10 PL/SQL Packages

What is a Package?

A package is a schema object that groups logically related PL/SQL types, variables, constants, subprograms, cursors, and exceptions. A package is compiled and stored in the database, where many applications can share its contents.

A package always has a specification, which declares the public items that can be referenced from outside the package. For more information about the package specification, see "Package Specification".

If the public items include cursors or subprograms, then the package must also have a body. The body must define queries for public cursors and code for public subprograms. The body can also declare and define private items that cannot be referenced from outside the package, but are necessary for the internal workings of the package. Finally, the body can have an initialization part, whose statements initialize variables and do other one-time setup steps, and an exception-handling part. You can change the body without changing the specification or the references to the public items; therefore, you can think of the package body as a black box. For more information about the package body, see "Package Body".

In either the package specification or package body, you can map a package subprogram to an external Java or C subprogram by using a call specification, which maps the external subprogram name, parameter types, and return type to their SQL counterparts. For details, see "Function Declaration and Definition" and "Procedure Declaration and Definition".

The AUTHID clause of the package specification determines whether the subprograms and cursors in the package run with the privileges of their definer (the default) or invoker, and whether their unqualified references to schema objects are resolved in the schema of the definer or invoker. For more information, see "Invoker's Rights and Definer's Rights (AUTHID Property)".

The ACCESSIBLE BY clause of the package specification lets you specify a white list" of PL/SQL units that can access the package. You use this clause in situations like these:

• You implement a PL/SQL application as several packages—one package that provides the application programming interface (API) and helper packages to do the work. You want clients to have access to the API, but not to the helper packages. Therefore, you omit the ACCESSIBLE BY clause from the API package specification and include it in each helper package specification, where you specify that only the API package can access the helper package.

• You create a utility package to provide services to some, but not all, PL/SQL units in the same schema. To restrict use of the package to the intended units, you list them in the ACCESSIBLE BY clause in the package specification.

Reasons to Use Packages

Packages support the development and maintenance of reliable, reusable code with the following features:

• Modularity

  Packages let you encapsulate logically related types, variables, constants, subprograms, cursors, and exceptions in named PL/SQL modules. You can make each package easy to understand, and make the interfaces between packages simple, clear, and well defined. This practice aids application development.

• Easier Application Design

  When designing an application, all you need initially is the interface information in the package specifications. You can code and compile specifications without their bodies. Next, you can compile standalone subprograms that reference the packages. You need not fully define the package bodies until you are ready to complete the application.

• Hidden Implementation Details

  Packages let you share your interface information in the package specification, and hide the implementation details in the package body. Hiding the implementation details in the body has these advantages:

  • You can change the implementation details without affecting the application interface.

  • Application users cannot develop code that depends on implementation details that you might want to change.

• Added Functionality

  Package public variables and cursors can persist for the life of a session. They can be shared by all subprograms that run in the environment. They let you maintain data across transactions without storing it in the database. (For the situations in which package public variables and cursors do not persist for the life of a session, see "Package State".)

• Better Performance

  The first time you invoke a package subprogram, Oracle Database loads the whole package into memory. Subsequent invocations of other subprograms in same the package require no disk I/O.

  Packages prevent cascading dependencies and unnecessary recompiling. For example, if you change the body of a package function, Oracle Database does not recompile other subprograms that invoke the function, because these subprograms depend only on the parameters and return value that are declared in the specification.

• Easier to Grant Roles

  You can grant roles on the package, instead of granting roles on each object in the package.

Package Specification

A package specification declares public items. The scope of a public item is the schema of the package. A public item is visible everywhere in the schema. To reference a public item that is in scope but not visible, qualify it with the package name. (For information about scope, visibility, and qualification, see "Scope and Visibility of Identifiers".)

Each public item declaration has all information needed to use the item. For example, suppose that a package specification declares the function factorial this way:

FUNCTION factorial (n INTEGER) RETURN INTEGER; -- returns n!

The declaration shows that factorial needs one argument of type INTEGER and returns a value of type INTEGER, which is invokers must know to invoke factorial. Invokers need not know how factorial is implemented (for example, whether it is iterative or recursive).

