PL/SQL User’s Guide and Reference phần 6 docx

What Is a Package?

8-2 PL/SQL User’s Guide and Reference

What Is a Package?

A package is a schema object that groups logically related PL/SQL types, items, and

subprograms. Packages usually have two parts, a specification and a body, although

sometimes the body is unnecessary. The specification (spec for short) is the interface

to your applications; it declares the types, variables, constants, exceptions, cursors,

and subprograms available for use. The body fully defines cursors and subprograms,

and so implements the spec.

As Figure 8–1 shows, you can think of the spec as an operational interface and of

the body as a "black box." You can debug, enhance, or replace a package body

without changing the interface (package spec) to the package.

Figure 8–1 Package Interface

To create packages, use the CREATE PACKAGE statement, which you can execute

interactively from SQL*Plus. Here is the syntax:

CREATE [OR REPLACE] PACKAGE package_name

[AUTHID {CURRENT_USER | DEFINER}]

{IS | AS}

[PRAGMA SERIALLY_REUSABLE;]

[collection_type_definition ...]

[record_type_definition ...]

[subtype_definition ...]

[collection_declaration ...]

[constant_declaration ...]

[exception_declaration ...]

[object_declaration ...]

[record_declaration ...]

[variable_declaration ...]

Application Package Database

specification

body

What Is a Package?

Packages 8-3

[cursor_spec ...]

[function_spec ...]

[procedure_spec ...]

[call_spec ...]

[PRAGMA RESTRICT_REFERENCES(assertions) ...]

END [package_name];

[CREATE [OR REPLACE] PACKAGE BODY package_name {IS | AS}

[PRAGMA SERIALLY_REUSABLE;]

[collection_type_definition ...]

[record_type_definition ...]

[subtype_definition ...]

[collection_declaration ...]

[constant_declaration ...]

[exception_declaration ...]

[object_declaration ...]

[record_declaration ...]

[variable_declaration ...]

[cursor_body ...]

[function_spec ...]

[procedure_spec ...]

[call_spec ...]

[BEGIN

sequence_of_statements]

END [package_name];]

The spec holds public declarations, which are visible to your application. The body

holds implementation details and private declarations, which are hidden from your

application. Following the declarative part of the package body is the optional

initialization part, which typically holds statements that initialize package variables.

The AUTHID clause determines whether all the packaged subprograms execute 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 Rights versus Definer Rights" on page 7-29.

A call spec lets you publish a Java method or external C function in the Oracle data

dictionary. The call spec publishes the routine by mapping its name, parameter

types, and return type to their SQL counterparts. To learn how to write Java call

specs, see Oracle8i Java Stored Procedures Developer’s Guide. To learn how to write C

call specs, see Oracle8i Application Developer’s Guide - Fundamentals.

What Is a Package?

8-4 PL/SQL User’s Guide and Reference

In the example below, you package a record type, a cursor, and two employment

procedures. Notice that the procedure hire_employee uses the database sequence

empno_seq and the function SYSDATE to insert a new employee number and hire

date, respectively.

CREATE OR REPLACE PACKAGE emp_actions AS -- spec

TYPE EmpRecTyp IS RECORD (emp_id INT, salary REAL);

CURSOR desc_salary RETURN EmpRecTyp;

PROCEDURE hire_employee (

ename VARCHAR2,

job VARCHAR2,

mgr NUMBER,

sal NUMBER,

comm NUMBER,

deptno NUMBER);

PROCEDURE fire_employee (emp_id NUMBER);

END emp_actions;

CREATE OR REPLACE PACKAGE BODY emp_actions AS -- body

CURSOR desc_salary RETURN EmpRecTyp IS

SELECT empno, sal FROM emp ORDER BY sal DESC;

PROCEDURE hire_employee (

ename VARCHAR2,

job VARCHAR2,

mgr NUMBER,

sal NUMBER,

comm NUMBER,

deptno NUMBER) IS

BEGIN

INSERT INTO emp VALUES (empno_seq.NEXTVAL, ename, job,

mgr, SYSDATE, sal, comm, deptno);

END hire_employee;

PROCEDURE fire_employee (emp_id NUMBER) IS

BEGIN

DELETE FROM emp WHERE empno = emp_id;

END fire_employee;

END emp_actions;

Only the declarations in the package spec are visible and accessible to applications.

