PACKAGES

PACKAGES

A package is a container for related objects. It has specification and body. Each of them is stored separately in data dictionary.

PACKAGE SYNTAX
Create or replace package <package_name> is
— package specification includes subprograms signatures, cursors and global or
public variables.
End <package_name>;

Create or replace package body <package_name> is
— package body includes body for all the subprograms declared in the spec, private
Variables and cursors.
Begin
— initialization section
Exception
— Exception handling seciton
End <package_name>;
IMPORTANT POINGS ABOUT PACKAGES

  1. The first time a packaged subprogram is called or any reference to a packaged variable or type is made, the package is instantiated.
  2. Each session will have its own copy of packaged variables, ensuring that two sessions executing subprograms in the same package use different memory locations.
  3. In many cases initialization needs to be run the first time the package is instantiated within a session. This can be done by adding initialization section to the package body after all the objects.
  4. Packages are stored in the data dictionary and can not be local.
  5. Packaged subprograms has an advantage over stand alone subprogram.
  6. When ever any reference to package, the whole package p-code was stored in shared pool of SGA.
  7. Package may have local subprograms.
  8. You can include authid clause inside the package spec not in the body.
  9. The execution section of a package is know as initialization section.
  10. You can have an exception section at the bottom of a package body.
  11. Packages subprograms are not invalidated.

COMPILING PACKAGES
SQL> Alter package PKG compile;
SQL> Alter package PKG compile specification;
SQL> Alter package PKG compile body;

PACKAGE DEPENDENCIES

  • The package body depends on the some objects and the package header.
  • The package header does not depend on the package body, which is an advantage of packages.
  • We can change the package body with out changing the header.

PACKAGE RUNTIME STATE
Package runtime state is differ for the following packages.

  • Serially reusable packages
  • Non serially reusable packages

SERIALLY REUSABLE PACKAGES
To force the oracle to use serially reusable version then include PRAGMA SERIALLY_REUSABLE in both package spec and body, Examine the following package.

 

SQL> exec pkg.emp_proc
Ename = SMITH
Ename = ALLEN
Ename = WARD
Ename = JONES
Ename = MARTIN
SQL> exec pkg.emp_proc
Ename = SMITH
Ename = ALLEN
Ename = WARD
Ename = JONES
Ename = MARTIN

  • The above package displays the same output for each execution even though the cursor is not closed.
  • Because the serially reusable version resets the state of the cursor each time it was called.

NON SERIALL Y REUSABLE PACKAGES
This is the default version used by the oracle, examine the following package.

 

 

SQL> exec pkg.emp_proc
Ename = SMITH
Ename = ALLEN
Ename = WARD
Ename = JONES
Ename = MARTIN
SQL> exec pkg.emp_proc
Ename = BLAKE
Ename = CLARK
Ename = SCOTT
Ename = KING
Ename = TURNER

  • The above package displays the different output for each execution even though the cursor is not closed.
  • Because the non-serially reusable version remains the state of the cursor over database calls.

DEPENDENCIES OF PACKAGE RUNTIME STATE
Dependencies can exists between package state and anonymous blocks.
Examine the following program
Create this package in first session

Connect to second session, run the following code.

BEGIN
pkg.p;
END;

The above code wil work.
Go back to first session and recreate the package using create.
Then connect to second session and run the following code again.

BEGIN
pkg.p;
END;

  1. This above code will not work because of the following.
  2. The anonymous block depends on pkg. This is compile time dependency.
  3. There is also a runtime dependency on the packaged variables, since each session has its own copy of packaged variables.
  4. Thus when pkg is recompiled the runtime dependency is followed, which invalidates the block and raises the oracle error.
  5. Runtime dependencies exist only on package state. This includes variables and cursors declared in a package.
  6. If the package had no global variables, the second execution of the anonymous block would have succeeded.

PURITY LEVELS
In general, calls to subprograms are procedural, they cannot be called from SQL statements. However, if a stand-alone or packaged function meets certain restrictions, it can be called during execution of a SQL statement.
User-defined functions are called the same way as built-in functions but it must meet different restrictions. These restrictions are defined in terms of purity levels.
There are four types of purity levels.

  • WNDS — Writes No Database State
  • RNDS — Reads No Database State
  • WNPS — Writes No Package State
  • RNPS — Reads No Package State

In addition to the preceding restrictions, a user-defined function must also meet the following requirements to be called from a SQL statement.
The function has to be stored in the database, either stand-alone or as part of a
package.

