Understanding The Package Specification

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. They can be shared by all subprograms that execute in the environment. They let you maintain data across transactions without storing it in the database.

Better Performance

When you call a packaged subprogram for the first time, the whole package is loaded into memory. Later calls to related subprograms in the package require no disk I/O.

Packages stop cascading dependencies and avoid unnecessary recompiling. For example, if you change the body of a packaged function, Oracle does not recompile other subprograms that call the function; these subprograms only depend on the parameters and return value that are declared in the spec, so they are only recompiled if the spec changes.

Understanding The Package Specification

The package specification contains public declarations. The declared items are accessible from anywhere in the package and to any other subprograms in the same schema. Figure 9–1 illustrates the scoping.

9-4 PL/SQL User's Guide and Reference

Understanding The Package Specification

Figure 9–1 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).

If a spec declares only types, constants, variables, exceptions, and call specs, the package body is unnecessary. Only subprograms and cursors have an underlying implementation. In the following example, the package needs no body because it declares types, exceptions, and variables, but no subprograms or cursors. Such packages let you define global variables—usable by stored procedures and functions and triggers—that persist throughout a session.

CREATE PACKAGE trans_data AS -- bodiless package

TYPE TimeRec IS RECORD (

Referencing Package Contents

To reference the types, items, subprograms, and call specs declared within a package spec, use dot notation:

package_name.type_name package_name.item_name package_name.subprogram_name package_name.call_spec_name

Using PL/SQL Packages 9-5