Implementation details in the package body are hidden and inaccessible. So, you

can change the body (implementation) without having to recompile calling

programs.

Advantages of Packages

Packages 8-5

Advantages of Packages

Packages offer several advantages: modularity, easier application design,

information hiding, added functionality, and better performance.

Modularity

Packages let you encapsulate logically related types, items, and subprograms in a

named PL/SQL module. Each package is easy to understand, and the interfaces

between packages are simple, clear, and well defined. This aids application

development.

Easier Application Design

When designing an application, all you need initially is the interface information in

the package specs. You can code and compile a spec without its body. Then, stored

subprograms that reference the package can be compiled as well. You need not

define the package bodies fully until you are ready to complete the application.

Information Hiding

With packages, you can specify which types, items, and subprograms are public

(visible and accessible) or private (hidden and inaccessible). For example, if a

package contains four subprograms, three might be public and one private. The

package hides the implementation of the private subprogram so that only the

package (not your application) is affected if the implementation changes. This

simplifies maintenance and enhancement. Also, by hiding implementation details

from users, you protect the integrity of the package.

Added Functionality

Packaged public variables and cursors persist for the duration of a session. So, they

can be shared by all subprograms that execute in the environment. Also, they allow

you to maintain data across transactions without having to store it in the database.

Better Performance

When you call a packaged subprogram for the first time, the whole package is

loaded into memory. So, later calls to related subprograms in the package require no

disk I/O. Also, packages stop cascading dependencies and thereby avoid

unnecessary recompiling. For example, if you change the implementation of a

packaged function, Oracle need not recompile the calling subprograms because

they do not depend on the package body.

The Package Spec

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The Package Spec

The package spec contains public declarations. The scope of these declarations is

local to your database schema and global to the package. So, the declared items are

accessible from your application and from anywhere in the package. Figure 8–2

illustrates the scoping.

Figure 8–2 Package Scope

The spec lists the package resources available to applications. All the information

your application needs to use the resources is in the spec. For example, the

following declaration shows that the function named fac takes one argument of

type INTEGER and returns a value of type INTEGER:

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

That is all the information you need to call the function. You need not consider its

underlying implementation (whether it is iterative or recursive for example).

Only subprograms and cursors have an underlying implementation. So, if a spec

declares only types, constants, variables, exceptions, and call specs, the package

body is unnecessary. Consider the following bodiless package:

CREATE PACKAGE trans_data AS -- bodiless package

TYPE TimeRec IS RECORD (

minutes SMALLINT,

hours SMALLINT);

TYPE TransRec IS RECORD (

category VARCHAR2,

account INT,

amount REAL,

time_of TimeRec);

schema

package spec

package spec

other objects

package body

package body

procedure

function

procedure

function

function

procedure

The Package Spec

Packages 8-7

minimum_balance CONSTANT REAL := 10.00;

number_processed INT;

insufficient_funds EXCEPTION;

END trans_data;

The package trans_data needs no body because types, constants, variables, and

exceptions do not have an underlying implementation. Such packages let you

define global variables—usable by subprograms and database triggers—that persist

throughout a session.

Referencing Package Contents

To reference the types, items, subprograms, and call specs declared within a

package spec, use dot notation, as follows:

package_name.type_name

package_name.item_name

package_name.subprogram_name

package_name.call_spec_name

You can reference package contents from database triggers, stored subprograms,

3GL application programs, and various Oracle tools. For example, you might call

the packaged procedure hire_employee from SQL*Plus, as follows:

SQL> CALL emp_actions.hire_employee(’TATE’, ’CLERK’, ...);

In the example below, you call the same procedure from an anonymous PL/SQL

block embedded in a Pro*C program. The actual parameters emp_name and job_

title are host variables (that is, variables declared in a host environment).

EXEC SQL EXECUTE

BEGIN

emp_actions.hire_employee(:emp_name, :job_title, ...);

Restrictions

You cannot reference remote packaged variables directly or indirectly. For example,

you cannot call the following procedure remotely because it references a packaged

variable in a parameter initialization clause:

CREATE PACKAGE random AS

seed NUMBER;

PROCEDURE initialize (starter IN NUMBER := seed, ...);

Also, inside a package, you cannot reference host variables.