Topics

• Appropriate Public Items

• Creating Package Specifications

CREATE OR REPLACE PACKAGE trans_data AUTHID DEFINER AS
  TYPE TimeRec IS RECORD (
    minutes SMALLINT,
    hours   SMALLINT);
  TYPE TransRec IS RECORD (
    category VARCHAR2(10),
    account  INT,
    amount   REAL,
    time_of  TimeRec);
  minimum_balance     CONSTANT REAL := 10.00;
  number_processed    INT;
  insufficient_funds  EXCEPTION;
  PRAGMA EXCEPTION_INIT(insufficient_funds, -4097);
END trans_data;
/

CREATE OR REPLACE PACKAGE aa_pkg AUTHID DEFINER IS
  TYPE aa_type IS TABLE OF INTEGER INDEX BY VARCHAR2(15);
END;
/
CREATE OR REPLACE PROCEDURE print_aa (
  aa aa_pkg.aa_type
) AUTHID DEFINER IS
  i  VARCHAR2(15);
BEGIN
  i := aa.FIRST;
 
  WHILE i IS NOT NULL LOOP
    DBMS_OUTPUT.PUT_LINE (aa(i) || '  ' || i);
    i := aa.NEXT(i);
  END LOOP;
END;
/
DECLARE
  aa_var  aa_pkg.aa_type;
BEGIN
  aa_var('zero') := 0;
  aa_var('one') := 1;
  aa_var('two') := 2;
  print_aa(aa_var);
END;
/

1  one
2  two
0  zero

Package Body

If a package specification declares cursors or subprograms, then a package body is required; otherwise, it is optional. The package body and package specification must be in the same schema.

Every cursor or subprogram declaration in the package specification must have a corresponding definition in the package body. The headings of corresponding subprogram declarations and definitions must match word for word, except for white space.

To create a package body, use the "CREATE PACKAGE BODY Statement".

In Example 10-3, the headings of the corresponding subprogram declaration and definition do not match word for word; therefore, PL/SQL raises an exception, even though employees.hire_date%TYPE is DATE.

CREATE PACKAGE emp_bonus AS
  PROCEDURE calc_bonus (date_hired employees.hire_date%TYPE);
END emp_bonus;
/
CREATE PACKAGE BODY emp_bonus AS
  -- DATE does not match employees.hire_date%TYPE
  PROCEDURE calc_bonus (date_hired DATE) IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE
      ('Employees hired on ' || date_hired || ' get bonus.');
  END;
END emp_bonus;
/

Warning: Package Body created with compilation errors.

SHOW ERRORS

Errors for PACKAGE BODY EMP_BONUS:
 
LINE/COL ERROR
-------- -----------------------------------------------------------------
2/13     PLS-00323: subprogram or cursor 'CALC_BONUS' is declared in a
         package specification and must be defined in the package body

CREATE OR REPLACE PACKAGE BODY emp_bonus AS
  PROCEDURE calc_bonus
    (date_hired employees.hire_date%TYPE) IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE
      ('Employees hired on ' || date_hired || ' get bonus.');
  END;
END emp_bonus;
/

Package body created.

The cursors and subprograms declared in the package specification and defined in the package body are public items that can be referenced from outside the package. The package body can also declare and define private items that cannot be referenced from outside the package, but are necessary for the internal workings of the package.

Finally, the body can have an initialization part, whose statements initialize public variables and do other one-time setup steps. The initialization part runs only the first time the package is referenced. The initialization part can include an exception handler.

You can change the package body without changing the specification or the references to the public items.

Package Instantiation and Initialization

When a session references a package item, Oracle Database instantiates the package for that session. Every session that references a package has its own instantiation of that package.

When Oracle Database instantiates a package, it initializes it. Initialization includes whichever of the following are applicable:

• Assigning initial values to public constants

• Assigning initial values to public variables whose declarations specify them

• Executing the initialization part of the package body

Package State

The values of the variables, constants, and cursors that a package declares (in either its specification or body) comprise its package state. If a PL/SQL package declares at least one variable, constant, or cursor, then the package is stateful; otherwise, it is stateless.

Each session that references a package item has its own instantiation of that package. If the package is stateful, the instantiation includes its state. The package state persists for the life of a session, except in these situations:

• The package is SERIALLY_REUSABLE.

  For details, see "SERIALLY_REUSABLE Packages".

• The package body is recompiled.

