
insert into t_manager
values
(
2,
'John Doe',
100000.0,
'HR'
);

/*
A "method" defined for the "employee" class,
which should be inherited by the "manager" class.
*/
create function getTax( t_employee ) returns float8 as '
declare
    emp alias for $1;
begin
    return emp.salary * 0.2;
end;
' language 'plpgsql';

/*
A "method" defined for the "manager" class,
which should override that defined for the 
"employee" class.
*/
create function getTax( t_manager ) returns float8 as '
declare
    mgr alias for $1;
begin
    return mgr.salary * 0.25;
end;
' language 'plpgsql';

test1.sql:
----------

/*
Calculate tax for all employees.
*/
select getTax( t_employee ) from t_employee;

test2.sql:
----------

/*
The "method" for managers has to be explicitly called.
The "method" defined for managers should be implicitly called
even if the record is in the "employee" table.
*/
select getTax( t_manager ) from t_manager;

------ End of scripts --------


Ganesh Prasad
Chief Web Architect
Reply2(tm) Ltd
Tel  (+ 61 2)  9339 2912

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