Package Spec

Question

1. what is the purpose of creating bodyless package ? what are the advantages of creating like this ?
2. what is the reason for using "CREATE OR REPLACE PACKAGE" instead of dropping and creating ?

Answer 1

Balarenganathan wrote:
1. what is the purpose of creating bodyless package ? what are the advantages of creating like this ?Here's one example of a user for creating a bodyless package...
Packages tend to fail because of their "package state". A package has a "state" when it contains package level variables/constants etc. and the package is called. Upon first calling the package, the "state" is created in memory to hold the values of those variables etc. If an object that the package depends upon e.g. a table is altered in some way e.g. dropped and recreated, then because of the database dependencies, the package takes on an INVALID status. When you next make a call to the package, Oracle looks at the status and sees that it is invalid, then determines that the package has a "state". Because something has altered that the package depended upon, the state is taken as being out of date and is discarded, thus causing the "Package state has been discarded" error message.
If a package does not have package level variables etc. i.e. the "state" then, taking the same example above, the package takes on an INVALID status, but when you next make a call to the package, Oracle sees it as Invalid, but knows that there is no "state" attached to it, and so is able to recompile the package automatically and then carry on execution without causing any error messages. The only exception here is if the thing that the package was dependant on has changes in such a way that the package cannot compile, in which case you'll get an Invalid package type of error.
And if you want to know how to prevent discarded package states....
Move all constants and variables into a stand-alone package spec and reference those from your initial package. Thus when the status of your original package is invlidated for whatever reason, it has no package state and can be recompiled automatically, however the package containing the vars/const will not become invalidated as it has no dependencies, so the state that is in memory for that package will remain and can continue to be used.
As for having package level cursors, you'll need to make these local to the procedures/functions using them as you won't be able to reference cursors across packages like that (not sure about using REF CURSORS though.... there's one for me to investigate!)
This first example shows the package state being invalided by the addition of a new column on the table, and causing it to give a "Package state discarded" error...
SQL> set serveroutput on
SQL>
SQL> create table dependonme (x number)
  2  /
Table created.
SQL>
SQL> insert into dependonme values (5)
  2  /
1 row created.
SQL>
SQL> create or replace package mypkg is
  2    procedure myproc;
  3  end mypkg;
  4  /
Package created.
SQL>
SQL> create or replace package body mypkg is
  2    v_statevar number := 5; -- this means my package has a state
  3
  4    procedure myproc is
  5      myval number;
  6    begin
  7      select x
  8      into myval
  9      from dependonme;
10
11      myval := myval * v_statevar;
12      DBMS_OUTPUT.PUT_LINE('My Result is: '||myval);
13    end;
14  end mypkg;
15  /
Package body created.
SQL>
SQL> exec mypkg.myproc
My Result is: 25
PL/SQL procedure successfully completed.
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        VALID
SQL>
SQL>
SQL> alter table dependonme add (y number)
  2  /
Table altered.
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        INVALID
SQL>
SQL> exec mypkg.myproc
BEGIN mypkg.myproc; END;
ERROR at line 1:
ORA-04068: existing state of packages has been discarded
ORA-04061: existing state of package body "SCOTT.MYPKG" has been invalidated
ORA-06508: PL/SQL: could not find program unit being called: "SCOTT.MYPKG"
ORA-06512: at line 1
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        INVALID
SQL>
SQL> exec mypkg.myproc
PL/SQL procedure successfully completed.
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        VALIDAnd this next example shows how having the package variables in their own package spec, allows the package to automatically recompile when it is called even though it became invalidated by the action of adding a column to the table.
SQL> drop table dependonme
  2  /
Table dropped.
SQL>
SQL> drop package mypkg
  2  /
Package dropped.
SQL>
SQL> set serveroutput on
SQL>
SQL> create table dependonme (x number)
  2  /
Table created.
SQL>
SQL> insert into dependonme values (5)
  2  /
1 row created.
SQL>
SQL> create or replace package mypkg is
  2    procedure myproc;
  3  end mypkg;
  4  /
Package created.
SQL>
SQL> create or replace package mypkg_state is
  2    v_statevar number := 5; -- package state in seperate package spec
  3  end mypkg_state;
  4  /
Package created.
SQL>
SQL> create or replace package body mypkg is
  2    -- this package has no state area
  3
  4    procedure myproc is
  5      myval number;
  6    begin
  7      select x
  8      into myval
  9      from dependonme;
10
11      myval := myval * mypkg_state.v_statevar;  -- note: references the mypkg_state package
12      DBMS_OUTPUT.PUT_LINE('My Result is: '||myval);
13    end;
14  end mypkg;
15  /
Package body created.
SQL>
SQL> exec mypkg.myproc
My Result is: 25
PL/SQL procedure successfully completed.
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        VALID
SQL>
SQL> alter table dependonme add (y number)
  2  /
Table altered.
SQL>
SQL> select object_name, object_type, status from user_objects where object_name = 'MYPKG'
  2  /
OBJECT_NAME
OBJECT_TYPE         STATUS
MYPKG
PACKAGE             VALID
MYPKG
PACKAGE BODY        INVALID
SQL>
SQL> exec mypkg.myproc
My Result is: 25
PL/SQL procedure successfully completed.The other uses are of course that you just want to create a load of type definitions etc. to be used by multiple packages elsewhere. ;)
2. what is the reason for using "CREATE OR REPLACE PACKAGE" instead of dropping and creating ?What's the reason for dropping and recreating when you've got the option to create or replace? ;)