  If the body of an instantiated, stateful package is recompiled (either explicitly, with the "ALTER PACKAGE Statement", or implicitly), the next invocation of a subprogram in the package causes Oracle Database to discard the existing package state and raise the exception ORA-04068.

  After PL/SQL raises the exception, a reference to the package causes Oracle Database to re-instantiate the package, which re-initializes it. Therefore, previous changes to the package state are lost. (For information about initialization, see "Package Instantiation and Initialization".)

• Any of the session's instantiated packages are invalidated and revalidated.

  All of a session's package instantiations (including package states) can be lost if any of the session's instantiated packages are invalidated and revalidated. For information about invalidation and revalidation of schema objects, see Oracle Database Development Guide.

As of Oracle Database 11g Release 2 (11.2.0.2), Oracle Database treats a package as stateless if its state is constant for the life of a session (or longer). This is the case for a package whose items are all compile-time constants.

A compile-time constant is a constant whose value the PL/SQL compiler can determine at compilation time. A constant whose initial value is a literal is always a compile-time constant. A constant whose initial value is not a literal, but which the optimizer reduces to a literal, is also a compile-time constant. Whether the PL/SQL optimizer can reduce a nonliteral expression to a literal depends on optimization level. Therefore, a package that is stateless when compiled at one optimization level might be stateful when compiled at a different optimization level. For information about the optimizer, see "PL/SQL Optimizer".

SERIALLY_REUSABLE Packages

SERIALLY_REUSABLE packages let you design applications that manage memory better for scalability.

If a package is not SERIALLY_REUSABLE, its package state is stored in the user global area (UGA) for each user. Therefore, the amount of UGA memory needed increases linearly with the number of users, limiting scalability. The package state can persist for the life of a session, locking UGA memory until the session ends. In some applications, such as Oracle Office, a typical session lasts several days.

If a package is SERIALLY_REUSABLE, its package state is stored in a work area in a small pool in the system global area (SGA). The package state persists only for the life of a server call. After the server call, the work area returns to the pool. If a subsequent server call references the package, then Oracle Database reuses an instantiation from the pool. Reusing an instantiation re-initializes it; therefore, changes made to the package state in previous server calls are invisible. (For information about initialization, see "Package Instantiation and Initialization".)

Topics

• Creating SERIALLY_REUSABLE Packages

• SERIALLY_REUSABLE Package Work Unit

• Explicit Cursors in SERIALLY_REUSABLE Packages

-- Create bodiless SERIALLY_REUSABLE package:
 
CREATE OR REPLACE PACKAGE bodiless_pkg AUTHID DEFINER IS
  PRAGMA SERIALLY_REUSABLE;
  n NUMBER := 5;
END;
/
 
-- Create SERIALLY_REUSABLE package with specification and body:
 
CREATE OR REPLACE PACKAGE pkg AUTHID DEFINER IS
  PRAGMA SERIALLY_REUSABLE;
  n NUMBER := 5;
END;
/
 
CREATE OR REPLACE PACKAGE BODY pkg IS
  PRAGMA SERIALLY_REUSABLE;
BEGIN
  n := 5;
END;
/

CREATE OR REPLACE PACKAGE pkg IS
  n NUMBER := 5;
END pkg;
/

CREATE OR REPLACE PACKAGE sr_pkg IS
  PRAGMA SERIALLY_REUSABLE;
  n NUMBER := 5;
END sr_pkg;
/

BEGIN
  pkg.n := 10;
  sr_pkg.n := 10;
END;
/

BEGIN
  DBMS_OUTPUT.PUT_LINE('pkg.n: ' || pkg.n);
  DBMS_OUTPUT.PUT_LINE('sr_pkg.n: ' || sr_pkg.n);
END;
/

pkg.n: 10
sr_pkg.n: 5

DROP TABLE people;
CREATE TABLE people (name VARCHAR2(20));
 
INSERT INTO people (name) VALUES ('John Smith');
INSERT INTO people (name) VALUES ('Mary Jones');
INSERT INTO people (name) VALUES ('Joe Brown');
INSERT INTO people (name) VALUES ('Jane White');

CREATE OR REPLACE PACKAGE sr_pkg IS
  PRAGMA SERIALLY_REUSABLE;
  CURSOR c IS SELECT name FROM people;
END sr_pkg;
/
 