  • The function can take only in parametes.
  • The formal parameters must use only database types, not PL/SQL types such as boolean or record.
  • The return type of the function must also be a database type.
  • The function must not end the current transaction with commit or rollback, or rollback to a savepoint prior to the function execution.
  • It also must not issue any alter session or alter system commands.

RESTRICT_REFERENCES
For packaged functions, however, the RESTRICT_REFERENCES pragma is required to specify the purity level of a given function.
Syntax:
PRAGMA RESTRICT_REFERENCES(subprogram_name or package_name, WNDS [,WNPS]
[,RNDS] [,RNPS]);
Ex:

The above package body will not be created as it will give the following errors.
PLS-00452: Subprogram ‘FUN1’ violates its associated pragma
PLS-00452: Subprogram ‘FUN2’ violates its associated pragma 

Now the package body will be created.

DEFAULT
If there is no RESTRICT_REFERENCES pragma associated with a given packaged function, it will not have any purity level asserted. However, you can change the default purity level for a package. The DEFAULT keyword is used instead of the subprogram name in the pragma.

Ex:

The above package body will not created, it will give the following erros because the pragma will apply to all the functions.
PLS-00452: Subprogram ‘FUN1’ violates its associated pragma
PLS-00452: Subprogram ‘FUN2’ violates its associated pragma

Now the package body will be created.

TRUST
If the TRUST keyword is present, the restrictions listed in the pragma are not enforced. Rather, they are trusted to be true.
Ex:

The above package will be created successfully.

IMPORTANT POINTS ABOUT RESTRICT_REFERENCES

  1. This pragma can appear anywhere in the package specification, after the function declaration.
  2. It can apply to only one function definition.
  3. For overload functions, the pragma applies to the nearest definition prior to the Pragma.
  4. This pragma is required only for packages functions not for stand-alone functions.
  5. The Pragma can be declared only inside the package specification.
  6. The pragma is checked at compile time, not runtime.
  7. It is possible to specify without any purity levels when trust or combination of default and trust keywords are present.

PINNING IN THE SHARED POOL
The shared pool is the portion of the SGS that contains, among other things, the p-code of compiled subprograms as they are run. The first time a stored a store subprogram is called, the p-code is loaded from disk into the shared pool. Once the object is no longer referenced, it is free to be aged out. Objects are aged out of the shared pool using an LRU(Least Recently Used) algorithm.

The DBMS_SHARED_POOL package allows you to pin objects in the shared pool. When an object is pinned, it will never be aged out until you request it, no matter how full the pool gets or how often the object is accessed. This can improve performance, as it takes time to reload a package from disk.

DBMS_SHARED_POOL has four procedures

  • KEEP
  • UNKEEP
  • SIZES
  • ABORTED_REQUEST_THRESHOLD

KEEP
The DBMS_SHARED_POOL.KEEP procedure is used to pin objects in the pool.
Syntax:
PROCEDURE KEEP(object_name varchar2,flag char default ‘P’);
Here the flag represents different types of flag values for different types of objects.

P — Package, function or procedure
Q — Sequence
R — Trigger
C — SQL Cursor
T — Object type
JS — Java source
JC — Java class
JR — Java resource
JD — Java shared data

UNKEEP
UNKEEP is the only way to remove a kept object from the shared pool, without restarting the database. Kept objects are never aged out automatically.
Syntax:
PROCEDURE UNKEEP(object_name varchar2, flag char default ‘P’);

SIZES
SIZES will echo the contents of the shared pool to the screen.
Syntax:
PROCEDURE SIZES(minsize number);
Objects with greater than the minsize will be returned. SIZES uses DBMS_OUTPUT to return the data.

ABORTED_REQUEST_THRESHOLD
When the database determines that there is not enough memory in the shared pool to satisfy a given request, it will begin aging objects out until there is enough memory. It enough objects are aged out, this can have a performance impact on other database sessions. The ABORTED_REQUEST_THRESHOLD can be used to remedy this.
Syntax:
PROCEDURE ABORTED_REQUEST_THRESHOLD(threshold_size number);
Once this procedure is called, oracle will not start aging objects from the pool unless at least threshold_size bytes is needed.

DATA MODEL FOR SUBPROGRAMS AND PACKAGES

  • USER_OBJECTS
  • USER_SOURCE
  • USER_ERRORS
  • DBA_OBJECTS
  • DBA_SOURCE
  • DBA_ERRORS
  • ALL_OBJECTS
  • ALL_SOURCE
  • ALL_ERRORS

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