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Saturday 12 May 2012

PLSQL-IMPORTENT POINTS


IMPORTANT POINTS ABOUT SUBPROGRAMS

Ø  When a stored subprogram is created, it is stored in the data dictionary.
Ø  The subprogram is stored in compile form which is known as p-code in addition to the source text.
Ø  The p-code has all of the references in the subprogram evaluated, and the source code is translated into a form that is easily readable by PL/SQL engine.
Ø  When the subprogram is called, the p-code is read from the disk, if necessary, and executed.
Ø  Once it reads from the disk, the p-code is stored in the shared pool portion of the system global area (SGA), where it can be accessed by multiple users as needed.
Ø  Like all of the contents of the shared pool, p-code is aged out of the shared pool according to a least recently used (LRU) algorithm.
Ø  Subprograms can be local.
Ø  Local subprograms must be declared in the declarative section of PL/SQL block and called from the executable section.
Ø  Subprograms can not have the declarative section separately.
Ø  Stored subprograms can have local subprograms;
Ø  Local subprograms also can have local subprograms.
Ø  If the subprogram contains a variable with the same name as the column name of the table then use the dot method to differentiate (subprogram_name.sal).
Ø  Subprograms can be invalidated.

PROCEDURES V FUNCTIONS

Ø  Procedures may return through out and in out parameters where as function must return.
Ø  Procedures can not have return clause where as functions must.
Ø  We can use call statement directly for executing procedure where as we need to declare a variable in case of functions.
Ø  Functions can use in select statements where as procedures can not.
Ø  Functions can call from reports environment where as procedures can not.
Ø  We can use exec for executing procedures where as functions can not.
Ø  Function can be used in dbms_output where as procedure can not.
Ø  Procedure call is a standalone executable statement where as function call is a part of an executable statement.

STORED V LOCAL SUBPROGRAMS

Ø  The stored subprogram is stored in compiled p-code in the database, when the procedure is called it does not have to be compiled.
     The local subprogram is compiled as part of its containing block. If the containing
     block is anonymous and is run multiple times, the subprogram has to be compiled
     each time.

Ø  Stored subprograms can be called from any block submitted by a user who has execute privileges on the subprogram.
     Local subprograms can be called only from the block containing the subprogram.
Ø  By keeping the stored subprogram code separate from the calling block, the calling block is shorter and easier to understand.
     The local subprogram and the calling block are one and the same, which can lead to
     part confusion. If a change to the calling block is made, the subprogram will be
     recompiled as of the recompilation of the containing block.
Ø  The compiled p-code can be pinned in the shared pool using the DBMS_SHARED_POOL Package. This can improve performance.
     Local subprograms cannot be pinned in the shared pool by themselves.
Ø  Stand alone stored subprograms can not be overloaded, but packaged subprograms can be overloaded within the same package.
Ø  Local subprograms can be overloaded within the same block.

Ex1:
CREATE OR REPLACE PROCEDURE P IS
BEGIN
    dbms_output.put_line('Stored subprogram');
END;

Output:
SQL> exec p
Stored subprogram

Ex2:
DECLARE
      PROCEDURE P IS
     BEGIN
          dbms_output.put_line('Local subprogram');
      END;
BEGIN
     p;
END;

Output:
Local subprogram
COMPILING SUBPROGRAMS

Ø  SQL> Alter procedure P1 compile;
Ø  SQL> Alter function F1 compile;

SUBPROGRAMS DEPENDECIES

Ø  A stored subprogram is marked as invalid in the data dictionary if it has compile errors.
Ø  A stored subprogram can also become invalid if a DDL operation is performed on one of its dependent objects.
Ø  If a subprogram is invalidated, the PL/SQL engine will automatically attempt to recompile in the next time it is called.
Ø  If we have two procedures like P1 and P2 in which P1 depends on P2. If we compile P2 then P1 is invalidated.

SUBPROGRAMS DEPENDENCIES IN REMOTE DATABASES

Ø  We will call remote subprogram using connect string like P1@ORACLE;
Ø  If we have two procedures like P1 and P2 in which P1 depends on P2 but P2 was in remote database. If we compile P2 it will not invalidate P1 immediately because the data dictionary does not track remote dependencies.
Ø  Instead the validity of remote objects is checked at runtime. When P1 is called, the remote data dictionary is queried to determine the status of P2.
Ø  P1 and P2 are compared to see it P1 needs to be recompiled, there are two different methods of comparision
ü  Timestamp Model
ü  Signature Model

TIMESTAMP MODEL

Ø  This is the default model used by oracle.
Ø  With this model, the timestamps of the last modifications of the two objects are
      compared.
Ø  The last_ddl_time field of user_objects contains the timestamp.
Ø  If the base object has a newer timestamp than the dependent object, the
     dependent object will be recompiled.

ISSUES WITH THIS MODEL

Ø  If the objects are in different time zones, the comparison is invalid.
Ø  When P1 is in a client side PL/SQL engine such as oracle forms, in this case it may not possible to recompile P1, because the source for it may not be included with the forms.

SIGNATURE MODEL

Ø  When a procedure is created, a signature is stored in the data dictionary in addition to the p-code.
Ø  The signature encodes the types and order of the parametes.
Ø  When P1 is compiled the first time, the signature of P2 is included. Thus, P1 only needs to recompiled when the signature of P2 changes.
Ø  In order to use the signature model, the parameter REMOTE_DEPENDENCIES_MODE must be set to SIGNATURE. This is a parameter in the database initialization file.

THREE WAYS OF SETTING THIS MODE

Ø  Add the line REMOTE_DEPENDENCIES_MODE=SIGNATURE to the database initialization file. The next time the database is started, the mode will be set to SIGNATURE for all sessions.
Ø  Alter system set remote_dependencies_mode = signature;
     This will affect the entire database (all sessions) from the time the statement is
     issued. You must have the ALTER SYSTEM privilege to issue this command.
Ø  Alter session set remote_dependencies_mode = signature;
     This will only affect your session

ISSUES WITH THIS MODEL

Ø  Signatures don’t get modified if the default values of formal parameters are
     changed.

Ø  Suppose P2 has a default value for one of its parameters, and P1 is using this
     default value. If the default in the specification for P2 is changed, P1 will not be
          recompiled by default. The old value for the default parameter will still be used until
          P1 is manually recompiled.
Ø  If P1 is calling a packaged procedure P2, and a new overloaded version of P2 is added to the remote package, the signature is not changed. P1 will still use the old version(not the new overloaded one) until P1 is recompiled manually.

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