CREATE OR REPLACE PROCEDURE fetch_from_cursor IS
  name_  VARCHAR2(200);
BEGIN
  IF sr_pkg.c%ISOPEN THEN
    DBMS_OUTPUT.PUT_LINE('Cursor is open.');
  ELSE
    DBMS_OUTPUT.PUT_LINE('Cursor is closed; opening now.');
    OPEN sr_pkg.c;
  END IF;
 
  FETCH sr_pkg.c INTO name_;
  DBMS_OUTPUT.PUT_LINE('Fetched: ' || name_);
 
  FETCH sr_pkg.c INTO name;
    DBMS_OUTPUT.PUT_LINE('Fetched: ' || name_);
  END fetch_from_cursor;
/
 

BEGIN
  fetch_from_cursor;
  fetch_from_cursor;
END;
/

Cursor is closed; opening now.
Fetched: John Smith
Fetched: Mary Jones
Cursor is open.
Fetched: Joe Brown
Fetched: Jane White
 

BEGIN
  fetch_from_cursor;
  fetch_from_cursor;
END;
/

Cursor is closed; opening now.
Fetched: John Smith
Fetched: Mary Jones
Cursor is open.
Fetched: Joe Brown
Fetched: Jane White

Package Writing Guidelines

• Become familiar with the packages that Oracle Database supplies, and avoid writing packages that duplicate their features.

  For more information about the packages that Oracle Database supplies, see Oracle Database PL/SQL Packages and Types Reference.

• Keep your packages general so that future applications can reuse them.

• Design and define the package specifications before the package bodies.

• In package specifications, declare only items that must be visible to invoking programs.

  This practice prevents other developers from building unsafe dependencies on your implementation details and reduces the need for recompilation.

  If you change the package specification, you must recompile any subprograms that invoke the public subprograms of the package. If you change only the package body, you need not recompile those subprograms.

• Declare public cursors in package specifications and define them in package bodies, as in Example 10-7.

  This practice lets you hide cursors' queries from package users and change them without changing cursor declarations.

• Assign initial values in the initialization part of the package body instead of in declarations.

  This practice has these advantages:

  • The code for computing the initial values can be more complex and better documented.

  • If computing an initial value raises an exception, the initialization part can handle it with its own exception handler.

• If you implement a database application as several PL/SQL packages—one package that provides the API and helper packages to do the work, then make the helper packages available only to the API package, as in Example 10-8.

In Example 10-7, the declaration and definition of the cursor c1 are in the specification and body, respectively, of the package emp_stuff. The cursor declaration specifies only the data type of the return value, not the query, which appears in the cursor definition (for complete syntax and semantics, see "Explicit Cursor Declaration and Definition").

CREATE PACKAGE emp_stuff AS
  CURSOR c1 RETURN employees%ROWTYPE;  -- Declare cursor
END emp_stuff;
/
CREATE PACKAGE BODY emp_stuff AS
  CURSOR c1 RETURN employees%ROWTYPE IS
    SELECT * FROM employees WHERE salary > 2500;  -- Define cursor
END emp_stuff;
/

Example 10-8 creates an API package and a helper package. Because of the ACCESSIBLE BY clause in the helper package specification, only the API package can access the helper package.

CREATE OR REPLACE PACKAGE helper
  AUTHID DEFINER
  ACCESSIBLE BY (api)
IS
  PROCEDURE h1;
  PROCEDURE h2;
END;
/
 
CREATE OR REPLACE PACKAGE BODY helper
IS
  PROCEDURE h1 IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE('Helper procedure h1');
  END;
 
  PROCEDURE h2 IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE('Helper procedure h2');
  END;
END;
/
 
CREATE OR REPLACE PACKAGE api
  AUTHID DEFINER
IS
  PROCEDURE p1;
  PROCEDURE p2;
END;
/
 
CREATE OR REPLACE PACKAGE BODY api
IS
  PROCEDURE p1 IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE('API procedure p1');
    helper.h1;
  END;
 
  PROCEDURE p2 IS
  BEGIN
    DBMS_OUTPUT.PUT_LINE('API procedure p2');
    helper.h2;
  END;
END;
/
 

BEGIN
  api.p1;
  api.p2;
END;
/
 

API procedure p1
Helper procedure h1
API procedure p2
Helper procedure h2

BEGIN
  helper.h1;
END;
/
 

SQL> BEGIN
  2    helper.h1;
  3  END;
  4  /
  helper.h1;
  *
ERROR at line 2:
ORA-06550: line 2, column 3:
PLS-00904: insufficient privilege to access object HELPER
ORA-06550: line 2, column 3:
PL/SQL: Statement ignored

Package Example

Example 10-9 creates a table, log, and a package, emp_admin, and then invokes package subprograms from an anonymous block. The package has both specification and body.

The specification declares a public type, cursor, and exception, and three public subprograms. One public subprogram is overloaded (for information about overloaded subprograms, see "Overloaded Subprograms").

The body declares a private variable, defines the public cursor and subprograms that the specification declares, declares and defines a private function, and has an initialization part.

The initialization part (which runs only the first time the anonymous block references the package) inserts one row into the table log and initializes the private variable number_hired to zero. Every time the package procedure hire_employee is invoked, it updates the private variable number_hired.

-- Log to track changes (not part of package):

DROP TABLE log;
CREATE TABLE log (
  date_of_action  DATE,
  user_id         VARCHAR2(20),
  package_name    VARCHAR2(30)
);

-- Package specification:

CREATE OR REPLACE PACKAGE emp_admin AUTHID DEFINER AS
  -- Declare public type, cursor, and exception:
  TYPE EmpRecTyp IS RECORD (emp_id NUMBER, sal NUMBER);
  CURSOR desc_salary RETURN EmpRecTyp;
  invalid_salary EXCEPTION;

  -- Declare public subprograms:

  FUNCTION hire_employee (
    last_name       VARCHAR2,
    first_name      VARCHAR2,
    email           VARCHAR2,
    phone_number    VARCHAR2,
    job_id          VARCHAR2,
    salary          NUMBER,
    commission_pct  NUMBER,
    manager_id      NUMBER,
    department_id   NUMBER
  ) RETURN NUMBER;

  -- Overload preceding public subprogram:
  PROCEDURE fire_employee (emp_id NUMBER);
  PROCEDURE fire_employee (emp_email VARCHAR2);

  PROCEDURE raise_salary (emp_id NUMBER, amount NUMBER);
  FUNCTION nth_highest_salary (n NUMBER) RETURN EmpRecTyp;
END emp_admin;
/
-- Package body:

CREATE OR REPLACE PACKAGE BODY emp_admin AS
  number_hired  NUMBER;  -- private variable, visible only in this package

  -- Define cursor declared in package specification:

  CURSOR desc_salary RETURN EmpRecTyp IS
    SELECT employee_id, salary
    FROM employees
    ORDER BY salary DESC;

  -- Define subprograms declared in package specification:

  FUNCTION hire_employee (
    last_name       VARCHAR2,
    first_name      VARCHAR2,
    email           VARCHAR2,
    phone_number    VARCHAR2,
    job_id          VARCHAR2,
    salary          NUMBER,
    commission_pct  NUMBER,
    manager_id      NUMBER,
    department_id   NUMBER
  ) RETURN NUMBER
  IS
    new_emp_id NUMBER;
  BEGIN
    new_emp_id := employees_seq.NEXTVAL;
    INSERT INTO employees (
      employee_id,
      last_name,
      first_name,
      email,
      phone_number,
      hire_date,
      job_id,
      salary,
      commission_pct,
      manager_id,
      department_id
    )
    VALUES (
      new_emp_id,
      hire_employee.last_name,
      hire_employee.first_name,
      hire_employee.email,
      hire_employee.phone_number,
      SYSDATE,
      hire_employee.job_id,
      hire_employee.salary,
      hire_employee.commission_pct,
      hire_employee.manager_id,
      hire_employee.department_id
    );
    number_hired := number_hired + 1;
    DBMS_OUTPUT.PUT_LINE('The number of employees hired is ' 
                         || TO_CHAR(number_hired) );   
    RETURN new_emp_id;
  END hire_employee;

  PROCEDURE fire_employee (emp_id NUMBER) IS
  BEGIN
    DELETE FROM employees WHERE employee_id = emp_id;
  END fire_employee;

  PROCEDURE fire_employee (emp_email VARCHAR2) IS
  BEGIN
    DELETE FROM employees WHERE email = emp_email;
  END fire_employee;

  -- Define private function, available only inside package:

  FUNCTION sal_ok (
    jobid VARCHAR2,
    sal NUMBER
  ) RETURN BOOLEAN
  IS
    min_sal NUMBER;
    max_sal NUMBER;
  BEGIN
    SELECT MIN(salary), MAX(salary)
    INTO min_sal, max_sal
    FROM employees
    WHERE job_id = jobid;

    RETURN (sal >= min_sal) AND (sal <= max_sal);
  END sal_ok;

  PROCEDURE raise_salary (
    emp_id NUMBER,
    amount NUMBER
  )
  IS
    sal NUMBER(8,2);
    jobid VARCHAR2(10);
  BEGIN
    SELECT job_id, salary INTO jobid, sal
    FROM employees
    WHERE employee_id = emp_id;

    IF sal_ok(jobid, sal + amount) THEN  -- Invoke private function
      UPDATE employees
      SET salary = salary + amount
      WHERE employee_id = emp_id;
    ELSE
      RAISE invalid_salary;
    END IF;
  EXCEPTION
    WHEN invalid_salary THEN
      DBMS_OUTPUT.PUT_LINE ('The salary is out of the specified range.');
  END raise_salary;

  FUNCTION nth_highest_salary (
    n NUMBER
  ) RETURN EmpRecTyp
  IS
    emp_rec  EmpRecTyp;
  BEGIN
    OPEN desc_salary;
    FOR i IN 1..n LOOP
      FETCH desc_salary INTO emp_rec;
    END LOOP;
    CLOSE desc_salary;
    RETURN emp_rec;
  END nth_highest_salary;

BEGIN  -- initialization part of package body
   INSERT INTO log (date_of_action, user_id, package_name)
   VALUES (SYSDATE, USER, 'EMP_ADMIN');
   number_hired := 0;
END emp_admin;
/
-- Invoke packages subprograms in anonymous block:

DECLARE
  new_emp_id NUMBER(6);
BEGIN
  new_emp_id := emp_admin.hire_employee (
    'Belden',
    'Enrique',
    'EBELDEN',
    '555.111.2222',
    'ST_CLERK',
    2500,
    .1,
    101,
    110
  );
  DBMS_OUTPUT.PUT_LINE ('The employee id is ' || TO_CHAR(new_emp_id));
  emp_admin.raise_salary (new_emp_id, 100);

  DBMS_OUTPUT.PUT_LINE (
    'The 10th highest salary is '||
    TO_CHAR (emp_admin.nth_highest_salary(10).sal) ||
             ', belonging to employee: ' ||
             TO_CHAR (emp_admin.nth_highest_salary(10).emp_id)
  );

  emp_admin.fire_employee(new_emp_id);
  -- You can also delete the newly added employee as follows:
  -- emp_admin.fire_employee('EBELDEN');
END;
/

The number of employees hired is 1
The employee id is 210
The 10th highest salary is 11500, belonging to employee: 168

How STANDARD Package Defines the PL/SQL Environment

A package named STANDARD defines the PL/SQL environment. The package specification declares public types, variables, exceptions, subprograms, which are available automatically to PL/SQL programs. For example, package STANDARD declares function ABS, which returns the absolute value of its argument, as follows:

FUNCTION ABS (n NUMBER) RETURN NUMBER;

The contents of package STANDARD are directly visible to applications. You need not qualify references to its contents by prefixing the package name. For example, you might invoke ABS from a database trigger, stored subprogram, Oracle tool, or 3GL application, as follows:

abs_diff := ABS(x - y);

If you declare your own version of ABS, your local declaration overrides the public declaration. You can still invoke the SQL function by specifying its full name:

abs_diff := STANDARD.ABS(x - y);

Most SQL functions are overloaded. For example, package STANDARD contains these declarations:

FUNCTION TO_CHAR (right DATE) RETURN VARCHAR2;
FUNCTION TO_CHAR (left NUMBER) RETURN VARCHAR2;
FUNCTION TO_CHAR (left DATE, right VARCHAR2) RETURN VARCHAR2;
FUNCTION TO_CHAR (left NUMBER, right VARCHAR2) RETURN VARCHAR2;

PL/SQL resolves an invocation of TO_CHAR by matching the number and data types of the formal and actual parameters.